Majidreza Nazem

Efficiency of High-Order Elements in Large-Deformation Problems of Geomechanics

AbstractThis paper investigates the application of high-order elements within the framework of the arbitrary Lagrangian-Eulerian method for the analysis of elastoplastic problems involving large deformations. The governing equations of the method as well as its important aspects such as the nodal stress recovery and the remapping of state variables are discussed. The efficiency and accuracy of 6-, 10-, 15-, and 21-noded triangular elements are compared for the analysis of two geotechnical engineering problems, namely, the behavior of an undrained layer of soil under a strip footing subjected to large deformations and the soil behavior in a biaxial test. The use of high-order elements is shown to increase the accuracy of the numerical results and to significantly decrease the computational time required to achieve a specific level of accuracy. For problems considered in this study, the 21-noded elements outperform other triangular elements.

Dynamic analysis of free-falling penetrometers in soil deposits

The simulation of free falling objects penetrating seabed soil deposits is one of the most sophisticated and challenging problems encountered in numerical modelling when using the Finite Element Method. This paper describes a robust numerical method for dealing with such complex and difficult problems. The approach is based on the Arbitrary LagrangianEulerian (ALE) method of analysis, whose main features and challenges are described briefly in the paper. Application of this method to the simulation of dynamic penetration of instruments into undrained layers of uniform soil is discussed in some detail and comparisons with experimental observations are made.

On Application of the Maximum Entropy Meshless Method for Large Deformation Analysis of Geotechnical Problems

Traditional grid-based numerical techniques such as the Finite Element Method (FEM) are known to suffer when large deformations of the continuum are encountered. As such, there has been limited success using this class of methods to solve many of the complex problems encountered in computational geomechanics. The potential of Meshfree techniques for addressing this perceived deficiency has been recognised. This study presents a robust Maximum Entropy Meshless (MEM) method for the analysis of problems involving geometrical nonlinearity in computational geomechanics. The method is validated via simulation of an undrained layer of soil under a rigid and rough strip footing undergoing large deformations and its merit is demonstrated through a comparison of the results with those obtained via the FEM.

Arbitrary Lagrangian-Eulerian method for non-linear problems of geomechanics

In many geotechnical problems it is vital to consider the geometrical non-linearity caused by large deformation in order to capture a more realistic model of the true behaviour. The solutions so obtained should then be more accurate and reliable, which should ultimately lead to cheaper and safer design. The Arbitrary Lagrangian-Eulerian (ALE) method originated from fluid mechanics, but has now been well established for solving large deformation problems in geomechanics. This paper provides an overview of the ALE method and its challenges in tackling problems involving non-linearities due to material behaviour, large deformation, changing boundary conditions and time-dependency, including material rate effects and inertia effects in dynamic loading applications. Important aspects of ALE implementation into a finite element framework will also be discussed. This method is then employed to solve some interesting and challenging geotechnical problems such as the dynamic bearing capacity of footings on soft soils, consolidation of a soil layer under a footing, and the modelling of dynamic penetration of objects into soil layers.

Elasto‐plastic analysis of three‐dimensional structures

In this paper, quasi‐Tresca yield surfaces are reviewed. In order to do elasto‐plastic analysis, a new yield criterion is presented. The proposed yield surface can be used in nonlinear three‐dimensional analysis of structures. Function of the yield surface is presented in principal stress space and also Cartesian one. A computer program has been developed for nonlinear analysis in C++. Numerical examples have been solved by the proposed yield surface and good results have been obtained.

A Numerical Investigation of Sinkhole Subsidence Development over Shallow Excavations in Tectonised Weak Rocks: The Dolaei Tunnel’s Excavation Case

Roof collapses during the constructions of underground excavations in weak rocks is a serious problem. In particular, excavations in shallow depths and in incompetent rocks may initiate the caving of the overburden materials and sinkhole formation. Sinkholes have significant environmental impacts and more importantly, they threaten the stability of surface and subsurface infrastructures above the excavations. This study investigates the formation of sinkhole subsidences in shallow excavations in poor and problematic rocks. The progressive collapse and sinkhole subsidence in the Dolaei road tunnel is considered as a case history. Understanding the geological and geotechnical characteristics of rocks is the fundamental step for analysing the mechanisms of instability. Rock mass characteristics are reviewed, and the most effective factors impacting on the tunnel’s stability are identified and discussed. The role of the method of excavation and support in controlling ground movements are assessed through numerical modellings. The effect of pre-support as a practical technique for controlling ground movements and preventing sinkhole formation in weak rocks is also discussed. Outcomes of this study indicate that the rock mass surrounding the Dolaei tunnel consists of highly tectonised and foliated metamorphic rocks. Schistosity and foliation considerably impact on the strength and deformability of the rock mass. This study shows that the geological characteristics of the rocks in the Dolaei tunnel had substantial effect on the collapse and sinkhole formation. The numerical findings also reveal that employing pre-supporting techniques, particularly forepoling, using a staged excavation and applying composite support systems (consisting of rock bolts and reinforced shotcrete) are practical remedies to prevent the progressive collapse, and to avoid the formation of sinkholes during excavation in weak rocks in shallow depths.

Analysis of Soil Penetration Problems by High-Order Elements

This paper addresses the application of high-order elements in the analysis of soil penetration problems, particularly those involving inertia forces and large deformations. Among others, 15-node triangular elements are formulated within an Arbitrary Lagrangian-Eulerian finite element method. Preliminary studies indicate that high-order elements can significantly decrease the analysis time without significant loss of accuracy.

Rail Degradation Prediction Models for Tram System: Melbourne Case Study

Tram is classified as a light rail mode of transportation. Tram tracks experience high acceleration and deceleration forces of locomotives and wagons within their service life and also share their route with other vehicles. This results in higher rates of degradation in tram tracks compared to the degradation rate in heavy rail tracks. In this research, gauge deviation is employed as a representative of track geometry irregularities for the predication of the tram track degradation. Data sets used in this research were sourced from Melbourne’s tram system. For model development, the data of approximately 250 km of tram tracks are used. Two different models including a regression model and an Artificial Neural Networks (ANN) model have been applied for predicting tram track gauge deviation. According to the results, the performances of the regression models are similar to the ANN models. The determination coefficients of the developed models are above 0.7.

Dynamic Analysis of Unsaturated Soils Subjected to Large Deformations

This paper deals with the large deformation analysis of partially saturated soils subjected to dynamic loading. The so-called ‘mixture’ theory is employed to consider the hydro-mechanical coupling involved in this kind of problem. The finite element method is used to discretise the problem domain and the generalized-α algorithm is employed to integrate the governing equations over time. Some of the most challenging aspects of dynamic analysis of partially saturated soils will be discussed. One of the key challenges is selecting a consistent constitutive model within the theory of mixtures that can incorporate the pore suction forces into the description of stress. The necessity of such incorporation has frequently been reported in experimental studies of unsaturated soils. To tackle this problem, a unique strategy for integrating the constitutive model for unsaturated soils is adopted. Moreover, an absorbing boundary condition, which prevents wave reflection from rigid boundaries, is introduced and implemented into the numerical algorithm. Finally, a solution for the problem of dynamic compaction of soil in a partially saturated condition is presented.

On Application of the Third Medium Contact Method in Analysis of Geotechnical Problems

In computational contact mechanics, the contact constraints are usually applied using the Lagrange multiplier method, the penalty method, or alternative variants. Traditional contact approaches discretise the contact constraints in a weak sense, providing a stable interpolation scheme. However, they demand complicated search algorithms for contact detection at the interface between the intersecting bodies, and they usually lead to formulations that yield highly nonlinear tangent matrices, particularly for cases with realistic soil models and frictional contact. Recently, a new contact method based on the concept of a third medium has been developed, which overcomes the drawbacks of the conventional contact mechanics techniques. This new scheme is based on a space filling mesh in which the contacting bodies can move and interact. Contact constraints are enforced by changing the mechanical properties of medium with respect to the movements of the bodies. This new method has been developed for contact bodies undergoing large deformations using a hyper-elastic material law. In this study, the method is further extended to solve geomechanics problems in which the material behaviour is elastoplastic and the soil is subjected to large deformations. Potential merits of the third medium contact concept for analysing the geotechnical problems by the finite element method will also be addressed.