A constitutive model for unsaturated soil–structure interfaces

A constitutive model for simulation of the behavior of unsaturated interfaces is presented here. The model is an extension of an existing critical state compatible interface model for dry and saturated interfaces that was already proposed by one of the authors [Lashkari, A. 2013. Int. J. Numer. Anal. Meth. Geomech. 37 (8): 904–931]. For a proper simulation of the behavior of partially saturated interfaces, the extended model is formulated in terms of two pairs of work conjugate stress–strain‐like variables. The modified model simulations are compared with the existing data of dry, unsaturated, and saturated interfaces. For each interface type, it is shown that the proposed model can capture the essential elements of the behavior using a unique set of parameters. Copyright © 2015 John Wiley & Sons, Ltd.     

A new contact algorithm in the material point method for geotechnical simulations

Contact between stiff structural elements and soil is encountered in many applications in geotechnical engineering. Modelling of such contact is challenging as it often involves impact that would lead to large deformation and failure of the soil. The Material Point Method (MPM) is a mesh‐free method that has been applied to simulate such phenomena. However, the frictional contact algorithm commonly used in MPM only supports Coulomb friction and cannot model fully or partially rough contact conditions in terms of geotechnical engineering. Moreover, because of very different stiffness of contacting materials, the contact force predicted by the previous frictional contact algorithms usually suffers from severe oscillation when applied in structure–soil interaction. This paper presents a new contact algorithm, termed Geo‐contact, designed for geotechnical engineering. In Geo‐contact, a penalty function is incorporated to reduce the oscillation in contact computation, and a limited shear stress is specified along the contact interface. The proposed Geo‐contact algorithm has been implemented to simulate smooth, partially rough and rough contact in typical large deformation penetration problems. The resistance–displacement curves obtained using the Geo‐contact are compared with analytical solutions of limit analysis and large deformation finite element results to verify the accuracy and robustness of the proposed contact algorithm. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical simulation of desiccation cracking in a thin clay layer using 3D discrete element modeling

Desiccation cracking of clay soil is of critical importance in many applications, such as industrial waste containment, hydraulic barriers, road embankments, and agricultural operations. The factors that influence cracking are known qualitatively, but it is not clear how to predict the initiation and propagation of cracks. This study presents a discrete element approach to modeling desiccation cracking in thin clay layers, considering material property changes. First, an aggregate shrinkage model based on the aggregate structure of clay was proposed, and the drying shrinkage of clay soil was modeled by imposing drying shrinkage kinetics for each aggregate at the micro-scale. Second, the clay soil was represented by an assembly of aggregates linked by bonds, and desiccation cracking of the clay layer was modeled using a three-dimensional discrete element code (PFC3D), with the aid of the embedded programming language FISH. When the clay layer is sufficiently thin, the water content gradient along the section can be neglected; thus, the shrinkage kinetics are the same for all of the grains of clay. In the model based on the discrete element method (DEM), the bond strength and contact stiffness changed during drying. Their changes were determined by matching the simulation results with the experimental data. Third, the DEM approach was validated by reproducing experimental desiccation tests performed on a thin clay layer in a disk shape. The geometric parameters of surface cracks were quantified using image analysis techniques and were compared with experimental observations. Fourth, some factors of influence, such as the sample thickness, the properties of the soil–base interface, micro-mechanical parameters, and shrinkage parameters, were investigated using the DEM model. The results obtained from the DEM analyses were compared with the results of prior research in this field of study. The approach used in this study is very promising for simulating desiccation cracking in thin clay soil because the model captures the initiation and propagation mechanism of desiccation cracks. Although this study was carried out on surface cracking in a thin clay layer, the extension of this methodology is of potential benefit not only for predicting three-dimensional desiccation cracking in real clay liners but also for modeling cracking in other materials with properties that vary with water content or temperature, such as concrete and rock.     

Experimental Investigation of Frictional Behavior Between Cohesive Soils and Solid Materials Using Direct Shear Apparatus

Soil shearing resistance is very important while designing various structures which have direct contact with soil, for example, sheet piles, piles, retaining walls, reinforced earth structures and shallow foundations. Even though designers use empirical values for their design, it is very important to obtain more accurate values for soil–solid materials shearing resistance. In this work, laboratory tests have been carried out to investigate the effect of roughness interface and texture models on friction angle between cohesive soils and steel, as well as abrasive paper material, using direct shear tests. All tests were carried out under consolidated drained shear conditions. The behavior at the soils–solid interface was found to vary according to surface roughness. It also seems that the type of material (steel or paper of abrasive) used does not have a major influence on the shear strength. As far as roughness is concerned, friction behavior is likely to be generally classified into three failure modes, namely full sliding at the interface, shear failure within the soil, and a mixed behavior where interface sliding and shear deformation of the soil specimen proceed simultaneously. However, for the second mode, the shear strength at the interface soil-rough solid materials steel was found to be lower than the shear strength of the soil, for a soil that is classified as high plasticity clay. Furthermore, it was found that the interfacial shear strength is independent of the texture surface for a given roughness.     

Numerical modelling of a field soil desiccation test using a cohesive fracture model with Voronoi tessellations

Numerical modelling of a field soil desiccation test is performed using a hybrid continuum-discrete element method with a mix-mode cohesive fracture model and Voronoi tessellation grain assemblages. The fracture model considers material strength and contact stiffness degradation in both normal and tangential directions of an interface. It is found that the model can reasonably reproduce the special features of the field soil desiccation, such as curling and sub-horizontal crack. In addition, three significant factors controlling field desiccation cracking, fracture energy, grain heterogeneity and grain size are identified.     

非线性接触力学模型在地基-基础相互作用弹性分析中的应用

针对多体相互作用体系的非连续变形分析问题和接触问题,采用Mohr-Coulomb屈服准则和关联流动法则以及接触界面上的非线性应力分布模式,考虑接触界面特性提出了非线性接触力元模型,以结点位移和界面相互接触应力同时作为独立未知变量,建立了离散系统的总体控制方程.进而,通过数值求解能够直接确定变形体内的应力与变形、界面上的接触应力与离散体的位移与运动.将这种以接触力元为基础的多体系统分析方法具体应用于基础与地基相互作用分析,通过数值计算与分析探讨了地基与基础的相对刚度、荷载大小及其偏心距、地基与基础间界面力学参数对接触界面的应力分布和地基变形的影响,所得结果为工程中考虑基础与地基相互作用影响的设计与分析提供了参考依据.     

基于地质剖面构建三维地质模型的方法研究

地质剖面是三维地质建模的重要数据源,运用地质剖面构建三维地质模型的方法应用较为广泛。在模型构建之前,统一确定模数据的坐标系和比例足,建立原始资料数据库。构建三维地质模型的关键是不同地质界面。本文详细介绍了模型边界面、断层面、地层界面、岩体界面等4种主要地质界面的构建流程与方法,尤其对褶皱构造、地层界面的断层效应、复杂岩体界面等的构建进行了重点阐述按模型边界面(模型的底界面和四周边界面)、DEM面、断层面、其他地质界面的顺序依次构建地质界面,构建断层面和其他地质界面时严格按先新后老的顺序。运用已构建好的地质界面按先新后老的顺序逐个、依次建立单个地质体,再将所有地质体的面模型组合成整个模型的面模型。通过对面模型进行网格(实体)填充和对网格赋予相应的属性值,最终构建三维地质模型。     

接触角度对黄土-三趾马红土界面剪切力学特性影响研究

西北黄土高原第四系黄土广泛沉积于新近系三趾马红土之上形成粗糙接触的异质土界面,为典型的易滑层面。为探讨接触界面粗糙度对黄土-三趾马红土界面剪切力学特性影响,研制界面制样装置及剪切仪,开展简化黄土-三趾马红土界面直剪试验研究。结果表明:界面剪切破坏模式有齿间滑动、齿间滑动-齿面剪断、齿面剪断3种,界面接触角度越大,破坏模式越趋于齿面剪断,接触角度越小,破坏模式越趋于齿间滑动;界面剪切应力-剪切位移曲线演化规律表明界面脆性剪切破坏特征明显,且界面接触角度越大,峰值前剪切刚度与剪切破坏位移越大,峰值后剪切位移“跳跃”跌落现象越明显,界面脆性剪断破坏特征越显著;界面剪切过程产生明显剪胀效应,随界面接触角度增大,峰值剪胀角呈先减小而后增大趋势,反映了界面不同剪切破坏模式变化;受界面间初始黏聚强度与剪切破坏模式影响,界面抗剪强度随法向应力呈非线性变化,并受界面接触角度影响,峰值强度随界面角度增大呈先增大而后减小趋势,残余强度随界面角度增大呈增大趋势。     

Beam–solid contact formulation for finite element analysis of pile–soil interaction with arbitrary discretization

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. Copyright © 2014 John Wiley & Sons, Ltd.     

等应力增量比路径下接触面应变软化模型

等应力增量比路径单剪试验条件下,部分土体与结构接触面表现出明显的应变软化和剪胀特性。基于广义位势理论,将土与结构的接触面问题看作应力空间上的二维问题,势函数取法向应力和切向应力,用塑性状态方程取代传统的屈服面,建立等应力增量比路径条件下的土与结构接触面应力-应变软化模型。采用指数函数,拟合单向压缩试验中法向应力与法向应变的关系,采用复合指数函数,拟合应力比与切向应变的关系,采用另一复合指数函数,拟合法向剪胀分量与切向应变的关系。通过对拟合函数进行微分,确定模型中待定系数的求解方法。结合试验结果对模型进行验证,模型拟合效果良好,具有一定的合理性。     

Frictional contact of laminated elastic half‐spaces allowing interface cavities. Part 1: Analytical treatment

The paper deals with the plane problem on frictional contact of stratified elastic half‐spaces provided discontinuity of their direct touch. Imperfectness of contact of the bodies is assumed to be caused by surface unevenness of their surface layers. The problem is formulated within the framework of the homogenized model with microlocal parameters. Using the method of complex potentials in combination with the method of interface gaps the problem is reduced to a singular integral equation on the function of interface gap height. Copyright © 2001 John Wiley & Sons, Ltd.     

A two-mechanism soil–structure interface model for three-dimensional cyclic loading

A three-dimensional (3D) soil–structure interface model is proposed within the two-mechanism constitutive theory and bounding surface theory originally established for soils. The proposed model has two main characteristics: first, the model is formulated based on two different and superposed deformation mechanisms. The first mechanism is due to the stress ratio increment, and the second is due to the normal stress increment. Each mechanism induces a shear strain component and a normal strain component. The proposed model can be reduced to the conventional single-mechanism interface model. Second, the plastic modulus and stress dilatancy are defined using the bounding surface theory. The plastic flow rule under cyclic loading is modified and assumed to be dependent on both the stress state of the mapping point and the stress reversal loading direction. The proposed model was validated against the available 3D interface tests and was found to satisfactorily reflect the salient features of the interfaces under monotonic and cyclic loading paths with different normal boundaries. The problem in which the elastic normal stiffness in conventional single-mechanism interface models is often underestimated to enhance the simulation performance under varying normal stress conditions is solved by incorporating the second mechanism. And the effect of the second mechanism on the modeling behavior is discussed. The modified plastic flow direction accurately simulates the 3D cyclic shear response, and the difference between the model simulation and test result increases with the number of cycles by use of the plastic flow direction defined in conventional bounding surface theory.     

A dual mortar contact method for porous media and its application to clay-core rockfill dams

The clay-core rockfill dam is a multibody contact system in which the hydromechanical response of the clay core plays a crucial role. This complex problem is highly challenging to model numerically. We present a numerical approach that considers the multibody contact, consolidation, and strong geometric and material nonlinearities for the modeling of clay-core rockfill dams. Within the framework of the dual mortar finite element method, the presented approach considers the contact bodies as independent porous media continuums. The nonlinear contact conditions are derived based on the effective contact traction on contact interfaces and pore pressure continuity. The weak forms are obtained by introducing Lagrange multipliers as additional unknowns, which are then condensed through an extended general transformation. The presented method is first validated with a patch test considering the contact between two porous media. Then, a three-dimensional analysis of the Rumei clay-core rockfill dam is performed. The main numerical analysis concerns are the two observation galleries planned for construction inside the clay core. The galleries consist of dozens of tunnel-like concrete blocks, giving rise to complex concrete-concrete and concrete-clay contacts. The discontinuous separation and sliding between concrete blocks are investigated. For the concrete-concrete contact, both hard and soft joint approaches are evaluated and compared. The pore pressure results of the concrete structures are also analyzed.     

Finite element modeling of slab–foundation interaction on rigid pavement applications

Bending stresses produced in pavement concrete slabs are influenced by the contact condition along the slab–foundation interface. A finite element model for the analysis of frictional contact between unbonded pavement layers is presented. A 2D plate element is proposed for the idealization of composite slabs. Interface elements are used to connect the unbonded layers within the composite slab and to model the interface between the bottom slab layer and the foundation layer. Interface elements have the ability to capture the separation and sliding between pavement layers, due to thermal loads, and to calculate the frictional traction at their interface.     

Implementation of DSC model and application for analysis of field pile tests under cyclic loading

The disturbed state concept (DSC) model, and a new and simplified procedure for unloading and reloading behavior are implemented in a nonlinear finite element procedure for dynamic analysis for coupled response of saturated porous materials. The DSC model is used to characterize the cyclic behavior of saturated clays and clay–steel interfaces. In the DSC, the relative intact (RI) behavior is characterized by using the hierarchical single surface (HISS) plasticity model; and the fully adjusted (FA) behavior is modeled by using the critical state concept. The DSC model is validated with respect to laboratory triaxial tests for clay and shear tests for clay‐steel interfaces. The computer procedure is used to predict field behavior of an instrumented pile subjected to cyclic loading. The predictions provide very good correlation with the field data. They also yield improved results compared to those from a HISS model with anisotropic hardening, partly because the DSC model allows for degradation or softening and interface response. Copyright © 2000 John Wiley & Sons, Ltd.     

Modeling of pile installation using contact mechanics and quadratic elements

In geotechnical engineering, numerical analysis of pile capacity is often performed in such a way that piles are modeled using only the geometry of their final position in the ground and simply loaded to failure. In these analyses, the stress changes caused by the pile installation are neglected, irrespective of the installation method. For displacement piles, which are either pushed or hammered into the ground, such an approach is a very crude simplification. To model the entire installation process of displacement piles a number of additional nonlinear effects need to be considered. As the soil adjacent to the pile is displaced significantly, small deformation theory is no longer applicable and a large deformation finite element formulation is required. In addition, the continuously changing interface between the pile and the soil has to be considered. Recently, large deformation frictional contact has been used to model the pile installation and cone penetration processes. However, one significant limitation of the analysis was the use of linear elements, which have proven to be less accurate than higher order elements for nonlinear materials such as soils.

This paper presents a large deformation frictional contact formulation which can be coupled consistently with quadratic solid elements. The formulation uses the so-called mortar-type discretisation of the contact surfaces. The performance of this contact discretisation technique is demonstrated by accurately predicting the stress transfer between the pile and the soil surfaces.     

Frictionless contact formulation for dynamic analysis of nonlinear saturated porous media based on the mortar method

A finite element algorithm for frictionless contact problems in a two‐phase saturated porous medium, considering finite deformation and inertia effects, has been formulated and implemented in a finite element programme. The mechanical behaviour of the saturated porous medium is predicted using mixture theory, which models the dynamic advection of fluids through a fully saturated porous solid matrix. The resulting mixed formulation predicts all field variables including the solid displacement, pore fluid pressure and Darcy velocity of the pore fluid. The contact constraints arising from the requirement for continuity of the contact traction, as well as the fluid flow across the contact interface, are enforced using a penalty approach that is regularised with an augmented Lagrangian method. The contact formulation is based on a mortar segment‐to‐segment scheme that allows the interpolation functions of the contact elements to be of order N. The main thrust of this paper is therefore how to deal with contact interfaces in problems that involve both dynamics and consolidation and possibly large deformations of porous media. The numerical algorithm is first verified using several illustrative examples. This algorithm is then employed to solve a pipe‐seabed interaction problem, involving large deformations and dynamic effects, and the results of the analysis are also compared with those obtained using a node‐to‐segment contact algorithm. The results of this study indicate that the proposed method is able to solve the highly nonlinear problem of dynamic soil–structure interaction when coupled with pore water pressures and Darcy velocity. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhancement of a hypoplastic model for granular soil–structure interface behaviour

Modelling of interfaces in geotechnical engineering is an important issue. Interfaces between structural elements (e.g., anchors, piles, tunnel linings) and soils are widely used in geotechnical engineering. The objective of this article is to propose an enhanced hypoplastic interface model that incorporates the in-plane stresses at the interface. To this aim, we develop a general approach to convert the existing hypoplastic model with a predefined limit state surface for sands into an interface model. This is achieved by adopting reduced stress and stretching vectors and redefining tensorial operations which can be used in the existing continuum model with few modifications. The enhanced interface model and the previous model are compared under constant-load, stiffness and volume conditions. The comparison is followed by a verification of two the approaches for modelling the different surface roughness. Subsequently, a validation between available experimental data from the literature versus simulations is presented. The new enhanced model gives improved predictions by the incorporation of in-plane stresses into the model formulation.     

基于水平集的复杂三维地层模型建模及在地下工程中的应用研究

由于地质体存在断层、尖灭、出露等复杂地质现象,在三维地层建模时,为了表达这些现象,无论是面模型还是体模型或混合模型,都存在空间分割或曲面求交线的问题。由于地质体拓扑关系的复杂性、数据误差以及计算机精度问题,使得这些模型在实际建模过程中常常失效。运用水平集理论可以有效解决这一问题,水平集用隐函数表达曲面（或超曲面）,可以实现复杂地质体的表达及并、交、差等拓扑运算。在三维地层建模中,插值生成各种地质界面后,用水平集表达这些地质界面,利用水平理论完成各种复杂的拓扑操作,建立以水平集表达的三维地层模型。在此基础上,插入水平集表达的各种工程活动界面,利用水平集理论进行拓扑操作,可构建各种工程活动后的地层模型。再利用Marching cube算法抽取各种地质界面或工程活动界面,构建可用于实时可视化或用于工程评估（如有限元计算）等的NMTINF-BR地层模型或工程活动后的NMTINF-BR地层模型。     

正交各向异性岩体中非圆形水工隧洞的解析解

将围岩和衬砌分别视作均质、连续的线弹性正交各向异性和各向同性体,并充分考虑衬砌的支护滞后效应和隧洞运行时的内水压力作用,运用复变函数方法中的幂级数解法,提出了正交各向异性岩体中任意形状水工隧洞的力学解析方法。以直墙半圆拱形水工隧洞为例,所获得的解析解可精确满足衬砌内边界的应力边界条件以及围岩与衬砌接触面的应力、位移连续条件,同时还将解析结果与ANSYS数值结果对比分析,吻合良好。利用获得的解析解,讨论了围岩开挖面上不同的各向异性程度、不同的弹性对称面角度以及隧洞内不同的水压荷载对衬砌以及围岩上应力和位移分布的影响。