Numerical Analysis of Segmental Tunnel Linings employing a Hybrid Modeling Approach

The structural performance of shield-driven tunnel linings is considerably affected by the existence of segmental joints. Nevertheless, segmental tunnel linings are commonly modelled as isotropic structures in engineering practice, thereby ignoring the joint-induced stiffness reduction in numerical analysis. A more realistic approach is to consider the tunnel structure as continuous ring with adjusted rigidities which is also referred to as indirect-joint model. However, this modeling approach is a complicated task since it requires the definition of anisotropic stiffness parameters. In this context, the present paper introduces a hybrid modeling approach, which combines analytical solutions and numerical studies. Based on extensive numerical studies, anisotropic stiffness parameters are defined to model an existing drainage tunnel (SDT). Finally, a case study is discussed, where the developed indirect-joint model is used to investigate the structural response of the SDT. In this context, particular emphasis is placed on the deformation of the tunnel crown developing throughout the entire construction phase

A rigorous variant of the shear strength reduction method and its geotechical applications

This paper is focused on a new optimization variant of the shear strength reduction (OPT-SSR) in non-associated Mohr-Coulomb plasticity. The OPT-SSR method mimics the limit analysis problem and enables to compute the factor of safety without performing an elasto-plastic analysis. It is shown that this optimization problem is well defined and closely related to recently developed Davis approaches used in combination with the standard SSR method. Next, the duality between the static and kinematic principles of OPT-SSR is introduced. For the numerical solution, a regularization method is suggested

CPT Data Interpretation Employing Different Machine Learning Techniques

The classification of soils into categories with a similar range of properties is a fundamental geotechnical engineering procedure. At present, this classification is based on various types of cost- and time-intensive laboratory and/or in situ tests. These soil investigations are essential for each individual construction site and have to be performed prior to the design of a project. Since Machine Learning could play a key role in reducing the costs and time needed for a suitable site investigation program, the basic ability of Machine Learning models to classify soils from Cone Penetration Tests (CPT) is evaluated. To find an appropriate classification model, 24 different Machine Learning models, based on three different algorithms, are built and trained on a dataset consisting of 1339 CPT. The applied algorithms are a Support Vector Machine, an Artificial Neural Network and a Random Forest. As input features, different combinations of direct cone penetration test data (tip resistance qc, sleeve friction fs, friction ratio Rf, depth d), combined with “defined”, thus, not directly measured data (total vertical stresses σv, effective vertical stresses σ’v and hydrostatic pore pressure u0), are used. Standard soil classes based on grain size distributions and soil classes based on soil behavior types according to Robertson are applied as targets. The different models are compared with respect to their prediction performance and the required learning time. The best results for all targets were obtained with models using a Random Forest classifier. For the soil classes based on grain size distribution, an accuracy of about 75%, and for soil classes according to Robertson, an accuracy of about 97–99%, was reached

Significance of flow rule for the passive earth pressure problem

Determination of earth pressures is one of the fundamental tasks in geotechnical engineering. Although many different methods have been utilized to present passive earth pressure coefficients, the influence of non-associated plasticity on the passive earth pressure problem has not been discussed intensively. In this study, finite-element limit analysis and displacement finite-element analysis are applied for frictional materials. Results are compared with selected data from literature in terms of passive earth pressure coefficients, shape of failure mechanism and robustness of the numerical simulation. The results of this study show that passive earth pressure coefficients determined with an associated flow rule are comparable to the Sokolovski solution. However, comparison with a non-associated flow rule reveals that passive earth pressure coefficients are significantly over predicted when following an associated flow rule. Moreover, this study reveals that computational costs for determination of passive earth pressure are considerably larger following a non-associated flow rule. Additionally, the study shows that numerical instabilities arise and failure surfaces become non-unique. It is shown that this problem may be overcome by applying the approach suggested by Davis (Soil Mech 341–354, 1968).Ruhr-Universität Bochum (1007

Slope stability analysis: Barodesy vs linear elastic – perfectly plastic models

The results of slope stability analysis are not unique. Different factors of safety are obtained investigating the same slope. The differences result from different constitutive models including different failure surfaces. In this contribution, different strength reduction techniques for two different constitutive models (linear elastic - perfectly plastic model using a Mohr-Coulomb failure criterion and barodesy) have been investigated on slope stability calculations for two different slope inclinations. The parameters for Mohr – Coulomb are calibrated on peak states of element tests simulated with barodesy for different void ratios. For both slopes the predictions of the factors of safety are higher with barodesy than with Mohr-Coulomb. The difference is to some extend explained by the different shapes of failure surfaces and thus different values for peak strength under plane strain conditions. The plane strain predictions of Mohr-Coulomb are conservative compared to barodesy, where the failure surface coincides with Matsuoka-Nakai

Optimization and variational principles for the shear strength reduction method

In this paper, a modified shear strength reduction method (MSSR) and its optimization variant (OPT-MSSR) are suggested. The idea of MSSR is to approximate the standard shear strength reduction to be more stable and rigorous from the numerical point of view. The MSSR method consists of a simplified associated elasto-plastic model completed by the strength reduction depending on the dilatancy angle. Three Davis' modifications suggested by Tschuchnigg et al. (2015) are interpreted as special cases of MSSR and their factors of safety are compared. The OPT-MSSR method is derived from MSSR on the basis of rigid plastic assumption, similarly as in limit analysis. Using the variational approach, the duality between the static and kinematic principles of OPT-MSSR is shown. The numerical solution of OPT-MSRR is obtained by performing a regularization method in combination with the finite element method, mesh adaptivity and a damped Newton method. In-house codes (Matlab) are used for the implementation of this solution concept. Finally, two slope stability problems are considered, one of which follows from analysis of a real slope. The softwares packages Plaxis and Comsol Multiphysics are used for comparison of the results.Web of Science45162407238

Finite element analyses of slope stability problems using non-associated plasticity

In recent years, finite element analyses have increasingly been utilized for slope stability problems. In comparison to limit equilibrium methods, numerical analyses do not require any definition of the failure mechanism a priori and enable the determination of the safety level more accurately. The paper compares the performances of strength reduction finite element analysis (SRFEA) with finite element limit analysis (FELA), whereby the focus is related to non-associated plasticity. Displacement-based finite element analyses using a strength reduction technique suffer from numerical instabilities when using non-associated plasticity, especially when dealing with high friction angles but moderate dilatancy angles. The FELA on the other hand provides rigorous upper and lower bounds of the factor of safety (FoS) but is restricted to associated flow rules. Suggestions to overcome this problem, proposed by Davis (1968), lead to conservative FoSs; therefore, an enhanced procedure has been investigated. When using the modified approach, both the SRFEA and the FELA provide very similar results. Further studies highlight the advantages of using an adaptive mesh refinement to determine FoSs. Additionally, it is shown that the initial stress field does not affect the FoS when using a Mohr-Coulomb failure criterion. Keywords: Finite element limit analysis (FELA), Finite element method, Slope stability, Strength reduction technique, Non-associated plasticity, Adaptive mesh refinement, Initial stresse

Optimization variant of the shear strength reduction method and its usage for stability of embankments with unconfined seepage

In this paper, an optimization variant of the shear strength reduction method is introduced and used for the solution of embankment stability problems with unconfined seepage. The optimization framework is based on approximations of non-associated Mohr–Coulomb plastic models with associated ones, especially by using various Davis’ approaches. Next, the finite element method is considered and mesh adaptive solution concepts are developed for both the unconfined seepage and stability problems. In-house codes in Matlab are used for their implementation. Finally, two numerical examples inspired by geotechnical practice are investigated in order to demonstrate the accuracy of the optimization framework and to evaluate three different Davis’ approaches. The results are compared with commercial codes in Plaxis and Comsol Multiphysics.Web of Science281art. no. 10703