We describe a novel computational framework based on the cut finite element method (CutFEM) for process-oriented simulation in mechanized tunneling. The framework incorporates all relevant components required for the simulation of the tunnel advance process, namely the ground, the staged installation of the lining support, the tail void grouting and the tunnel boring machine. We demonstrate that CutFEM concepts can significantly facilitate the modeling, discretization and coupling of the different components, while maintaining the same accuracy as the standard boundary-fitted finite element method. The proposed CutFEM technology, which is being applied and investigated in the context of advancement simulations in mechanized tunneling for the first time, enables the seamless analysis of an arbitrary number of different tunnel alignment variants on the same structured background mesh without the need to set up a new model for each variant. This is a shift of paradigm in simulation-supported tunnel design, as the CutFEM based framework considerably facilitates a direct integration of geometric, building information and simulation models in early stages of a tunnel project. The simulation model allows for the damage assessment of the buildings during tunnel advancement with regards to different excavation scenarios, as shown in the numerical examples in this paper. © 2022 Elsevier Ltd

The solution of saddle-point problems, such as the Stokes equations, is a challenging task, especially in large-scale problems. Multigrid methods are one of the most efficient solvers for such systems of equations and can achieve convergence rates independent of the problem size. The smoother is a crucial component of multigrid methods and significantly affects its overall efficiency. We propose a Vanka-type smoother that we refer to as Restricted Additive Vanka and investigate its convergence in the context of adaptive geometric multigrid methods for the Stokes equations. The proposed smoother has the advantage of being an additive method and provides favorable properties in terms of algorithmic complexity, scalability and applicability to high-performance computing. We compare the performance of the smoother with two variants of the classical Vanka smoother using numerical benchmarks for the Stokes problem. We find that the restricted additive smoother achieves comparable convergence rates to the classical multiplicative Vanka smoother while being computationally less expensive per iteration, which results in faster solution runtimes. © 2022 Elsevier Inc.

During curing or hydration processes, materials such as polymers or fresh concrete undergo microstructural changes, which manifest themselves on the macroscopic scale as evolving material properties like strength or stiffness. Considering the increasing importance of additive manufacturing techniques using this type of “aging” materials, which typically undergo large deformations during the extrusion and deposition processes, a consistent finite strain model is required that takes evolving material properties and the proper characterization of the large deformation kinematics into account. In the proposed formulation, the problem of evolving stiffness is solved, in contrast to hypoelastic rate formulations typically used for this type of problems, by means of a multiplicative split of the deformation gradient into elastic and non-recoverable aging parts and the adoption of a hyperelastic potential. The existence of a hyperelastic potential is an advantage as it easily allows accounting for thermodynamic consistency. By introducing an internal aging parameter, a hyperelastic model based on principal logarithmic strains is adopted, to derive a novel and consistent evolution law for the aging part of the deformation gradient. The incremental and temporal discretization of the proposed constitutive model leads to a stress update scheme, which is reduced to a single multiplication of the principal logarithmic strains by a certain factor. As only minor adaptions are necessary, the proposed model is very attractive for implementations in already existing numerical models. In a benchmark study, the main aspects of the formulation are discussed, and the applicability of the proposed model is demonstrated by a computational analysis of a 3D printed concrete wall. © 2022 Elsevier B.V.

Introduction: Coracoid fractures after arthroscopic treatment of acromioclavicular (AC) joint separations lead to poor clinical outcomes. In this study, different configurations of bone tunnels in the lateral clavicle and coracoid were examined concerning the amount of stress induced in the coracoid. Methods: An authentic 3D finite element model of an ac joint was established. Three 2.4 mm bone tunnels were inserted in the lateral clavicle, which were situated above, medially and laterally of the coracoid. Then, two 2.4 mm bone tunnels were inserted in the latter, each simulating a proximal and a distal suture button position. Von Mises stress analyses were performed to evaluate the amount of stress caused in the coracoid process by the different configurations. Then, a clinical series of radiographs was examined, the placement of the clavicle drill hole was analyzed and the number of dangerous configurations was recorded. Results: The safest configuration was a proximal tunnel in the coracoid combined with a lateral bone tunnel in the clavicle, leading to an oblique traction at the coracoid. A distal bone tunnel in the coracoid and perpendicular traction as well as a proximal tunnel in the coracoid with medial traction caused the highest stresses. Anatomical placement of the clavicle drill hole does lead to configurations with smaller stresses. Conclusion: The bone tunnel placement with the smallest amount of shear stresses was found when the traction of the suture button was directed slightly lateral, towards the AC joint. Anatomical placement of the clavicle drill hole alone was not sufficient in preventing dangerous configurations. Level of evidence: Controlled laboratory study. © 2022, The Author(s).

Product quality and processing of additively manufactured concrete components strongly depend on the flow processes during material extrusion. To control layer deformations and enable purposeful design, numerical analyses with varying process and material parameters were performed to obtain a deeper understanding of flow processes and forces developing in the vicinity of the nozzle using the Lagrangian-based Particle Finite Element Method in association with a Bingham constitutive model. This model was validated by comparing the simulated layer geometries with those obtained from laboratory 3D-printing experiments. Within the investigated parameter range, the forces generated under the extrusion nozzle can be 6 times higher than those induced by self-weight and may cause deformations in substrate layers. Since the distribution of extrusion forces may change substantially under the nozzle for varying parameters, a novel indicator based on the yielding material is introduced to find optimal 3D-printing parameters to prevent plastic deformations in substrate layers. © 2022 Elsevier B.V.

In this contribution, a Finite Element modelling scheme for steel-fibre reinforced concrete (SFRC) is proposed with which the post-cracking response of fibre reinforced structural members can be predicted. In contrast to the common guidelines, the post-cracking response of SFRC is derived from the actual fibre properties instead of indirectly from bending tests. The numerical model is designed to directly track the influence of design parameters such as fibre type, fibre orientation, fibre content and concrete strength on the structural response. For this purpose, sub models on the single fibre level are combined into a crack bridging model, considering the fibre orientation and the fibre content, and are integrated into a finite element model for the purpose of numerical structural analysis. The predictive capability of the proposed numerical multi-level model for SFRC is systematically validated by means of test series performed on the fibre, crack and the structural level. The experimental study comprises pull-out tests of Dramix 3D fibres, uniaxial tension tests involving different fibre contents and fibre types as well as three-point bending tests on notched beams with 23 and 57 kg/m3 Dramix 3D fibres. Furthermore, the results are compared to the modelling approach presented in the fib model code 2010 and an inverse analysis approach. © 2022, RILEM.

The risk assessment of tunneling-induced damage in buildings is a challenging task in geotechnical and structural engineering. It is important to consider the soil-structure interaction during the tunnel construction process. In this paper, finite-element (FE) simulation models of mechanized tunneling processes are combined with FE models of buildings to predict tunneling-induced damage. The soil-structure interaction is taken into account by considering the building stiffness in the tunneling process simulation model and by applying the computed foundation settlements as boundary conditions of the building model. The building damage risk is assessed by means of strains in the structural members and a corresponding category of damage is determined. Uncertainties of the geotechnical parameters and the structural parameters are quantified as random variables and intervals in the framework of polymorphic uncertainty modeling. For real-time predictions, the FE simulation models are approximated by artificial neural networks. This makes it possible to predict the structural damage risk according to scenarios of the operational tunneling process parameters in order to assist machine drivers during tunnel construction. © 2021 American Society of Civil Engineers.

The circular lining in mechanized tunnelling consists of concrete segments, which are exposed to different loading cases during tunnel construction. One of the most critical loading condition is the thrust jack force, which is induced to the lining segments by the Tunnel Boring Machine (TBM) during construction. Experimental campaigns showed that steel fibre reinforced concrete is suitable for bearing such loads and could replace conventional reinforcement schemes. In this contribution, a numerical model is presented, which allows to directly track the influence of important design parameters such as fibre type, fibre orientation, fibre content and concrete strength on the structural response of steel-fibre reinforced segments. For this purpose, submodels on the single fibre level are combined into a crack bridging model considering the fibre orientation and the fibre content. The submodels are integrated into a finite element model to perform numerical structural analyses. Two validation examples demonstrate that the modelling approach is capable to predict the failure loads as well as the crack development of fibre reinforced specimens subjected to localized loads. Finally, an optimization procedure is carried out to determine a robust and cost-effective design of a fibre reinforced segmental linings. A hybrid reinforcement scheme consisting of two layers of fibre reinforced concrete is employed in order to provide an improved material utilization. The thickness of the segment and the fibre content are optimized taking uncertainties of the material parameters and construction tolerances as uncertain a priori parameters into account. A sufficient load bearing capacity and serviceability performance under all possible conditions is ensured by the consideration of accepted failure probabilities as constraints in the optimization task. © 2022 Elsevier Ltd

In this paper, a topology optimization approach is presented, where uncertain load and uncertain material parameters are considered. The concept of compliance minimization, i.e., stiffness maximization, is applied based on a plane stress finite element formulation. In order to take uncertain structural load parameters and uncertain material behavior into account, the topology optimization is embedded into a reliability-based design optimization approach. Uncertain structural parameters and design variables are quantified as random variables, intervals and probability boxes (p-boxes). This allows to consider aleatory and epistemic uncertainties by means of polymorphic uncertainty models within the topology optimization. Solving optimization problems with random variables, intervals and p-boxes leads to a high computational effort, because the objective functions and constraints have to evaluated millions of times. To speed up the optimization process, the finite element simulation of the topology optimization is replaced by artificial neural networks. This includes the topology dependent maximal stresses and displacements of the structure, which are used as constraints, and also the material density distribution inside the design domain. The reliability-based optimization of structural topologies approach is applied to a cantilever structure and a single span girder. © 2022 Elsevier Ltd

The description of the freezing characteristics of porous media is one of the most conspicuous ingredients in flow and heat transport models that involve freezing and thawing processes. Unfrozen liquid content (ULC) shows strong hysteresis during freezing and thawing cycles in different types of soils and other porous media. We discuss the possible mechanisms of hysteresis in porous media and develop a numerical model for the unfrozen liquid content that is capable of describing the hysteresis phenomenon in freezing and thawing cycles. We present a coupled finite element model as the framework for the numerical simulation of fluid flow and heat transport in partially frozen porous media. The implementation aspects of the ULC model as well as its integration into numerical codes are discussed in detail. We investigate the potential impact of the hysteresis phenomenon on the numerical simulation of transport processes in porous media through benchmark examples and validate the behavior of the model against available laboratory measurement data. © 2021

A mixed u–p edge-based smoothed particle finite element formulation is proposed for computational simulations of viscous flow. In order to improve the accuracy of the standard particle finite element method, edge-based and face-based smoothing operations on the displacement gradient are proposed for 2D and 3D analyses, respectively. Consequently, spatial integration involving the smoothing operator is performed on smoothing domains. The constitutive model is based on an elasto-viscoplastic formulation allowing for simulations of viscous fluid or fluid-like solid materials. The viscous response is modeled using an overstress function. The performance of the proposed edge-based smoothed particle finite element method (ES-PFEM) is demonstrated by several numerical benchmark studies, showing an excellent agreement with analytical and reference solutions and an improved accuracy and computational efficiency in comparison with results from the standard PFEM model. Finally, a numerical application of the ES-PFEM to the computational simulation of the extrusion process during 3D-concrete-printing is discussed. © 2021, The Author(s).

In this technical note, a consistent finite element formulation of the Hyperstatic Reaction Method (HRM) for tunnel linings design is proposed by introducing a variational consistently linearized formulation. It permits to consider a nonlinear interaction between a lining structure and the surrounding ground. Recent advances of the HRM in regard to the consideration of the nonlinear response of the segmented tunnel lining exposed to design loads use an iterative algorithm for solving the nonlinear system of equations. In the proposed Variationally consistent Hyperstatic Reaction Method (VHRM), a distributed nonlinear spring model representing the interaction between the lining and the ground soils is considered in a variationally consistent format. Computing the tangential spring stiffness via consistent linearization, and using Newton-Raphson iteration, requires significantly smaller number of iterations as compared to the original HRM model based on nodal springs. Furthermore, the method is applicable for simulations using solid finite elements (2D and 3D), as well as beam or finite shell elements, respectively. © 2021 The Authors. International Journal for Numerical and Analytical Methods in Geomechanics published by John Wiley & Sons Ltd.

Measurements have shown that the pressure distribution at the front of an EPB shield drilling in saturated granular material is not symmetric with respect to the vertical plane. This was confirmed recently by computational simulations of the material flow within different EPB pressure chambers. The fluid pressure is higher in the part where the cutting wheel is moving upwards, compared to the pressure in the part where the movement is downward. This unbalance may influence both the steering of the TBM and the stability of the tunnel face. The pressure distribution as measured with pore pressure gauges at the cutting wheel are used to calculate the centre of gravity of the muck in an EPB and the measured pressure differences are compared with theoretical calculations. The pore pressure distribution in the mixing chamber along lines parallel to the tunnel axis is discussed. © 2021 ISSMGE, London, UK

This paper presents a novel concept for vibration-based feature extraction to identify damages in cutting discs of Tunnel Boring Machines (TBM). Defect frequencies resulting from repeated interaction of rock and disc defects are analysed. The data set is represented by the normal force acting on the edge of a cutting disc and the rock. Two different methods, the Hilbert transform and the complex demodulation, are used to generate the envelope of the time series, which was used to analyse whether the signal shows a feature representing an existing defect in the frequency domain. For the first proof of concept two numerical models were used - a multi-body system and a peridynamics 3D model simulating time series of normal forces. With both models, the linear motion of the disc on a rock sample with constant velocity was simulated. An experimental setup, mechanically similar to the simulations, was used in two experiments for further comparison. All implemented defects could be detected using vibration data of forces and one of the proposed data analysis techniques. © 2021, Springer Nature Switzerland AG.

Concrete is a heterogeneous material with a disordered material morphology that strongly governs the behaviour of the material. In this contribution, we present a computational tool called the Concrete Mesostructure Generator (CMG) for the generation of ultra-realistic virtual concrete morphologies for mesoscale and multiscale computational modelling and the simulation of concrete. Given an aggregate size distribution, realistic generic concrete aggregates are generated by a sequential reduction of a cuboid to generate a polyhedron with multiple faces. Thereafter, concave depressions are introduced in the polyhedron using Gaussian surfaces. The generated aggregates are assembled into the mesostructure using a hierarchic random sequential adsorption algorithm. The virtual mesostructures are first calibrated using laboratory measurements of aggregate distributions. The model is validated by comparing the elastic properties obtained from laboratory testing of concrete specimens with the elastic properties obtained using computational homogenisation of virtual concrete mesostructures. Finally, a 3D-convolutional neural network is trained to directly generate elastic properties from voxel data. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

The process of ASR (Alkali-Silica Reaction) induced expansion and damage in pavement concrete specimens is investigated using laboratory experiments and computational modeling. In the experimental program, the concrete specimens are subject to CS-CPT (climate simulation concrete prism test) to obtain ASR induced expansion with and without external supply of alkali. The dissolution rates of the granodiorite used in the concrete mix and the gel formation rates are determined under concrete-like conditions (pH 13.8, with/without Ca(OH)2 and NaCl) at different temperatures. A micromechanics based computational model with aggregate-scale diffusion and reaction kinetics coupled to an Eigenstrain based micromechanics damage model is developed for the simulation of ASR induced expansion and damage. Data from the experimental program are used to calibrate and validate the computational model. Model predictions show that for the given concrete mixture, ASR induced expansion in the specimen exposed to water is predominantly governed by microcracking in the aggregate, while the expansion in the specimen subjected to external alkali supply is governed by microcracking in both the aggregates and the cement paste. © 2021 Elsevier Ltd

In the present study, the capability of high-strength short steel fibers to control the degrada-tion in high-performance concrete was experimentally examined and numerically simulated. To this end, notched prismatic high-performance concrete specimens with (HPSFRC) and without (HPC) short steel fibers were subjected to static and cyclic tensile tests up to 100,000 cycles. The cyclic tests showed that the rate of strain increase was lower for HPSFRC specimens and that the strain stagnated after around 10,000 cycles, which was not the case with HPC specimens. The microscopic examinations showed that in HPSFRC, a larger number of microcracks developed, but they had a smaller total surface area than the microcracks in the HPC. To further investigate the influence of fibers on the behavior of HPSFRC in the cracked state, displacement-controlled crack opening tests, as well as numerical simulations thereof, were carried out. Experiments have shown, and the numerical simulations have confirmed, that the inclusion of short steel fibers did not significantly affect the ultimate strength; however, it notably increased the post-cracking ductility of the material. Finally, the unloading/reloading behavior was examined, and it was observed that the unloading stiffness was stable even for significant crack openings; however, the hysteresis loops due to unloading/reloading were very small. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Large infrastructure projects involving the construction of tunnels in urban areas constitute complex, integrated and multi-disciplinary systems, which require building and construction information modelling as well as computational design assessment tools for decision making during all project phases and during their complete life cycle. Even if the underlying information needed for computational analysis is stored in an information model, the translation to computational models is still cumbersome and requires significant manual work for model generation and set-up as well as excessive computing resources and time. To address these shortcomings, this paper presents a systematic summary of concepts for integrated information modelling, numerical analysis and visualisation for urban mechanized tunnelling. Our first approach “BIM-to-FEM” is characterised by a fully automated link for error-free data exchange between a standalone Tunnelling Information Model and the process-oriented simulation model for mechanized tunnelling “ekate”. In the second approach “SATBIM”, a fully automated data exchange workflow is established between a parametric multi-level information model for tunnelling and multi-level numerical models based on both Finite Element and Isogeometric Analysis, where meta models are employed for real-time design assessment. We discuss the different applications of these concepts, such as scenario-based exploration of design alternatives, real-time design assessment within a TIM based on meta-models, and the potentials of using these models for the process control during construction. Furthermore, we present two case studies where real project data has been used for the integration of information and numerical modelling. The examples in this paper indicate clear advantages of this approach compared to traditional approaches in terms of efficiency of modelling achieved by reduced user interactions and error-free information exchange, and show the benefits of multi-level model representation and real-time analysis tasks. © 2020 Elsevier Ltd

In the paper, the main steps involved in the development of an object-oriented computational framework for the 3D bending and free vibration analysis of multilayer plates are presented. The mathematical formulation for layered finite elements is based on Reddy's plate theory for laminated composites. The analysis model has been implemented into Matlab, and the pre- and post-processing phases are performed using GiD. The proposed solver is characterized by a fast assembly procedure of sparse matrices using matrix vectorization, and a novel algorithm for the evaluation of interlaminar stresses satisfying continuity at layer interfaces. The performance, efficiency and accuracy of the computational framework are demonstrated through a number of validation examples by comparing the obtained results against the exact solution. Results from both static and dynamic analyses of multilayer panels are shown. © 2020 Elsevier Ltd

In the case of a sudden change in the geology in front of the Tunnel Boring Machine (TBM) during mechanized tunneling processes, non-appropriate investigation and process adaptation may result in non-desirable situations that can induce construction and machine defects. Therefore, subsurface anomalies detection is necessary to trigger alarm to update the process. This paper presents an approach for geological anomaly detection using data produced by the TBM. The data observations are continuously produced at a motion of 10 to 15 s from hundreds of sensors around the TBM. Unsupervised machine learning techniques are applied to analyze the online streaming data. As a result, a model, which is able to learn the system characteristics from normal operational condition and to flag any unanticipated or unexpected behavior, is established. The proposed approach has been tested on the data of the Wehrhahn-Linie metro project in Düsseldorf in Germany. The model can accurately detect the presence of concrete walls in the ground domain with a distance up to around one meter before the TBM approaches the walls. The developed method can thus be used as a monitoring system for ground risks detection to ensure safe and sustainable constructions in mechanized tunneling. © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG.

In peridynamic models for fracture, the dissipated fracture energy is regularized over a non-local region denoted as the peridynamic horizon. This paper investigates the influence of this parameter on the dynamic fracture process in brittle solids, using two as well as three dimensional simulations of dynamic fracture propagation in a notched plate for two loading cases. The predicted crack speed for the various scenarios of the initially stored energy, also known as the velocity toughening behavior as well as characteristics of the crack surface topology obtained in different crack propagation regimes in 3D computational simulations are compared with the experimentally observed crack velocity and fracture surfaces for Polymethyl Methacrylate (PMMA) specimens. In addition, we investigate the influence of the specimen size on the dynamic fracture process using two dimensional peridynamic simulations. The fracture strengths and the velocity toughening relationship obtained from different specimen sizes are compared with the Linear Elastic Fracture Mechanics (LEFM) size effect relationship and with results from experiments, respectively. © 2021, The Author(s).

Until now, vibration immissions caused by dynamically excited compactors are usually predicted using empirical formulas, whose uncertainties come from lack of knowledge about the influences of machine parameters and soil properties. A novel predictive method is presented, which is based on the individual force emission of the machine depending on the machine-soil interaction and the measured transfer mobility. The transfer mobility, which is examined by artificial vibration excitation with an impulse generator, is the transfer function between force excitation and received vibration velocity. Emitted spectral forces, which can be used for prognosis, are previously determined by extensive measurements. The developed method provides vibration values in the frequency and time domain taking into account statistical variations of influencing parameters. The comparison of predicted and measured values shows an enhanced accuracy of the new method. © 2021, VDI Fachmedien GmBbH & Co.. All rights reserved.

Damage in concrete structures initiates as the growth of diffuse microcracks that is followed by damage localisation and eventually leads to structural failure. Weak changes such as diffuse microcracking processes are failure precursors. Identification and characterisation of these failure precursors at an early stage of concrete degradation and application of suitable precautionary measures will considerably reduce the costs of repair and maintenance. To this end, a reduced order multiscale model for simulating microcracking-induced damage in concrete at the mesoscale level is proposed. The model simulates the propagation of microcracks in concrete using a two-scale computational methodology. First, a realistic concrete specimen that explicitly resolves the coarse aggregates in a mortar matrix was generated at the mesoscale. Microcrack growth in the mortar matrix is modelled using a synthesis of continuum micromechanics and fracture mechanics. Model order reduction of the two-scale model is achieved using a clustering technique. Model predictions are calibrated and validated using uniaxial compression tests performed in the laboratory. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

This paper presents a finite element methodology for modelling solid media with any number of straight or curved reinforcing bars (rebars). The formulation can handle rebars arbitrarily located within a 2D or 3D finite element mesh without the need for nodal compatibility. A new continuous rod–solid interface (or joint) element is developed to connect bulk elements to the rebars, including a method that directly considers the relative displacements. An arbitrary constitutive model can be used in conjunction with the proposed interface element to represent slip behaviour. In this regard, we also present a cyclic nonlinear constitutive model. Unlike existing embedded approaches, the proposed method allows for the direct application of boundary conditions at reinforcements’ endpoints, including tip bearing. The formulation is verified by comparing with literature examples and analytical solutions. In all cases, excellent agreement is observed. The approach can be conveniently applied to the study of complex reinforced concrete structures or any solid medium where reinforcing bars improve its mechanical properties. © 2021 Elsevier B.V.

Ultrasonic measurements are used in civil engineering for structural health monitoring of concrete infrastructures. The late portion of the ultrasonic wavefield, the coda, is sensitive to small changes in the elastic moduli of the material. Coda Wave Interferometry (CWI) correlates these small changes in the coda with the wavefield recorded in intact, or unperturbed, concrete specimen to reveal the amount of velocity change that occurred. CWI has the potential to detect localized damages and global velocity reductions alike. In this study, the sensitivity of CWI to different types of concrete mesostructures and their damage levels is investigated numerically. Realistic numerical concrete models of concrete specimen are generated, and damage evolution is simulated using the discrete element method. In the virtual concrete lab, the simulated ultrasonic wavefield is propagated from one transducer using a realistic source signal and recorded at a second transducer. Different damage scenarios reveal a different slope in the decorrelation of waveforms with the observed reduction in velocities in the material. Finally, the impact and possible generalizations of the findings are discussed, and recommendations are given for a potential application of CWI in concrete at structural scale. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

During the planning phase of tunneling projects, in particular in urban areas, it is crucial to assess the likely extent of structural damage caused by the tunnel construction in close vicinity to existing surface structures. Tunneling inevitably causes ground movements which in turn may have an impact on deformations and stresses of the above-ground structures. For this reason, a reliable estimate of the soil-structure interactions due to tunnelling-induced settlements is essential. In this contribution, various approaches that differ in precision and complexity are employed to predict the magnitude of expected settlements and the vulnerability of structures with regard to tunneling induced damage. In addition, a three-step damage assessment concept adjustable to the necessary level of detail is suggested. Firstly, ground movements are predicted using analytical or numerical approaches. Secondly, the above-ground structures are idealized by means of surrogate beam-, slab- or 3D-models. Finally, structural damage is assessed according to the computed strain pattern or the tilt of the building. This method enables the evaluation of the potential damage to above ground structures associated with planned tunnel alignment. When developing 3D numerical models, the main focus will be to ensure that the building and the soil-structure interactions are represented with an appropriate level of detail. To this end, this paper aims to provide recommendations for a sufficient level of detail (LOD) of surface structures for the assessment of tunneling induced damage in computational simulations in mechanized tunneling. © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Tunnel linings are designed to permanently fulfill basic structural, serviceability and durability requirements throughout the lifetime of a tunnel. In order to ensure structural stability, it is important to correctly assess the response of tunnel linings with respect to the loading from the ground and process loads to which lining structures are subjected. For the design of segmental tunnel linings, precise structural models are needed, as the segmentation imbues the lining system with non-trivial kinematics. In this contribution, a technique for modeling the segment-wise installation of tunnel linings in the context of a 3D tunnel advance simulation is proposed in order to better predict the time dependent structural forces that develop in segmental lining systems during tunnel advance. The segments of the lining ring are explicitly modeled as separate bodies, and the interactions between segments at the longitudinal and ring joints are modeled by means of a surface-to-surface frictional contact algorithm. In order to examine the 3D stress distribution in the segmental concrete lining under realistic, time-dependent process loadings, the lining model is integrated into the process oriented finite element simulation model ekate. The influence of the joint arrangement and the segmentation is investigated through comparison with simulations in which a standard, continuous lining modeling technique is employed and with standard structural beam models used in engineering practice. It is shown that the magnitude of structural forces obtained by the explicit modelling of segmental lining joints and their time-dependent installation process within a 3D structural model diverges significantly from those obtained using standard methods, i.e. bedded beam models. © 2021 Elsevier Ltd

This paper presents an extension of the variational interface fracture model proposed in Khisamitov and Meschke (2018) to model fluid driven fracture in porous materials. The fluid saturated material is described by the theory of poroelasticity and the effective stress concept, while the coupling between fluid flow and fracture initiation and propagation is accomplished via a variational interface element formulation, using a damage variable c as an additional degree of freedom in conjunction with linear momentum and mass balance equations along the interface zones. Introducing the damage variable c, fracture propagates according to the values of the minimizers of the total potential energy expressed in terms of effective stresses. Hence, neither local crack propagation criteria nor tracking algorithms are required. Taking into account that a fracture grows quasi-statically, the discretized system of PDEs is solved by the backward Euler-scheme, ignoring the contribution from inertia forces. In the Newton–Raphson iterative solution procedure, an operator splitting algorithm is employed to solve first the poroelastic equations, and updating subsequently the damage variable c. The proposed model for hydraulic fracturing is validated first by means of one-dimensional analytical solutions in toughness as well as in viscosity dominated regimes. The performance and the applicability of the model to simulate interactions between propagating and existing fractures is demonstrated by means of 2D and 3D numerical examples. © 2020 Elsevier B.V.

A numerical model based on the particle finite element method (PFEM) combined with a hypoplastic constitutive model is proposed for the analysis of soft soil excavations by means of single and multiple excavation tools. The PFEM allows to efficiently account for large deformations of the excavated soil material and free surfaces characterizing the simulation of tool–soil interaction during excavation. The utilization of a hypoplastic model, formulated in rate form, allows for a straightforward coupling with the standard velocity-based PFEM. Furthermore, effects such as pressure and density dependency of the soil stiffness are consistently incorporated into the formulation. The solution of the governing equations is performed implicitly, while an adaptive sub-stepping scheme is employed for the explicit time integration of the constitutive equation. Thus, the accuracy of the solution is improved at both global and local (constitutive) levels. The performance of the method is evaluated by means of the numerical re-analysis of selected geotechnical benchmark examples and soft soil excavations in 2D and 3D setups. © 2019, OWZ.

3D computational simulations of the tunnel advancement in mechanized tunneling usually demand high computational resources. This paper addresses strategies to enable large scale simulations of the advancement process by means of iterative, parallel solvers, considering frictional contact between the tunnel boring machine and the soil, fully saturated soil with different permeabilities and a number of additional features inherently connected with tunnel simulations, which require special attention in regards to robustness. To improve the computational performance of mechanized tunnel simulations a solution strategy based on domain decomposition is proposed to solve the resulting discretized linear system in parallel using an appropriately accelerated iterative solver. The interaction between the TBM and the ground is modeled by means of robust contact algorithm based on penalty method. The domain decomposition scheme accounts for contact treatment in parallel, which becomes relevant when the TBM traverses through different domains in the mesh. Different variants of block preconditioners are investigated in regards to their performance to speed-up the convergence of the iterative solver for the block linear system arising from simulations of tunnel advancement in saturated soft soil, considering different permeabilities. A validation of the computational model and selected numerical examples to showcase the efficiency of the proposed method and to verify the applicability of this strategy to high performance tunneling simulations are presented. © 2019 Elsevier Ltd

Tunnelling induced surface settlements can cause damage in buildings located in the vicinity of the tunnel. Currently, surface settlements and associated building damage risks usually are estimated based on empirical equations, e.g. by assuming Gaussian curves for the settlement trough and by applying the Limit Tensile Strain Method or the tilt-based method to evaluate and categorise the expected building damage. In this paper, finite element simulations are used to predict the soil-structure interaction in mechanised tunnelling during the tunnel advancement. The time variant surface settlement field and the corresponding tunnelling induced strains in the facade of a building are computed by two independent finite element models. Coupling both models allows predicting the expected category of damage (cod) for the building, given the operational parameters of the tunnel drive. Based upon this coupled approach, a method is proposed in the paper, which provides optimised operational parameters (e.g. tail void grouting pressure and face support pressure) during the advancement of tunnel boring machines below vulnerable buildings, such that the risk of damage for existing buildings is minimised. For real-time applicability of this method two different types of Artificial Neural Networks in combination with the Proper Orthogonal Decomposition approach are generated as surrogate models of the finite element simulations. The surrogate models are finally linked and implemented into a user-friendly application, which can be used as an assistant tool to adjust the operational parameters of the tunnel boring machine at the construction site. © 2020 Elsevier Ltd

Both planning and design phase of large infrastructural project require analysis, modelling, visualization, and numerical analysis. To perform these tasks, different tools such as Building Information Modelling (BIM) and numerical analysis software are commonly employed. However, in current tunnel engineering practice, there are no systematic solutions for the exchange between design and analysis models, and these tasks usually involve manual and error-prone model generation, setup and update. In this paper, focussing on tunnelling engineering, we demonstrate a systematic and versatile approach to efficiently generate a tunnel design and analyse the lining in different practical scenarios. To this end, a BIM-based approach is developed, which connects a user-friendly industry-standard BIM software with effective simulation tools for high-performance computing. A fully automatized design-through-analysis workflow solution for segmented tunnel lining is developed based on a fully parametric design model and an isogeometric analysis software, connected through an interface implemented with a Revit plugin. The IGA-Revit interface implements a reconstruction algorithm based on sweeping teachnique to construct trivariate NURBS lining segment geometry, which avoids the burden to deal with trimmed geometries. © 2020 Elsevier Ltd

Experimental research has shown the extraordinary potential of the addition of short fibers to cement-based materials by improving significantly the behavior of concrete structures for serviceability and ultimate limit states. Software based on the finite element method has been used for the simulation of the material nonlinear behavior of fiber-reinforced concrete (FRC) structures. The applicability of the existing approaches has often been assessed by simulating experimental tests with structural elements, in general of a small scale, where the parameter values of the material constitutive laws are adjusted for the aimed predicting level, which constitutes an inverse technique of arguable utility for structural design practice. For assessing the predictive performance of these approaches, a blind simulation competition was organized. Two twin T-cross section steel FRC beams, flexurally reinforced with steel bars and without conventional shear reinforcement in the critical shear span, were experimentally tested up to failure. Despite the experimental data provided for the definition of the relevant model parameters, inaccuracies on the load capacity, deflection, and strain at peak load attained 40, 113, and 600%, respectively. Inadequate failure modes and highly different results were estimated with the same commercial software, indicating the need for deeper analysis and understanding of the models and influence of their parameters on their predictive performance. © 2020 fib. International Federation for Structural Concrete

There is an increasing need for the development of novel technologies for tunnel construction in difficult geological conditions to protect segmental linings from unexpected large deformations. In the context of mechanized tunneling, one method to increase the damage tolerance of tunnel linings in such conditions is the integration of a compressible two-component grout for the annular gap between the segmental linings and the deformable ground. In this regard, expanded polystyrene (EPS) lightweight concrete/mortar has received increasing interest as a potential “candidate material” for the aforementioned application. In particular, the behavior of the EPS lightweight composites can be customized by modifying their pore structure to accommodate deformations due to specific geological conditions such as squeezing rocks. To this end, novel compressible cementitious EPS-based composite materials with high compaction potential have been developed. Specimens prepared from these composites have been subjected to compressive loads with and without lateral confinement. Based on these experimental data a computational model based on the Discrete Element Method (DEM) has been calibrated and validated. The proposed calibration procedure allows for modeling and prognosis of a wide variety of composite materials with a high compaction potential. The calibration procedure is characterized by the identification of physically quantifiable parameters and the use of phenomenological submodels. Model prognoses show excellent agreement with new experimental measurements that were not incorporated in the calibration procedure. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

Cut-cell quadrature based on the moment fitting scheme generates an accurate numerical integration rule for each cut element with the same small number of point evaluations as a standard Gauss quadrature rule. It therefore significantly increases the efficiency of unfitted finite element schemes such as the finite cell method that have often relied on cut-cell integration with prohibitively many quadrature points. Moment fitting, however, does not directly apply to inhomogeneous integrands as they result from nonlinear material behavior. In this article, we describe a novel modification of moment fitting approach that opens the door for its application in materially nonlinear analysis. The basic idea is the decomposition of each cut cell into material subdomains, each of which can be assigned a physically valid location where constitutive integration and the update of local history variables can be performed. We formulate a moment fitting scheme for each material subdomain using the same quadrature points, such that the resulting weights from all material subdomains can be added and the total number of point evaluations remains the same as in standard Gauss quadrature. We discuss numerical details of the modified scheme, including its ramifications for consistent linearization, and demonstrate its optimal performance in the context of the finite cell method and elastoplasticity. © 2020 Elsevier B.V.

This paper studies elasto-plastic large deformation behaviour of thin shell structures using the isogeometric computational approach with the main focus on the efficiency in modelling the multi-patches and arbitrary material formulation. In terms of modelling, we employ the bending strip method to connect the patches in the structure. The incorporation of bending strips allows to eliminate the strict demand of the C1 continuity condition, which is postulated in the Kirchhoff-Love theory for thin shell, and therefore it enables us to use the standard multi-patch structure even with C0 continuity along the patch boundaries. Furthermore, arbitrary nonlinear material models such as hyperelasticity and finite strain plasticity are embedded in the shell formulation, from which a unified thin shell formulation can be achieved. In terms of analysis, the Bézier decomposition concept is used to retain the local support of the traditional finite element. The performance of the presented approach is verified through several numerical benchmarks. © 2020

Earth Pressure Balance (EPB) shield machines are widely used in mechanized tunneling operations in soft grounds. In contrast to slurry shield machines, the pressure distribution in EPB excavation chambers is not well undersood, as it is influenced by the muck properties and the chamber design. A computational model specifically developed in Dang and Meschke (2018) for the numerical simulation of material transport in EPB pressure chambers is employed to investigate the pressure distribution in two different types of EPB chambers, characterized by a different design of the mixing components, the rotators and the screw conveyor. The numerical model allows for the description of all rotating and fixed components of the EPB chambers including the screw conveyor and considers adequately the compressibility and the consistency of the soil paste. The spatio-temporal pressure distribution is investigated with respect to the influence of chamber design and the properties of the soil paste. Essential characteristics of the pressure distribution, such as the pressure unbalance between the left and right side and pressure fluctuations observed in in situ measurements are captured by the numerical analyses. The key factors influencing the pressure unbalance and pressure fluctuations are identified by parametric studies for two different EPB chamber designs. © 2020 Elsevier Ltd

In current tunnelling practice, Finite Element (FE) simulations form an integral element in the planning and the design phase of mechanised tunnelling projects. The generation of adequate computational models is often time consuming and requires data from many different sources, in particular, when manually generated using 2D-CAD drawings. Incorporating Building Information Modelling (BIM) concepts offers opportunities to simplify this process by using geometrical BIM sub-models as a basis for structural analyses. This paper presents a Tunnel Information Model (TIM) as a BIM specifically tailored to fit the needs of mechanised tunnelling projects and a ”BIM-to-FEM“ technology, that automatically extracts relevant information (geology, alignment, lining, material and process parameters) needed for FE simulations from BIM sub-models and subsequently performs FE analysis of the tunnel drive. The results of the analysis are stored centrally on a data server to which the user has continuous access. A case study from the Wehrhahn-Metro line project in Düsseldorf, Germany, is presented and discussed to demonstrate the efficiency and the applicability of the proposed BIM-to-FEM workflow. © 2020, Ernst und Sohn. All rights reserved.

The solution of optimization problems with polymorphic uncertain data requires combining stochastic and non-stochastic approaches. The concept of uncertain a priori parameters and uncertain design parameters quantified by random variables and intervals is presented in this paper. Multiple runs of the nonlinear finite element model solving the structural mechanics with varying a priori and design parameters are needed to obtain a solution by means of iterative optimization algorithms (e.g. particle swarm optimization). The combination of interval analysis and Monte Carlo simulation is required for each design to be optimized. This can only be realized by substituting the nonlinear finite element model by numerically efficient surrogate models. In this paper, a multilevel strategy for neural network based surrogate modeling is presented. The deterministic finite element simulation, the stochastic analysis as well as the interval analysis are approximated by sequentially trained artificial neural networks. The approach is verified and applied to optimize the concrete cover of a reinforced concrete structure, taking the variability of material parameters and the structural load as well as construction imprecision into account. © 2019 Elsevier Inc.

Numerical multi-level model for fiber-reinforced concrete – Multi-level validation based on an experimental study on high-strength concrete. The contribution systematically examines the predictive capability of a numerical multi-level model for steel fiber reinforced concrete made of high-strength concrete by means of test series performed on the fiber as well as the structure level. The experimental study comprises pull-out tests of Dramix 3D fibers in high-strength concrete with different embedment lengths and inclinations to the crack surface as well as three-point bending tests on notched beams with three significantly different fiber contents. The numerical model is designed to directly track the influence of design parameters such as fiber type, fiber orientation, fiber content and concrete strength on the structural response. For this purpose, submodels on the single fiber level are combined into a crack bridging model, considering the fiber orientation and the fiber content, and are integrated into a finite element model for the purpose of numerical structural analysis. The validation of the models for hooked-end steel fibers shows that the interaction mechanisms between fiber and high-strength concrete are realistically captured for all investigated cases (fiber inclinations, embedment lengths). On the structural level, the load-displacement diagrams from the numerical simulations show a good agreement of the peak load as well as the post-peak behavior for all fiber contents. © 2020, Ernst und Sohn. All rights reserved.

Generation of appropriate computational meshes in the context of numerical methods for partial differential equations is technical and laborious and has motivated a class of advanced discretization methods commonly referred to as unfitted finite element methods. To this end, the finite cell method (FCM) combines high-order FEM, adaptive quadrature integration and weak imposition of boundary conditions to embed a physical domain into a structured background mesh. While unfortunate cut configurations in unfitted finite element methods lead to severely ill-conditioned system matrices that pose challenges to iterative solvers, such methods permit the use of optimized algorithms and data patterns in order to obtain a scalable implementation. In this work, we employ linear octrees for handling the finite cell discretization that allow for parallel scalability, adaptive refinement and efficient computation on the commonly regular background grid. We present a parallel adaptive geometric multigrid with Schwarz smoothers for the solution of the resultant system of the Laplace operator. We focus on exploiting the hierarchical nature of space tree data structures for the generation of the required multigrid spaces and discuss the scalable and robust extension of the methods across process interfaces. We present both the weak and strong scaling of our implementation up to more than a billion degrees of freedom on distributed-memory clusters. © Springer Nature Switzerland AG 2020.

In this paper, a concept for safety and durability assessment of reinforced concrete structures is presented. It is based on a multilevel modeling approach, where three different models are combined using simple linear elastic beam models for the reinforcement design, low fidelity nonlinear finite element models to compute the deformation at the full structural scale and finally high fidelity nonlinear finite element models to compute the crack widths at critical hot spots of the structure. The uncertainty of structural parameters is quantified by means of polymorphic uncertainty models combining stochastic and non-stochastic approaches. The multilevel approach is applied to the safety assessment and the crack widths prediction of a reinforced concrete beam structure. Copyright © 2019 European Safety and Reliability Association.

Finite Element simulation is a possible tool to investigate interactions between the Tunnel Boring Machine and the surrounding soil. Surface settlements can be predicted in real-time based on simulation results by machine learning surrogate models. However, to train such models, large amounts of computationally intensive simulations are required. To accomplish this step with minimal costs, we propose a hybrid active learning approach to select the minimal amount of simulations necessary to build an accurate model. During the tunnel construction, the real-time settlements prediction model will be used to analyze associated risks to ensure safe and sustainable constructions in urban areas. © 2019 The Authors. Published by Elsevier Ltd.

A new concept, which combines machine and monitoring data analysis with numerical simulation of mechanized tunneling processes, is presented in this work. The main idea is to fuse sensors data collected from the TBM and the settlement measurements with finite element simulations capturing the physical behavior of the tunneling process. Both data and simulation based prediction models are developed to assist the TBM steering with respect to the settlement behavior in real-time. The real-time machine and monitoring data analysis offers the possibility of detecting anomalies, e.g. in the soil layers, which are not considered a priori in the simulation models. First results show that most anomalies created by concrete walls in the ground can be detected up to 1 m ahead of the cutting wheel by using the specific torque (a combination of torque and penetration rate data) together with time series preprocessing and drift detection methods. © 2019 Taylor & Francis Group, London.

The design of complex underground infrastructure projects involves various empirical, analytical, or numerical models with different levels of complexity. The use of simulation models in the current state-of-the-art tunnel design process can be cumbersome when significant manual time-consuming preparation, analysis, and excessive computing resources are required. This paper addresses the challenges connected with minimizing the user workload and computational time, as well as enabling real-time computations during construction. To ensure a seamless workflow during design and to minimize the computation time of the analysis, we propose a novel concept for Building Information Modeling (BIM)-based numerical simulations enabling the modeling of the tunnel advance with different levels of detail in terms of geometrical representation, material modeling, and modeling of the advancement process. To ensure computational efficiency, simulation software was developed with special emphasis on efficient implementation, including parallelization strategies for shared and distributed memory systems. For real-time on-demand calculations, simulation-based metamodels were integrated into the software platform. The components of the BIM-based multilevel simulation concept were described and evaluated in detail by means of representative numerical examples. © 2019 American Society of Civil Engineers.

This paper proposes a numerical model for the simulation of fracture propagation in brittle materials based on the theory of configurational forces. Configurational forces are adopted as the driving force for fracture in association with a tensile stretch criterion to identify crack tips of propagating cracks. Fracture surfaces are approximated by zero-thickness interface elements embedded into the finite element mesh between the bulk elements in the entire domain. In order to eliminate the mesh bias in regards to the fracture pattern, a new fracture segment is allowed to grow along several interface elements at once. The minimal fracture surface is found using an algorithm based on Graph Theory. The solution strategy is explicit, and the fracture path using this algorithm is identified in a post-processing step. The proposed configurational interface model (CIM) for fracture is validated by the analytical solution for a mode I benchmark and by comparisons with results from a variety of laboratory experiments on cylindrical granite specimens containing one and two initial cracks at different angles, which are characterized by complex curved and partially irregular crack patterns. © 2019 Elsevier B.V.

A multi-scale micromechanics model is proposed to describe the expansion and deterioration of concrete due to Alkali-Silica Reaction (ASR). The mechanics of ASR induced deterioration of a Representative Elementary Volume (REV) of concrete is modeled through a synthesis of distributed microcracking and mean-field homogenization. At the microscale, ASR-gel-pressure induced microcrack growth in and around the reactive aggregates is modeled using the framework of linear elastic fracture mechanics. Mean-field homogenization across multiple scales is used to obtain the overall expansion and degradation of the material. By specifying the spatial distribution of the pressurizing gel, two different ASR mechanisms associated with “slowly” and “rapidly” reactive aggregates can be modeled. Experimental data for concrete degradation as a function of the macroscopic expansion is found to lie within the theoretical upper and lower bounds that characterize the distribution of the gel in the aggregate or the cement paste. © 2018 Elsevier Ltd

Systematic investigations of hardened cement paste, high-performance concrete and mortar with and without microfibers, subjected to static and cyclic tensile loadings, were conducted. The material degradation was investigated by means of microscopic analyses of the microcrack development. Notched specimens were subjected to a predefined number of load cycles. A nonsteady increase of microcracking with increasing load cycles was observed in high-strength concrete, whereas the addition of steel fibers lead to a steady increase of microcracks. High-strength mortar often showed premature failure, while addition of steel micro fibers allowed completion of the cyclic tests. To obtain a deeper insight into physical mechanisms governing fatigue and structural failure, high-performance concrete (HPC) and fiber-reinforced concrete (FRC) under static and cyclic tensile loadings have been modeled using cohesive interface finite elements, micromechanics, and a fiber-bundle model. Analysis of model predictions shows the significance of strength disorder and fiber properties on the structural behavior. © 2019 The Authors. Structural Concrete published by John Wiley & Sons Ltd on behalf of International Federation for Structural Concrete

Multiscale analyses require to consider the scale bridging influences of uncertain parameters. In this paper, approaches for polymorphic uncertainty quantification at different length scales are presented. Especially, the effect of uncertain material parameters computed at lower structural scales is investigated with respect to the resulting macroscopic structural behavior. Also the dependencies of uncertain parameters at the macroscale on the structural response evaluated with submodels are discussed. Three examples are shown, where interval and stochastic uncertainty quantification approaches are combined. Two examples deal with material modeling of concrete and the durability of reinforced concrete structures under consideration of polymorphic uncertainties. A mesoscale model of concrete is developed based on a representative volume element containing aggregates, pores, and the cement phase, where the values of the Young's moduli for mortar and the aggregates as well as the volume fraction of the cement phase to aggregates are considered as intervals. Within the durability assessment of reinforced concrete structures, the influence of stochastic distributed loading and concrete material parameters onto the cracking behavior is analyzed by means of a submodeling strategy. In another application, the metal forming process of dual-phase steel sheets is investigated using statistical information of the microscopic material behavior in combination with epistemic uncertainties of the failure criterion and the friction coefficient between the sheet metal and the forming tools. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this contribution, a numerical design strategy for the optimization under polymorphic uncertainty is introduced and applied to a self-weight minimization of a framework structure. The polymorphic uncertainty, which affects the constraint function of the optimization problem, is thereby modeled in terms of stochastic variables, fuzzy sets, and intervals to account for variability, imprecision and insufficient information. The stochastic quantities are computed using polynomial chaos expansion resulting in a purely fuzzy-valued formulation of the constraint functions which can be computed using α-cut optimization. Afterward, the constraint function can be interpreted in a possibilistic manner, resulting in a flexible formulation to include expert knowledge and to achieve a robust design. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The improper estimation of design ground loadings as well as the exceedance of installation tolerances during segmental lining construction often results in unwanted segment damage. Incurred damages rarely impact the structural stability of the tunnel ring, but can significantly impact the serviceability state of a finished construction. For this reason, existing design codes often provide limits on allowable crack widths. In this contribution, a newly developed Finite Element modeling scheme for hybrid reinforced segmental tunnel linings is proposed with which the non-linear local cracking response can be predicted. By subjecting this model to various loading scenarios (e.g. jack forces, steady-state) and by modeling the exceedance of installation tolerances during the ring-build phase, the response of the segment can be analyzed. Using the proposed method, various traditionally reinforced and fiber-reinforced designs are investigated. Furthermore, hybrid-reinforced designs combining the strengths of both traditional and fiber reinforced concrete are proposed. © 2019 Taylor & Francis Group, London.

Loading assumptions used for the structural design of segmental linings often improperly reflect the complex load combinations that develop during the construction of a bored tunnel. Therefore segment designs used in practice tend to be on the safe side and often rely on conventional reinforcement methods instead of including other reinforcement concepts, such as steel fibres. In this contribution, a multi-scale computational modelling framework is proposed to investigate the response of steel-fibre reinforced, traditionally reinforced, and hybrid-reinforced lining segments to radial loadings with an emphasis on the longitudinal joints. This modelling approach offers an opportunity to directly investigate the influence of type and content of steel fibres on the performance of segmented linings at the structural scale. Using this framework, a method for robust optimization is applied in order to generate damage-tolerant hybrid segment designs. © 2019 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin

During the planning phase of tunneling projects, it is crucial to ensure that existing buildings are protected from damage when tunneling in urban environments. Tunneling inevitably causes ground movements whose impact on above-ground structures must be assessed. Various approaches that differ in precision and complexity may be employed to predict settlements, depending on the magnitude of expected settlements and the vulnerability of the structures with regard to tunneling induced damage. This contribution presents a threestep damage assessment concept adjustable to the level of detail necessary. First, ground movements are predicted using analytical or numerical approaches. Second, the above-ground structures are idealized by means of surrogate beam-, slab-or 3D-models. Lastly, structural damage is assessed according to strain information or tilt. This method enables the evaluation of the potential damage to above ground structures associated with various tunnel alignments during the planning phase. BIM concepts offer opportunities to streamline and simplify this process by using geometrical BIM sub-models as a basis for performing structural calculations. This paper aims to provide recommendations for a sufficient level of detail (LOD) of surface structures for the assessment of the induced damage in computational simulations of mechanized tunneling process. © 2019 Taylor & Francis Group, London.

In current mechanized tunneling practice, the position of the shield machine during TBM-advancement is controlled by the shield driver with the aid of monitoring-based guidance systems. This results in an uneven movement of the machine. This contribution proposes a computational model able to support the TBM-steering during tunnel drives along curved alignments. A computational framework is developed to simulate the advancement and excavation processes, and to enable an efficient and realistic 3D-modeling of the advancement process and the shield-soil interactions during tunneling along arbitrarily curved alignments using the finite element method. The proposed framework combines a newly developed steering algorithm to simulate the TBM-movement during the excavation process. The steering algorithm serves as an artificial guidance system to automatically determine the exact position and the driving direction of the TBM in 3D-space. This modeling approach allows for better prediction of the shield behavior and thrusting forces during the advancement process. © 2019 Taylor & Francis Group, London.

In mechanised tunnelling, it is important to perform reliability analyses with respect to the tunnel face collapse and the damage risks of the tunnel lining and existing structures on the ground surface due to the tunnelling induced settlements. The reliability assessment requires to deal with limited information describing the local geology and the soil parameters due to the availability of only a small number of borehole data. In this paper, it is focused on real-time reliability analyses in mechanised tunnelling considering different types of uncertain data, i.e. combining epistemic and aleatoric sources of uncertainty within polymorphic uncertainty models. The system output of interest in these analyses is time variant tunnelling induced surface settlement fields, which are computed by a finite element simulation model. However, for real-time predictions with uncertain data, efficient and reliable surrogate models are required. A new surrogate modelling strategy is developed to predict time variant high dimensional fuzzy settlement fields in real-time. The predicted results of the new surrogate model show similar accuracy compared to the results obtained by optimisation based fuzzy analyses. Meanwhile, the computation time is significantly reduced especially in case of high dimensional outputs and in combination with the p-box approach in the case of polymorphic uncertain data. Copyright © 2018 Inderscience Enterprises Ltd.

The behavior of high performance fiber reinforced cementitious composites is simulated using semi-analytical and computational sub-models specified at multiple scales. At the scale of a single fiber, a semi-analytical model is developed to characterize the microslip behavior at the interface between the matrix and the fiber. The microcrack bridging and arresting mechanisms of fiber bundles is taken into account within the framework of linear elastic fracture mechanics. Upscaling from the level of distributed microcracks to the macroscopic level is achieved using continuum micromechanics. According to the proposed model, the macroscopic hardening and softening constitutive characteristics is resulting from the microcrack-fiber interaction, the microcrack growth and the evolution of the microcrack density. Model predictions for FRC concrete are validated against experimental data. For the finite element analyses of failure behavior at the structural level, interface solid elements supplemented by a fiber bridging law specified according to the fiber pull-out mechanics are used to represent the cracking process. Selected numerical examples demonstrate that the crack pattern as well as the structural response can be well replicated by the proposed model. © 2018, RILEM.

Earth Pressure Balance (EPB) machines are widely used in mechanized tunneling operations in soft soils in a variety of soil conditions. In contrast to slurry shield machines, the pressure distribution at the tunnel face is not well defined. This paper proposes a computational model for the numerical simulation of the material transport inside the pressure chamber of EPB machines. The governing equations of the soil flow are discretized by using the Finite Element (FE) method and the Fractional Step (FS) scheme. A pressure-dependent viscoplastic fluid is adopted to model the behavior of the soil paste, which is assumed to be a compressible homogeneous mixture. The flow behavior along the boundaries of the chamber wall and the cutterhead rotators is described by the Navier slip law. The moving boundaries associated with the rotators and the mixing arms inside the pressure chamber are imposed in the numerical model by means of the Shear-Slip Mesh Update (SSMU) method. The description of the rotating screw in the conveyor system is enabled by using the Immersed Boundary (IB) method. The combination of the two methods enables to exploit the advantages and to mutually compensate for the disadvantages associated with each of the two methods. The numerical components in the model are presented and verified by several numerical benchmark examples. The setup for the chamber model is presented in detail. Selected results from the simulation of a pressure chamber of a tunnel boring machine are shown and some important observations and remarks are discussed. © 2017 Elsevier B.V.

Numerical methods for predicting localized shear failure in elasto-plastic solids have experienced considerable advancements in the last decades. Among these approaches, the so-called “Embedded Strong Discontinuity (ESD)” method is often successfully used to accurately simulate the post-localization response with negligible dependence on the finite element discretization. However, it was observed that the employed discontinuity tracking strategy plays a crucial role in the successful localization analysis. In this contribution, we propose a novel strategy for the global tracking of discontinuity surfaces. It is based on exploiting information obtained from the enhanced parameters employed in Enhanced Assumed Strain (EAS) formulations. It is well known, that enhanced strain element formulations are able to better capture localized shear deformations as compared to standard finite elements. This can be explained as a consequence of the improved performance in bending. We observed, that the approximation of the strain jumps delimiting the shear band is connected with a deformation field characterized by opposite bending curvatures across these two discontinuities. Hence, in view of the relations existing between the kinematics of strong and weak discontinuities, we formulate a proper scalar function of the enhanced parameters to identify potential strong discontinuity surfaces, which are evaluated in each step of the analysis with negligible computational cost. This proposed approach has a global character, as it is based upon evaluating discontinuity surfaces defined in the complete analysis domain that are, by construction, continuous across elements. We demonstrate that the tracking algorithm correctly identifies the potential strong discontinuity surface already in early loading stages, even before a localization condition is fulfilled. In those elements which are crossed by the potential failure surface and which also satisfy the localization condition, the kinematics of embedded strong discontinuities is activated to capture the shear failure surface. The performance of the new tracking algorithm is demonstrated by means of several numerical shear localization analyses using associative and non-associative Drucker–Prager elastoplastic models to simulate 2-D and 3-D benchmarkanalyses. © 2017 Elsevier B.V.

The contribution takes a glance at the application of BIM technologies in the design and construction phases of shield tunnelling projects. The intention is to show how Building Information Modelling can be translated into actual benefit, not only in the operation phase but also in the design and construction phases of bored tunnels. Emphasising the integrative character of referencing data uniformly in space and time, examples are given of seamless communication between 3D geometrical modelling, efficient numerical simulation and model adaptation based on measured data acquired during the boring process. The article covers the complete range from predesign through structural analysis and detailed design until the actual excavation process including its interactions with the environment. Special emphasis is given to data management, which is the key to transforming a mere 3D visualisation into a Building Information Model. The article therefore presents a concept for database-supported, web-based integration of software modules for geometrical modelling in various levels of detail, efficient numerical simulation tools that are based upon this representation, and process controlling that manages all data acquired during the construction process in a spatially and temporally coordinated reference system. Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin

The Folgar–Tucker fiber orientation model coupled with weakly compressible Smoothed Particle Hydrodynamics is used to simulate the process of casting of fiber reinforced concrete and to predict the spatial-temporal evolution of the probability density function of fiber orientation. The flowable concrete-fiber mix is modeled as a viscous Bingham-type fluid. Model predictions qualitatively agree with fiber orientations observed in an L-box test with fibers suspended in transparent gel. Important factors and assumptions regarding the fiber flow are reviewed and conclusions are drawn based on numerical experiments. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.

We investigate the influence of distributed microcracks on the overall diffusion properties of a porous material using the self-similar cascade continuum micromechanics model within the framework of mean-field homogenization and computational homogenization of diffusion simulations using a high-resolution pixel finite element method. In addition to isotropic, also anisotropic crack distributions are considered. The comparison of the results from the cascade continuum micromechanics model and the numerical simulations provides a deeper insight into the qualitative transport characteristics such as the influence of the crack density on the complexity and connectivity of crack networks. The analysis shows that the effective diffusivity for a disordered microcrack distribution is independent of the absolute length scale of the cracks. It is observed that the overall effective diffusivity of a microcracked material with the microcracks oriented in the direction of transport is not necessarily higher than that of a material with a random orientation of microcracks, independent of the microcrack density. © 2018, Springer Nature B.V.

The paper proposes a variational approach to model brittle fracture propagation based on zero-thickness finite elements. Similar to the phase-field model for fracture, the problem of a fractured structure is variationally formulated by considering a minimization problem involving bulk and fracture surface energies. With the help of a damage variable used as an additional degree of freedom, the fracture propagates according to the values of the minimizers of the total potential energy. This damage variable is restricted to dimensionally reduced interface elements inserted between element boundaries. Crack opening is predicted when the elastic energy within the interface surface exceeds the critical energy release rate. The solution of the discretized system of equations is performed in a staggered scheme, solving first for the displacement field and then searching for the solution for the updated nodal damage variables. Selected numerical examples, including re-analyses of laboratory tests characterized by rather complex crack paths, are presented to demonstrate the performance of the proposed variational interface model. © 2017 Elsevier B.V.

A prototype modeling framework for the coupled simulation of excavation processes at the tunnel face and the subsequent transport of the foam-soil mixture within the pressure chamber of EPB shield machines is proposed. The discrete element method is used for the modeling of soil excavation and the stabilized finite element method, using a non-Newtonian fluid model, is employed for the modeling of fluid transport. A variational approach is applied to directly obtain interparticle parameters of the DEM from a macroscopic strength criterion. A 2D numerical simulation model for a simplified representation of the cutting process at the tunnel face and the transport of the excavated soil-foam mixture is used to demonstrate the proposed coupled excavation-transport modeling approach. According to the proposed coupled DEM-FEM model, the mass flow obtained from the excavation simulation by means of the DEM serves as the input for the finite element flow simulation to generate the pressure distribution within the excavation chamber. It is shown that the proposed approach helps to obtain insight into the coupled excavation and transport processes at the tunnel face and the spatiotemporal distribution of the face pressure. © 2017 by Begell House, Inc.

The paper proposes a novel computational method for real-time simulation and monitoring-based predictions during the construction of machine-driven tunnels to support decisions concerning the steering of tunnel boring machines (TBMs). The proposed technique combines the capacity of a process-oriented 3D simulation model for mechanized tunnelling to accurately describe the complex geological and mechanical interactions of the tunnelling process with the computational efficiency of surrogate (or meta) models based on artificial neural networks. The process-oriented 3D simulation model with updated model parameters based on acquired monitoring data during the advancement process is used in combination with surrogate models to determine optimal tunnel machine-related parameters such that tunnelling-induced settlements are kept below a tolerated level within the forthcoming process steps. The performance of the proposed strategy is applied to the Wehrhahn-line metro project in Düsseldorf, Germany and compared with a recently developed approach for real-time steering of TBMs, in which only surrogate models are used. © 2016 Elsevier Ltd

Soil freezing is often used to provide temporary support of soft soils in geotechnical interventions. During the freezing process, the strength properties of the soil–water–ice mixture change from the original properties of the water-saturated soil to the properties of fully frozen soils. In the paper, a multiscale homogenization model for the upscaling of the macroscopic strength of freezing soil based upon information on three individual material phases—the solid particle phase (S), the crystal ice phase (C) and the liquid water phase (L)—is proposed. The homogenization procedure for the partially frozen soil–water–ice composite is based upon an extension of the linear comparison composite (LCC) method for a two-phase matrix–inclusion composite, using a two-step homogenization procedure. In each step, the LCC methodology is implemented by estimating the strength criterion of a two-phase nonlinear matrix–inclusion composite in terms of an optimally chosen linear elastic comparison composite with a similar underlying microstructure. The solid particle phase (S) and the crystal ice phase (C) are assumed to be characterized by two different Drucker–Prager strength criteria, and the liquid water phase (L) is assumed to have zero strength capacity under drained conditions. For the validation of the proposed upscaling strategy, the predicted strength properties for fully and partially frozen fine sands are compared with experimental results, focussing on the investigation of the influence of the porosity and the degree of ice saturation on the predicted failure envelope. © 2017 Springer-Verlag Berlin Heidelberg

The effective analysis of the nonlinear behavior of cement-based engineering structures not only demands physically-reliable models, but also computationally-efficient algorithms. Based on a continuum interface element formulation that is suitable to capture complex cracking phenomena in concrete materials and structures, an adaptive mesh processing technique is proposed for computational simulations of plain and fiber-reinforced concrete structures to progressively disintegrate the initial finite element mesh and to add degenerated solid elements into the interfacial gaps. In comparison with the implementation where the entire mesh is processed prior to the computation, the proposed adaptive cracking model allows simulating the failure behavior of plain and fiber-reinforced concrete structures with remarkably reduced computational expense. © 2017 by the authors.

The effective permeability of microcracked heterogeneous materials such as rocks, ceramics and concrete can be determined using analytical and computational homogenization methods. While in the companion paper (Timothy and Meschke, 2017) a semi-analytical Cascade Continuum Micromechanics (CCM) model is proposed to predict the effective permeability and the percolation threshold for porous materials with microcracks, the focus of this paper is to validate the CCM model by means of direct numerical meso-scale simulations of representative elementary volumes concerned with water flow through porous materials. Algorithms are developed to generate overlapping and non-overlapping microcrack morphologies within the framework of the finite element method to analyse the effective permeability as a function of the microcrack volume fraction. The numerical results confirm the predictions from the CCM model for different scenarios, including a low and high permeable matrix and different microcrack morphologies, both qualitatively as well as quantitatively. It is also shown, that in computational homogenization procedures, the effective permeability around the percolation threshold is strongly dependent on the discretization of the numerical REV. © 2017 Elsevier Ltd

The transport and fluid flow in heterogeneous materials such as rocks, ceramics and concrete with a distributed random microcrack network is strongly influenced by the density and the topology (distribution and connectivity) of microcracks. The overall fluid flow characteristics of such microcracked solids can be quantified in terms of an effective permeability. In the paper, a semi-analytical formulation for the effective permeability is proposed within the framework of the mean-field homogenization method using the cascade continuum micromechanics model considering long range and short range interactions. We compare model predictions of the percolation threshold i.e. critical volume fraction of microcracks beyond which a solid with distributed microcracks becomes permeable, using results from numerical simulations. The model reveals a new perspective into the self-similar characteristics of the microcrack morphology near the threshold volume fraction of microcracks at which the microcrack structure changes from multiple disconnected microcracks to a connected self-similar microcracked structure. © 2016 Elsevier Ltd

According to the objectives of the research group 1498, this paper deals with degradation effects in concrete structures that are caused by cyclic flexural loading. The goal is to determine their influence on the fluid transport processes within the material on the basis of experimental results and numerical simulations. The overall question was, to which extent the ingress of externally supplied alkalis and subsequently an alkali-silica reaction are affected by such modifications in the microstructure. Degradation in the concrete microstructure is characterized by ultrasonic wave measurements as well as by microscopic crack analysis. Furthermore, experiments on the penetration behavior of water into the investigated materials were performed. The penetration behavior into predamaged concrete microstructures was examined by the classical Karsten tube experiment, nuclear magnetic resonance method, and time domain reflectometry techniques. In order to create an appropriate model of the material's degradation on the water transport, the Darcy law was applied to describe the flow in partially saturated concrete. Material degradation is taken into account by an effective permeability that is dependent on the state of degradation. This effective permeability is obtained by the micromechanical homogenisation of the flow in an Representative Elementary Volume (REV) with distributed ellipsoidal microcracks embedded in a porous medium. The data gained in the microscopic crack analysis is used as input for the micromechanical model. Finite element simulations for unsaturated flow using the micromechanical model were compared with the experimental results showing good qualitative and quantitative agreement. © 2017 fib. International Federation for Structural Concrete

A surrogate modelling strategy for predictions of interval settlement fields in real time during machine driven construction of tunnels, accounting for uncertain geotechnical parameters in terms of intervals, is presented in the paper. Artificial Neural Network and Proper Orthogonal Decomposition approaches are combined to approximate and predict tunnelling induced time variant surface settlement fields computed by a process-oriented finite element simulation model. The surrogate models are generated, trained and tested in the design (offline) stage of a tunnel project based on finite element analyses to compute the surface settlements for selected scenarios of the tunnelling process steering parameters taking uncertain geotechnical parameters by means of possible ranges (intervals) into account. The resulting mappings of time constant geotechnical interval parameters and time variant deterministic steering parameters onto the time variant interval settlement field are solved offline by optimisation and online by interval analyses approaches using the midpoint-radius representation of interval data. During the tunnel construction, the surrogate model is designed to be used in real-time to predict interval fields of the surface settlements in each stage of the advancement of the tunnel boring machine for selected realisations of the steering parameters to support the steering decisions of the machine driver. © 2017 Elsevier Ltd.

In the design of machine driven tunnels, the loadings acting on the segmental lining are often adopted according to simplified assumptions, which improperly reflect the actual loading on the linings developing during the construction of a bored tunnel. A coupled 3D Finite Element model of the tunnel advancement process including the ring-wise installation of the lining and the hardening process of the grouting material serves as the basis for the analysis of the actual spatio-temporal evolution of the loading on the lining during tunnel construction. The distribution of the loadings in the different construction phases is calculated using a modified surface-to-surface contact condition imposed between the solidifying grouting material in the tail gap and the lining elements. An extensive parametric study investigates the influence of the initial grouting pressure, the pressure gradient, the temporal stiffness evolution, the soil permeability as well as the interface conditions between the grouting material and the tunnel shell on the temporal evolution of the loading on linings. © 2016 Elsevier Ltd

The paper presents a synthesis of analytical modeling and computational simulations of the intrinsic permeability of microcracks, embedded in porous materials taking into account the interaction of the fluid flow in the microcrack with the surrounding porous material. In the first part of the paper, using the DARCY, STOKES, BRINKMAN, and the BEAVERS–JOSEPH approximations, we derive the intrinsic permeability of a plain non-rough microcrack in terms of the microcrack geometry and the permeability of the porous material surrounding the microcrack. In the second part of the paper, the intrinsic permeability of a microcrack is determined by means of computational simulations using the framework of the lattice Boltzmann method with partial bounceback conditions. The comparison of predictions from the analytical model and the numerical simulations show an excellent agreement. Copyright © 2017 John Wiley & Sons, Ltd. Copyright © 2017 John Wiley & Sons, Ltd.

Peridynamics is a nonlocal continuum model which offers benefits over classical continuum models in cases, where discontinuities, such as cracks, are present in the deformation field. However, the nonlocal characteristics of peridynamics leads to a dispersive dynamic response of the medium. In this study we focus on the dispersion properties of a state-based linear peridynamic solid model and specifically investigate the role of the peridynamic horizon. We derive the dispersion relation for one, two and three dimensional cases and investigate the effect of horizon size, mesh size (lattice spacing) and the influence function on the dispersion properties. We show how the influence function can be used to minimize wave dispersion at a fixed lattice spacing and demonstrate it qualitatively by wave propagation analysis in one- and two-dimensional models of elastic solids. As a main contribution of this paper, we propose to associate peridynamic non-locality expressed by the horizon with a characteristic length scale related to the material microstructure. To this end, the dispersion curves obtained from peridynamics are compared with experimental data for two kinds of sandstone. © 2017, Springer-Verlag GmbH Germany.

The elastic properties of porous materials with a disordered pore structure are estimated using the mean-field Eshelby homogenization scheme together with the principle of recurrence to generate a cascade of effective microstructures as a function of the porosity and the cascade level n. Starting with the Hashin-Shtrikman upper bound for porous materials, the proposed cascade micromechanics model generates a hierarchy of micro-structures which evolve from an initial configuration of a porous material with spherical pores embedded within an elastic solid phase consistent with the Mori-Tanaka matrix inclusion morphology to a porous material characterized by a hierarchic distribution of spherical elastic grains. The model is explicit and allows for an easy computational implementation. It predicts physically consistent threshold porosities, characteristic for the specific morphology of the porous material under consideration, beyond which the material loses its stiffness. The validity of the cascade micromechanics model is evaluated against experimental data for various materials ranging from foam to ceramics with different pore structures. © 2015 Elsevier Ltd.

In the paper, a simple and efficient algorithm to track a moving delamination front of arbitrary shape, using a laminated finite plate element model in conjunction with the Virtual Crack Closure Technique (VCCT), is proposed. The solution requires the calculation of the virtually closed area in front of the delamination, which is approximated by means of a 6-node-polygon, the delamination opening behind this front and the reaction forces in the nodes at the delamination front. These quantities are calculated in a local coordinate system (LCS) defined in the nodes along the delamination front. Using the proposed algorithm, arbitrary meshes composed of 4- and 9-node quadrilateral finite elements can be considered. The proposed model is developed in the context of a layered finite element plate model. To prevent interlaminar penetration of adjacent layers in the delaminated region, an algorithm recently proposed by Marjanović et al. (2015) is adopted. The model performance is demonstrated by re-analyses of the Double-Cantilever-Beam problem, for which analytical solutions exist, and by transient analyses of laminated composite plates with propagating delamination fronts. © 2016 Elsevier Ltd.

The micro-damage associated with diffuse fracture processes in quasi-brittle materials can be described by continuum damage mechanics. In order to overcome the mesh dependence of local damage formulations, non-local and gradient-enhanced approaches are often employed. In this manuscript, a higher-order stress-based gradient-enhanced formulation is proposed, which exploits the higher-order continuity of B-spline functions in isogeometric analysis (IGA). The proposed formulation does not require the decomposition of the fourth-order model into two second-order models. Two numerical examples are presented to demonstrate the performance of the formulation and to compare the obtained solutions with results from conventional gradient-enhanced damage formulation. (C) 2016 Elsevier B.V. All rights reserved.

Molecular diffusion in fully saturated porous materials is strongly influenced by the pore space, which, in general, is characterized by a complex topological structure. Hence, information on macroscopic diffusion properties requires up-scaling of transport processes within nano-pores and micro-pores over several spatial scales. A new model in the framework of continuum micromechanics is proposed for predicting the effective molecular diffusivity in porous materials. Considering a representative volume element, characterizing a porous material without any information about the pore space microstructure complexity, the uniform flux is perturbed by recursively embedding shape information hierarchically in the form of the ESHELBY matrix-inclusion morphology to obtain the effective diffusivity as a function of the recurrence level and the porosity. The model predicts a threshold value for the porosity, below which no molecular diffusion can occur because of the presence of isolated pore clusters that are not connected and unavailable for transport. The maximum porosity, below which no molecular transport is possible, is predicted as one-third for spherical inclusions. The model allows for extensions to more complex morphologies of the inclusions. We also identify, that the effects of the micro-structure on molecular transport are characterized by porosity dependent long-range and short-range interactions. The developed framework is extended to incorporate realistic pore size distributions across several spatial scales by means of a distribution function within the hierarchical homogenization scheme. Available experimental results assert the model predictions. Copyright © 2016 John Wiley & Sons, Ltd.

The objective of this paper is to develop an accurate and efficient solution procedure for elastoplastic problems in structural mechanics in the framework of a two-field mixed variational principle. A novel solution algorithm is proposed and applied to the elastoplastic analysis of Mindlin plates. The Hellinger–Reissner principle is adopted to obtain the global finite-element equations of the problem. Instead of a static condensation, the stress-type field variables are preserved during the solution. According to the proposed approach, the strain increments within a nonlinear solution step are obtained directly at the nodal points from matrix operations instead of gradients of a displacement field. In the present implementation, the von Mises yield criterion with linear hardening is adopted. For the integration of the elastoplastic constitutive rate equations at the nodal points, a 3D fully implicit algorithm is employed. A layered approach is followed to enable the resolution of the plastic strains through the plate thickness. The mixed formulation of the Mindlin plate theory is shear-locking free by construction. The proposed solution strategy is verified by solving several benchmark problems that demonstrate the high accuracy and convergence rate of the presented layered mixed formulation for elastoplastic analyses. © 2015, Springer Science+Business Media Dordrecht.

Mechanized tunneling is characterized by complex interactions between the shield machine and the surrounding ground during the TBM advancement process. In this paper, a new computational framework is developed to enable an efficient and realistic three-dimensional modeling of the tunneling process for arbitrary alignments using the finite element method. A new steering algorithm for the advancement of the Tunnel Boring Machine (TBM) for arbitrary alignments during shield tunneling is incorporated in the proposed model. This algorithm simulates the shield behavior and accordingly provides the numerical model with the required information to keep the TBM on track during the simulation. However, the utilization of this algorithm is only possible using a finite element discretization which adapts to the actual motion path of the shield machine. For this purpose, a re-meshing technique is proposed in order to automate the process of mesh generation in the vicinity of the tunnel face, denoted as the region of interest, within the advancing process. The combination of a computational steering algorithm and a 3D automatic adaptive mesh generation procedure form a novel framework for process oriented finite element simulations of the mechanized tunnel construction process. The applicability of the proposed modeling technique for predicting the shield behavior and the soil-tunnel interactions during tunneling along curved alignments is demonstrated by means of selected examples. © 2016 Elsevier Ltd

Within the framework of mean-field homogenization methods, a lattice version of the cascade micromechanics model for the estimation of the effective permeability of microcracked materials with a physically consistent percolation threshold is proposed. Also, a link is investigated between the cascade scheme, self-similarity, and critical phenomena. The cascade lattice micromechanics model is compared with the dilute, the Mori-Tanaka and the self-consistent lattice schemes and validated by means of numerical experiments using a computational lattice microcrack-pore network model. Different scenarios of the microcrack probability and the permeability contrast between the pore channels and the microcracks are investigated. © 2016 American Society of Civil Engineers.

Glaucoma is among the leading causes of blindness worldwide. The disease involves damage to the retinal ganglion cell axons that transmit visual information from the eye to the brain. Experimental evidence indicates that biomechanical mechanisms at different length scales are involved in pathophysiology of glaucoma, where chronic intraocular pressure (IOP) elevation at the organ level initiates axonal insult at the level of the lamina cribrosa. The lamina cribrosa consists of a porous collagen structure through which the axons of retinal ganglion cells (RGCs) pass on their path from the retina to the brain. The extent to which the structural mechanics of the lamina cribrosa contribute to the axonal insult remains unclear. In this book chapter, we give a short review of the present understanding of the structural mechanics of the lamina cribrosa and its role in glaucoma. The main aim is to present a first computationally coupled two-scale analysis of the lamina cribrosa that translates the IOP load at the macroscale to the mechanical insult of the axons within the mesostructure of the lamina cribrosa. The numerical results of two-scale analysis suggest that the collagen structures of the lamina cribrosa and its surrounding peripapillary sclera effectively provide mechanical support to the axons by protecting them from high tensile stresses even at elevated IOP levels. However, in-plane shear stresses in the axonal tissue may increase with increasing IOP at the posterior lamina insertion region and contribute to a mechanical insult of the RGC axons in glaucoma. © Springer Science+Business Media, LLC 2016.

A multilevel modeling framework for the failure analyses of structures made of steel-fiber-reinforced concrete (SFRC), which allows researchers to follow the effects of design parameters such as fiber type, distribution, and orientation from the scale of fiber-matrix interaction to the structural behavior, is proposed. The basic ingredient at the level of single fibers is an analytical model for the prediction of the pullout response of straight or hooked-end fibers. For an opening crack in a specific SFRC composite, the fiber bridging effect is computed via the integration of the pullout response of all fibers intercepting the crack, taking anisotropic fiber orientations into consideration. For the finite-element analysis of the failure behavior of SFRC structures, interface solid elements are used to represent cracks. The softening behavior of opening cracks is governed by cohesive tractions and the fiber bridging effect. The use of an implicit/explicit integration scheme enhances the computational robustness considerably. Numerical analyses of selected benchmark problems demonstrate that the model is able to predict the structural response for different fiber cocktails in good agreement with experimental results. © 2016 American Society of Civil Engineers.

Artificial ground freezing is an environmentally friendly technique to provide temporary excavation support and groundwater control during tunnel construction under difficult geological and hydrological ground conditions. Evidently, groundwater flow has a considerable influence on the freezing process. Large seepage flow may lead to large freezing times or even may prevent the formation of a closed frozen soil body. For safe and economic design of freezing operations, this paper presents a coupled thermo-hydraulic finite element model for freezing soils integrated within an optimization algorithm using the Ant Colony Optimization (ACO) technique to optimize ground freezing in tunneling by finding the optimal positions of the freeze pipe, considering seepage flow. The simulation model considers solid particles, liquid water and crystal ice as separate phases, and the mixture temperature and liquid pressure as primary field variables. Through two fundamental physical laws and corresponding state equations, the model captures the most relevant couplings between the phase transition associated with latent heat effect, and the liquid transport within the pores. The numerical model is validated by means of laboratory results considering different scenarios for seepage flow. As demonstrated in numerical simulations of ground freezing in tunneling in the presence of seepage flow connected with the ACO optimization algorithm, the optimized arrangement of the freeze pipes may lead to a substantial reduction of the freezing time and of energy costs. © 2016.

The Strong Discontinuity Approach (SDA) is a popular method to incorporate cracks as displacement discontinuities into finite elements. In the first part of the paper, different SDA formulations (denoted as SOS, KOS and SKON) are assessed numerically based upon different error norms including a norm to evaluate the variational consistency of SDA elements. Results are compared with analytical solutions as well as with results from interface elements and the element erosion technique for cohesionless cracks. In the second part of the paper, a new method to improve the computational robustness of SDA analyses is proposed. In addition to using arc-length control to solve the nonlinear equation system, a sequential un- and reloading scheme is proposed with the crack state frozen during unloading to avoid, that more than one new crack segment is activated within an increment. The performance of the algorithm is demonstrated by numerical analyses of a tension test on a dog-bone shaped specimen by means of different variants of the SDA and, for comparison, also using interface elements. © 2016

In computational simulations of hydraulic fracturing problems, consideration of interactions between the propagating fracture zone and the fluid flow through the porous material requires an appropriate up-scaling procedure from the spatial scale of the local crack, which usually is much smaller compared to the scale of typical finite elements in poromechanics problems. This scale transition refers to both the displacement field (discontinuity across cracks) as well as to the fluid flow (accelerated flow within cracks and the interaction with the flow in the bulk material). To resolve the small and the large scale portion of the solution, the Generalized Finite Element Method (GFEM) exploiting the partition of unity property of shape functions is used. Accordingly, the displacements u and the liquid pressure p<inf>l</inf> are locally enriched to better resolve their distribution in the vicinity of cracks by means of an additive decomposition into a large and a small scale part. As far as the representation of cracks is concerned, the Extended Finite Element Method (XFEM) is used by enriching the displacement field by means of a jump function as well as crack tip functions. In the framework of the GFEM physically motivated enrichment functions for the local enrichment of the C1 discontinuity of the liquid pressure field across cracks are proposed in the paper. The space and time variant analytical solutions obtained from the 1D transient response of saturated porous materials subjected to the liquid pressure within the crack are applied as enrichment functions to locally improve the approximation of the liquid pressure field at discontinuities. Applying these space and time variant functions, which are the exact solutions of the pressure field in the vicinity of cracks, as local enrichment functions lead to a significant improvement in the local approximation of the pressure field at discontinuities. The new GFEM model is formulated in a poromechanics framework for fully saturated porous materials. Representative analyses demonstrate the improvement of the solution quality compared to existing FEM and XFEM models. © 2015 Elsevier B.V.

This paper presents a computational multi-level model for the description of alkali and moisture transport in concrete structures coupled to a macroscopic ASR induced phase-field damage model. Concrete is modeled as a heterogeneous material consisting of a partially saturated pore space with diffusively distributed microcracks and the solid skeleton (cement paste and potentially reactive and inert aggregates). The influence of the topology of the pore space and the presence of oriented microcracks on ion diffusion and moisture transport is taken into account through a novel continuum micromechanics homogenization model. The transport model is connected to a phenomenological reaction kinetics model to account for the ASR induced volume expansion of the affected aggregates. At the macroscopic scale, crack propagation and effects of induced topological changes on the fluid and ion transport are taken into account using a phase-field model. © ASCE.

According to the goals of the research group 1498, this paper deals with the effects of cyclic flexural loading in a four-point bending test on the fluid transport processes within a concrete structure. Therefore, the degradation of the microstructure is characterized through ultrasonic wave measurements as well as microscopic crack analysis. In order to numerically model these processes, experiments on the penetration behavior of water into the concrete were carried out. The penetration behavior over time as well as the influence of degradation on the water transport were investigated. To predict the influence of concrete degradation on alkali diffusivity, a multi-scale continuum micromechanics model is incorporated into the numerical model, which accounts for the topology and the three-dimensional distribution of microcracks. As expected, the numerical simulation predicts larger alkali-penetration in pre-damaged concrete. Regarding the micro-crack distribution, an anisotropic distribution of micro-cracks tangential to the direction of the alkali and water flux increases their penetration depth. © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

In the paper, a computational model for the transient response of laminated composite and sandwich plates with existing zones of partial delamination, subjected to dynamic pulse loading is proposed. Laminated composite and sandwich plates are modeled using the extended version of the Generalized Laminated Plate Theory. For the numerical solution, the finite element method with layered finite elements is used. Delamination between individual layers is considered as discontinuities in the displacement field using Heaviside step functions. Since the status of delaminated layers may change in dynamic loading conditions, leading to the so-called "breathing" phenomenon, contact conditions allowing for the opening/closing of delaminated layers are proposed. Nonlinear kinematics in the sense of small strains and moderately large rotations is accounted for according to the von Kármán assumptions. The material of the individual layers is assumed as orthotropic and linearly elastic. The governing spatial-temporal partial differential equations are integrated in time by means of the implicit Newmark's method. After verification of the proposed model for intact plates, the effect of the size and the position of embedded delamination zones on the transient response of geometrical nonlinearity of composite and sandwich plates is investigated numerically by means of a number of numerical applications. © 2014 Elsevier Ltd.

For numerical reliability analyses of engineering structures, highly efficient computational models are required, in particular if numerical simulations of construction processes are to be performed in real time during the construction. Adopting process-oriented finite element simulations in mechanised tunnelling as a specific field of application, surrogate models are proposed to substitute inevitable expensive complex finite element simulations with uncertain parameters. The focus in this paper is laid on the uncertainty of geotechnical and tunnelling process parameters described by stochastic numbers, intervals and interval stochastic numbers. A new hybrid surrogate modelling concept is presented, which is based on combining artificial neural networks and the proper orthogonal decomposition method. Thereby, uncertain time variant surface settlements of several monitoring points are predicted by recurrent neural networks. Based on these predictions, the complete surface settlement field of each time step is computed using the gappy proper orthogonal decomposition approach. The hybrid surrogate model is applied for reliability analyses of a mechanised tunnelling process, where limit states at multiple surface positions are evaluated. Copyright © 2015 Inderscience Enterprises Ltd.

A method for the simulation supported steering of the mechanized tunneling process in real time during construction is proposed. To enable real-time predictions of tunneling induced surface settlements, meta models trained a priori from a comprehensive process-oriented computational simulation model for mechanized tunneling for a certain project section of interest are introduced. For the generation of the meta models, Artificial Neural Networks (ANN) are employed in conjunction with Particle Swarm Optimization (PSO) for the model update according to monitoring data obtained during construction and for the optimization of machine parameters to keep surface settlements below a given tolerance. To provide a rich data base for the training of the meta model, the finite element simulation model for tunneling is integrated within an automatic data generator for setting up, running and postprocessing the numerical simulations for a prescribed range of parameters. Using the PSO-ANN for the inverse analysis, i.e. identification of model parameters according to monitoring results obtained during tunnel advance, allows the update of the model to the actual geological conditions in real time. The same ANN in conjunction with the PSO is also used for the determination of optimal steering parameters based on target values for settlements in the forthcoming excavation steps. The paper shows the performance of the proposed simulation-based model update and computational steering procedure by means of a prototype application to a straight tunnel advance in a non-homogeneous soil with two soil layers separated by an inclined boundary. © 2014 Elsevier Ltd.

In soft, partially or fully saturated ground conditions, machine-driven tunnel construction causes short- and long-term ground deformations resulting from the disturbance of the virgin stress state of the soil and changes in the pore water conditions. These variations are, in turn, influenced by the heading face support, shield skin friction and by the gap grouting. Realistic large-scale 3D simulations are, therefore, increasingly required to investigate the interaction between machine-driven tunnel construction and the surrounding soil in order to provide reliable estimates of the expected settlements and associated risks of damage for existing structures, respectively, in particular in urban tunneling projects. If performed properly, these simulations involve complex interactions between individual components of the numerical model. The presented paper is concerned with recent advances in the process-oriented adaptive computational simulation of the excavation and steering processes in mechanized tunneling in soft soils using the finite element method. A novel automated adaptive mesh-refinement procedure is proposed to allow a refined resolution of the region of interest in the vicinity of the tunnel face during the TBM advancement. This procedure allows for an accurate assessment of the tunnel face stability and for the investigation of the immediate soil deformation and pore pressure changes around the tunnel. Furthermore, selected aspects of the numerical treatment - such as the stabilization of low order, two-phase, finite elements and the sub-stepping schemes inherent in the numerical integration of elasto-plastic models -,are also addressed in the presentation. © 2014 American Society of Civil Engineers.

A new finite element method is proposed to simulate two-phase mixture problems with free surfaces and/or large moving boundaries. The two phases are treated as interpenetrating continua within a two-fluid model. In contrast to the conventional two-fluid model formulations, in which the governing equations are written in the Eulerian-Eulerian description, the modified two-fluid model has the governing equations of one fluid phase written in the Lagrangian description while the respective equations of the other phase is formulated in the Arbitrary Lagrangian-Eulerian (ALE) description with relative velocities defined with respect to the moving mesh associated with the first fluid phase. The Finite Element Method (FEM) together with stabilization techniques-the Characteristic-Based Split (CBS) and the Algebraic-Flux Correction (AFC) method-is employed to solve the equations numerically in space and time. The nodes of the computational mesh are updated together with the flow of the phase described in the Lagrangian description. The computational mesh is updated by applying techniques used in the so-called Particle Finite Element Method (PFEM), characterized by regenerating the mesh in each computational step according to the updated nodal positions while the current boundary of the domain is identified using α-shape method. The performance of the method is demonstrated by means of three 2D and 3D examples, including a mixing process of two fluids in a stirred tank, a free surface flow, a 3D two-phase channel flow, a phase separation problem and the flow of two immiscible fluids. © 2014 Elsevier B.V.

In the context of multiscale-oriented computational analyses of fiber-RC (FRC) structures, the modeling of single fiber pullout behavior represents the basic constituent to provide traction-displacement relations to be used for the modeling of FRC on a macroscopic scale. This essential ingredient needs to be formulated such that it only requires minimal computational effort. To this end, an analytical model for the pullout behavior of single fibers embedded in a concrete matrix for various configurations of fiber type, matrix strength, and embedment condition is proposed. An interface law is developed for the frictional behavior between the fiber and matrix. In the case of inclined fibers, the plastic deformation of the fiber and the local damage of concrete are also considered. For hooked-end fibers, the anchorage effect due to the deformed topology of the fiber ends is taken into account in the formulation. By combining these submodels, the pullout response of single fibers embedded in a concrete matrix is predicted. In addition, numerical simulations of pullout tests are performed to obtain insight into the local fiber-concrete interactions and provide supporting information for analytical modeling. The model is successfully validated by means of representative experimental results. © 2014 American Society of Civil Engineers.

This paper presents an embedded beam formulation for discretization independent finite element (FE) analyses of interactions between pile foundations or rock anchors and the surrounding soil in geotechnical and tunneling engineering. Piles are represented by means of finite beam elements embedded within FEs for the soil represented by 3D solid elements. The proposed formulation allows consideration of piles and pile groups with arbitrary orientation independently from the FE discretization of the surrounding soil. The interface behavior between piles and the surrounding soil is represented numerically by means of a contact formulation considering skin friction as well as pile tip resistance. The pile-soil interaction along the pile skin is considered by means of a 3D frictional point-to-point contact formulation using the integration points of the beam elements and reference points arbitrarily located within the solid elements as control points. The ability of the proposed embedded pile model to represent groups of piles objected to combined axial and shear loading and their interactions with the surrounding soil is demonstrated by selected benchmark examples. The pile model is applied to the numerical simulation of shield driven tunnel construction in the vicinity of an existing building resting upon pile foundation to demonstrate the performance of the proposed model in complex simulation environments. © 2014 John Wiley & Sons, Ltd.

The excavation process ofearth-pressure balance (EPB) shield machines involves the cutting of the ground at the tunnel face and the transport of the soil paste in the excavation chamber. For the numerical simulation of these two processes, a computational strategy, characterized by the coupling of two partial models using the discrete element method (DEM) and the finite element method (FEM) has been developed (Wessels et al., 2013). Excavation is simulated using DEM, with the fracture process being represented by the release of interaction forces between the particles. The transport of the excavated soil mixed with the soil conditioning foam, yielding a pasty soil-foam mixture within the pressure chamber, is simulated by means of a two-phase fluid model in Eulerian description. The two momentum and mixture mass equations are discretized in time by the characteristic-based split method (CBS) (Zienkiewicz et al., 2005). The phase volume fraction equations are solved using upwind weighting functions according to the improved Mizukami-Hughes method (Knobloch, 2006). The proposed model is applied to a coupled analysis of the excavation and transport-mixing flow inside a simplified pressure chamber with foam injections. The mixing process, due to the rotation in the chamber, is preliminarily investigated by the mixing flow in a 2D cavity test case. © 2014 American Society of Civil Engineers.

The pullout behaviour of single steel fibres embedded in a concrete matrix is investigated for various configurations of fibre types and embedment lengths and angles by means of laboratory tests and analytical models. Laboratory tests for fibre pullout are performed to investigate the fibre-matrix bond mechanisms. Parameters influencing the fibre pullout response, such as fibre shape, fibre tensile strength, concrete strength and fibre inclination angle are systematically studied. The effects of these parameters on the pullout force versus displacement relationship, fibre efficiency and fibre/matrix failure response are analysed based on the experimental results. For the analytical modelling of the fibre pullout behaviour of straight fibres, an interface law is proposed for the frictional behaviour between fibre and matrix. In the case of inclined fibres, the plastic deformation of the fibre and the local damage to the concrete are also considered. For hooked-end fibres, the anchorage effect due to the hook is analysed. Combining these sub-models allows the pullout response of single fibres embedded in a concrete matrix to be predicted. In addition, numerical simulations of pullout tests are performed to obtain insights into the local fibre-concrete interactions and to provide supporting information for the analytical modelling. The models are successfully validated with the experimental results. Copyright © 2014 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

In geotechnical applications of artificial ground freezing, safe design and execution require a correct prediction of the coupled thermo-hydro-mechanical behavior of soils subjected to freezing. In the context of thermo-poro- plasticity (Coussy, 2005), a three-phase finite element model of freezing soils is presented: (1) considering solid particles, liquid water and crystal ice as separate phases; and (2) mixture temperature, liquid pressure, and solid displacement as primary field variables. Through three fundamental physical laws (overall entropy balance, mass balance of liquid water and crystal ice, and overall momentum balance) and corresponding state relations, the model captures the most relevant couplings between the phase transition, the liquid transport within the pores, and the accompanying mechanical deformation. Particularly for the description of the poro-plastic mechanical behavior of the soil model, the enhanced Barcelona Basic Model (Nishimura et al., 2009) is adopted within a unified effective-stress-based framework. The macroscopic strength criterion of the freezing soil composite is improved through multi-scale strength homogenization based upon the linear comparison composite method (Ortega et al. 2011). The performance of the proposed model is demonstrated by re-analysis of a soil freezing test and AGF processes during tunneling. © ASCE 2014.

Computational reliability analyses of engineering structures are often time-consuming, in particular, if time-dependent structural behavior is considered. If (almost) real time prognoses are required, surrogate models may be used to approximate the structural behavior described by advanced numerical models. In this paper, a hybrid surrogate modeling strategy based on a combination of Proper Orthogonal Decomposition (POD) and Artificial Neural Networks (ANN) is introduced. The hybrid approach is developed for the approximation of time variant surface settlements in mechanized tunneling due to uncertain geological and process parameters. The approximation capabilities are demonstrated by means of an example. The new hybrid surrogate model can be applied for numerical, efficient, real-time reliability analyses in mechanized tunneling. © 2014 American Society of Civil Engineers.

A homogenization procedure to estimate the macroscopic strength of nonlinear matrix-inclusion composites with different strength characteristics of the matrix and inclusions, respectively, is presented in this paper. The strength up-scaling is formulated within the framework of the yield design theory and the linear comparison composite (LCC) approach, introduced by Ponte Castaneda (2002) and extended to frictional models by Ortega et al. (2011), which estimates the macroscopic strength of composite materials in terms of an optimally chosen linear thermo-elastic comparison composite with a similar underlying microstructure. In the paper various combinations for the underlying material behavior for the individual phases of the composite are considered: The matrix phase can be a quasi frictional material characterized either by a Drucker-Prager-type (hyperbolic) or an elliptical strength criterion, which predicts a strength limit also in hydrostatic compression, while the inclusion phase either may represent empty pores, pore voids filled with a pore fluid, rigid inclusions, or solid inclusions, whose strength characteristics also maybe described by a Drucker-Prager-type or an elliptical strength criterion. For generating the homogenized strength criterion efficiently in such general cases of matrix-inclusion composites, a novel algorithm is proposed in the paper. The validation of the proposed strength homogenization procedure for selected combinations of strength characteristics of the matrix material and the inclusions is conducted by comparisons with experimental results and alternative existing strength homogenization models. © 2013 Elsevier Ltd. All rights reserved.

Artificial ground freezing (AGF) is a commonly used technique in geotechnical engineering for ground improvement such as ground water control and temporary excavation support during tunnel construction in soft soils. The main potential problem connected with this technique is that it may produce heave and settlement at the ground surface, which may cause damage to the surface infrastructure. Additionally, the freezing process and the energy needed to obtain a stable frozen ground may be significantly influenced by seepage flow. Evidently, safe design and execution of AGF require a reliable prediction of the coupled thermo-hydro-mechanical behavior of freezing soils. With the theory of poromechanics, a three-phase finite element soil model is proposed, considering solid particles, liquid water, and crystal ice as separate phases and mixture temperature, liquid pressure, and solid displacement as the primary field variables. In addition to the volume expansion of water transforming into ice, the contribution of the micro-cryo-suction mechanism to the frost heave phenomenon is described in the model using the theory of premelting dynamics. Through fundamental physical laws and corresponding state relations, the model captures various couplings among the phase transition, the liquid transport within the pore space, and the accompanying mechanical deformation. The verification and validation of the model are accomplished by means of selected analyses. An application example is related to AGF during tunnel excavation, investigating the influence of seepage flow on the freezing process and the time required to establish a closed supporting frozen arch. © 2013 John Wiley & Sons, Ltd.

The edge-based smoothed finite element method (ES-FEM) was proposed recently in Liu, Nguyen-Thoi, and Lam to improve the accuracy of the FEM for 2D problems. This method belongs to the wider family of the smoothed FEM for which smoothing cells are defined to perform the numerical integration over the domain. Later, the face-based smoothed FEM (FS-FEM) was proposed to generalize the ES-FEM to 3D problems. According to this method, the smoothing cells are centered along the faces of the tetrahedrons of the mesh. In the present paper, an alternative method for the extension of the ES-FEM to 3D is investigated. This method is based on an underlying mesh composed of tetrahedrons, and the approximation of the field variables is associated with the tetrahedral elements; however, in contrast to the FS-FEM, the smoothing cells of the proposed ES-FEM are centered along the edges of the tetrahedrons of the mesh. From selected numerical benchmark problems, it is observed that the ES-FEM is characterized by a higher accuracy and improved computational efficiency as compared with linear tetrahedral elements and to the FS-FEM for a given number of degrees of freedom. © 2013 John Wiley & Sons, Ltd.

Abstract A method to design finite elements that imbricate with each other while being assembled, denoted as imbricate finite element method, is proposed to improve the smoothness and the accuracy of the approximation based upon low order elements. Although these imbricate elements rely on triangular meshes, the approximation stems from the shape functions of bilinear quadrilateral elements. These elements satisfy the standard requirements of the finite element method: continuity, delta function property, and partition of unity. The convergence of the proposed approximation is investigated by means of two numerical benchmark problems comparing three different schemes for the numerical integration including a cell-based smoothed FEM based on a quadratic shape of the elements edges. The method is compared to related existing methods. © Springer-Verlag Berlin Heidelberg 2013.

The diffusion properties of fracturing porous materials, such as concrete or geological materials, are strongly influenced by the complex and random topological structure of the pore space, the state of distributed micro-cracks inevitably caused by processes such as autogenous and drying shrinkage of concrete, and finally by propagating cracks caused by various loading conditions. Information on macroscopic diffusion properties of the porous material requires up-scaling of transport processes within nano- and micro-pores over several spatial scales. The macroscopic transport coefficients are computed using a cascade continuum micromechanics model recently proposed by the authors. The cascade continuum micromechanics model recursively embeds shape information in the form of the ESHELBY matrix-inclusion problem to obtain the homogenized effective diffusion coefficient. The model is able to predict mathematically and physically consistent percolation thresholds. To consider the effects of oriented, diffusely distributed micro-cracks on the diffusion properties, the homogenization scheme for in intact concrete is enhanced by representing micro-cracks as additional ellipsoidal inclusions within the aforementioned homogenized porous matrix. Finally, the effect of propagating macro-cracks on the diffusion process is taken into consideration by weakly coupling the diffusion model and a fracture energy based staggered phase-field model to simulate brittle fracture. © 2013 American Society of Civil Engineers.

The construction of tunnels in soft ground causes short and long term ground deformations resulting from the disturbance of the virgin stress state of the soil and the changing pore water conditions. In particular in urban tunneling, in each stage of the construction process, interactions between the construction process, the soil and existing building infrastructure need to be evaluated to limit the risk of damage on existing buildings and to decide on appropriate mitigation measures. Besides conventional tunneling, mechanized tunneling is a well established and flexible technology in particular in urban areas, which allows for tunnel advances in a wide range of soils and difficult conditions. The paper presents a finite element model for the simulation of interactions between mechanized tunnel construction, the surrounding soil and existing buildings resting on pile foundations in the framework of a process-oriented simulation model for mechanized tunneling. The performance of the model is demonstrated by means of selected prototype analyses. As a consequence of the high computational demand connected with this type of spatio-temporal simulations, problem specific parallelization techniques are investigated to increase the numerical efficiency of the numerical analyses. © 2012 Elsevier Ltd.

Adopting the linear comparison composite (LCC) method within the framework of yield design theory [1], this paper presents a novel multi-scale strength homogenization procedure for a three-phase, partially-frozen soil composite where the solid particle phase (S) and the crystal ice phase (C) are assumed to be characterized by two different Drucker-Prager strength criteria and the liquid water phase (L) has zero shear strength capacity. Based upon a multi-scale thought model shown in Fig. 1, the macroscopic strength criterion for partially frozen soil can be upscaled through a two-step homogenization process. For each step, the LCC methodology is implemented by estimating the strength criterion of a two-phase nonlinear matrix-inclusion composite in terms of an optimally chosen linear thermo-elastic comparison composite with a similar underlying microstructure. In addition, the pressure melting of ice is considered in this model by incorporating a temperature and pressure dependent phase transition between ice and water. The predicted macroscopic strength criterion for partially frozen soil shows qualitatively a good agreement with observed phenomena, such as strengthening of soil during freezing and weakening of soil during pressure melting. © 2013 American Society of Civil Engineers.

An Edge-based Imbricate Finite Element Method (EI-FEM), aiming at combining the ease of mesh generation using triangular elements and the improved accuracy of quadrilateral elements, is proposed. Two finite elements for 2D analyses that imbricate each other while being assembled are designed based upon the triangulation of the domain. By means of two numerical benchmark analyses, we show that similar improvements with respect to numerical accuracy as obtained by the recently proposed Edge-based Smoothed Finite Element Method (ES-FEM) can be obtained, without sacrificing the standard concepts of the Finite Element Method. © 2012 Elsevier Ltd. All rights reserved.

Mechanized tunneling is characterized by a staged procedure of excavation and lining erection and continuous support of the soil by means of supporting fluids (or compressed air) at the tunnel face and pressurized grouting of the tail gap. The interactions between the tunnel boring machine (TBM), the support measures, and the soil, including the groundwater, determine the efficiency, safety, and effects on the existing infrastructure. In this paper, a process-oriented numerical simulation model for mechanized tunneling and its integration in the context of an integrated optimization platform for tunneling (IOPT) is addressed. The simulation model is based upon the finite-element method and considers the transient excavation process and all relevant components, support measures, and processes, along with their interactions during tunnel advance. In particular, the model allows the investigation of the effects of drilling and stand-still periods upon the generation of a filter cake at the tunnel face. This is demonstrated by the numerical analysis of a straight tunnel advance by means of a hydroshield machine in water-saturated soft soil. © 2011 American Society of Civil Engineers.

Glaucoma is among the leading causes of blindness worldwide. The ocular disease is characterized by irreversible damage of the retinal ganglion cell axons at the level of the lamina cribrosa (LC). The LC is a porous, connective tissue structure whose function is believed to provide mechanical support to the axons as they exit the eye on their path from the retina to the brain. Early experimental glaucoma studies have shown that the LC remodels into a thicker, more posterior structure which incorporates more connective tissue after intraocular pressure (IOP) elevation. The process by which this occurs is unknown. Here we present a microstructure motivated growth and remodeling (G&R) formulation to explore a potential mechanism of these structural changes. We hypothesize that the mechanical strain experienced by the collagen fibrils in the LC stimulates the G&R response at the micro-scale. The proposed G&R algorithm controls collagen fibril synthesis/degradation and adapts the residual strains between collagen fibrils and the surrounding tissue to achieve biomechanical homeostasis. The G&R algorithm was applied to a generic finite element model of the human eye subjected to normal and elevated IOP. The G&R simulation underscores the biomechanical need for a LC at normal IOP. The numerical results suggest that IOP elevation leads to LC thickening due to an increase in collagen fibril mass, which is in good agreement with experimental observations in early glaucoma monkey eyes. This is the first study to demonstrate that a biomechanically-driven G&R mechanism can lead to the LC thickening observed in early experimental glaucoma. © 2011 Elsevier Ltd. All rights reserved.

During the design and construction of shield-driven tunnels, a reliable analysis of the construction process is required for the prognosis of the process-induced surface settlements, changes in soil stresses, and changes in groundwater conditions, as well as for the determination of the loads acting on the tunnel tube and on the tunnel-boring machine. In this context, numerical simulation methods like the finite-element method allow for a realistic description of the construction process and its impact on the surrounding underground. The investigated problem is governed by the interactions between the tunneling process and the surrounding underground and its constituents-soil grains, groundwater, and pore air. The tunnel-construction process interacts with the surrounding underground via the heading face support, by frictional contact between shield skin and soil, and because of grouting of the annular gap. Considering these interactions, a holistic simulation model is presented for the process-oriented simulation of shield-supported tunnel advance and its interactions with fully saturated, partially saturated, or nonsaturated soft soil. Its applicability is demonstrated by selected simulations of real-scale examples. Parametric studies are performed to investigate the influence of soil conditions and of process parameters on the time-variant settlements and groundwater conditions, showing its capabilities with respect to the simulation of the soil-process interactions in front, above, and behind the tunnel-boring machine. © 2012 American Society of Civil Engineers.

Adequate consideration of the various interactions between the Tunnel Boring Machine (TBM) and the surrounding underground is a pre-requisite for reliable prognoses in shield supported tunneling based upon numerical analysis. In addition to face support and the grouting of the annular gap the contact conditions along the shield skin between the moving TBM and the surrounding, deforming soil constitute the most relevant component of TBM-soil interactions in mechanized tunneling. This paper is concerned with the analysis of the interface conditions between the shield skin and the soil and its adequate numerical representation in the context of a process-oriented numerical simulation model for mechanized tunneling. The situation around the shield skin is influenced by the design of the Tunnel Boring Machine, the deformational behavior of the surrounding underground and by a possible inflow of process liquids into the steering gap. A novel simulation method is proposed which allows to model the viscous flow of the process liquids into the steering gap and its interactions with the face support, the tail void grouting, the deforming soil and the moving TBM. The proposed numerical model for the TBM-soil interaction is part of a recently developed three-dimensional, process-oriented finite element model for shield tunneling (Nagel et al., 2010). It allows to investigate the effects of the inflow of process liquids into the steering gap during TBM advance considering realistic machine-related and geological conditions. It is, in particular, capable to compute the pressure distribution within the developing liquid film in association with the face support and grouting conditions and to predict its influence on the surface settlements and the overall TBM-soil interaction affecting, e.g. the hydraulic jack forces or shield deformations. © 2010 Elsevier Ltd.

The biomechanics of the optic nerve head is assumed to play an important role in ganglion cell loss in glaucoma. Organized collagen fibrils form complex networks that introduce strong anisotropic and nonlinear attributes into the constitutive response of the peripapillary sclera (PPS) and lamina cribrosa (LC) dominating the biomechanics of the optic nerve head. The recently presented computational remodeling approach (Grytz and Meschke in Biomech Model Mechanobiol 9:225-235, 2010) was used to predict the micro-architecture in the LC and PPS, and to investigate its impact on intraocular pressure-related deformations. The mechanical properties of the LC and PPS were derived from a microstructure-oriented constitutive model that included the stretch-dependent stiffening and the statistically distributed orientations of the collagen fibrils. Biomechanically induced adaptation of the local micro-architecture was captured by allowing collagen fibrils to be reoriented in response to the intraocular pressure-related loading conditions. In agreement with experimental observations, the remodeling algorithm predicted the existence of an annulus of fibrils around the scleral canal in the PPS, and a predominant radial orientation of fibrils in the periphery of the LC. The peripapillary annulus significantly reduced the intraocular pressure-related expansion of the scleral canal and shielded the LC from high tensile stresses. The radial oriented fibrils in the LC periphery reinforced the LC against transversal shear stresses and reduced LC bending deformations. The numerical approach presents a novel and reasonable biomechanical explanation of the spatial orientation of fibrillar collagen in the optic nerve head. © 2010 Springer-Verlag.

Organized collagen fibrils form complex networks that introduce strong anisotropic and highly nonlinear attributes into the constitutive response of human eye tissues. Physiological adaptation of the collagen network and the mechanical condition within biological tissues are complex and mutually dependent phenomena. In this contribution, a computational model is presented to investigate the interaction between the collagen fibril architecture and mechanical loading conditions in the corneo-scleral shell. The biomechanical properties of eye tissues are derived from the single crimped fibril at the micro-scale via the collagen network of distributed fibrils at themeso-scale to the incompressible and anisotropic soft tissue at the macro-scale. Biomechanically induced remodeling of the collagen network is captured on the meso-scale by allowing for a continuous re-orientation of preferred fibril orientations and a continuous adaptation of the fibril dispersion. The presented approach is applied to a numerical human eye model considering the cornea and sclera. The predicted fibril morphology correlates well with experimental observations from X-ray scattering data. © Springer-Verlag 2009.

The paper presents a concept for life-time predictions of metallic structures subjected to ultra low cycle fatigue. The proposed hybrid strategy is characterized by a combination of unit cell analyses on a microstructural level and a micropore damage model used for structural analyses on the macroscopic level. To account for the large plastic deformations evolving during cyclic loading, an advanced elasto-plastic model using a Bari-Hassan-type kinematic hardening rule based on a superposition of several kinematic hardening laws according to Armstrong-Frederick is employed. Micromechanically oriented unit cell analyses are used for a calibration of the model parameters of a macroscopic Gurson-type model. Numerical results include the validation of the macroscopic Gurson model based on laboratory test results on steel specimens as well as a prototype application to a life-time prediction of a metallic spherical pressure vessel subjected to earthquake loading. © 2010 Elsevier Ltd. All rights reserved.

This paper presents a fully coupled finite element formulation for partially saturated soil as a triphasic porous material, which has been developed for the simulation of shield tunnelling with heading face support using compressed air. While for many numerical simulations in geotechnics use of a two-phase soil model is sufficient, the simulation of compressed air support demands the use of a three-phase model with the consideration of air as a separate phase. A multiphase model for soft soils is developed, in which the individual constituents of the soil-the soil skeleton, the fluid and the gaseous phase-and their interactions are considered. The triphasic model is formulated within the framework of the theory of porous media, based upon balance equations and constitutive relations for the soil constituents and their mixture. An elasto-plastic, cam-clay type model is extended to partially saturated soil conditions by incorporating capillary pressure according to the Barcelona basic model. The hydraulic properties of the soil are described via DARCY's law and the soil-water characteristic curve after VAN GENUCHTEN. Water is modelled as an incompressible and air as a compressible phase. The model is validated by means of selected benchmark problems. The applicability of the model to geotechnical problems is demonstrated by results from the simulation of a compressed air intervention in shield tunnelling. © 2009 John Wiley & Sons, Ltd.