Numerical simulation of lumpy soils using a hypoplastic model

The lumpy soil is a by product of the open-pit mining. A composite-lumpy material (in which, the lumps are randomly distributed in the reconstituted soil) is being created due to the degradation of the initial granular structure. In the present study, the compression and failure behaviour of an artificial lumpy material with randomly distributed inclusions are investigated using the finite element method. The computation results show that the stress ratio, defined as the ratio of the volume average stress between the lumps and the reconstituted soil within the inter-lump voids, is significantly affected by both the volume fraction and the preconsolidation pressure of the lumps under an isotropic compression path, while the volume fraction of the lumps plays a minor role under a triaxial compression path. Based on the simulation results, a homogenization law was proposed utilizing the secant stiffnesses.     

Employing a variable permeability model in numerical simulation of saturated sand behavior under earthquake loading

Despite advances in the numerical analysis of saturated sand behavior under earthquake loading, accurate prediction of liquefaction-related phenomena by numerical simulation remains a challenge. Variation of the coefficient of permeability is a key issue which has not obtained due attention in most previous modeling. In this study, a revised form of a recently proposed variable permeability function was implemented in a fully coupled dynamic model adopting modern two-surface plasticity constitutive law to evaluate the effects of permeability variations on the results of numerical modeling. The variable permeability model is comprised of a simple function relating the permeability coefficient of soil mass to the excess pore water ratio. In this study, the constants of the variable permeability function were attained based mainly on theoretical evidence and experimental observation. Well-documented centrifuge experiments were examined to evaluate how well the proposed model captures the main features of soil response to earthquake loading. The results indicate that the proposed function greatly enhanced the capability of numerical modeling to predict the behavior of saturated sand under cyclic loading. Particularly, the variable permeability model with proposed constants significantly improved the amount of liquefaction-induced settlement predicted by numerical modeling.     

Numerical simulation of cone penetration testing using a new critical state constitutive model for sand

A new perspective on the numerical simulation of cone penetration in sand is presented, based on an enhanced critical state model implemented in an explicit-integration finite element code. Its main advantage, compared to similar studies employing simpler soil models, is that sand compressibility can be described with a single set of model parameters, irrespective of the stress level and the sand relative density. Calibration is based on back-analysis of published centrifuge experiments, while results of the methodology are also compared against independent tests. Additional analyses are performed to investigate sand state effects on cone penetration resistance, in comparison with empirical expressions from the literature.     

On-chip earthquake simulation model using potentials

A two-dimensional (2-D) Cellular Automata (CA) dynamic system constituted of cells-charges has been proposed for the simulation of the earthquake process. The CA model has been calibrated with the use of real data. The calibration incorporates major seismic characteristics of the area under test. The simulation results are found in good quantitative and qualitative agreement with the recorded Gutenberg–Richter (GR) scaling relations. The model is enriched with a powerful multi-parameter interface that enables the user to load real data from different regions. This paper examines the on-chip realisation of the model and its instrumentation. The CA model hardware implementation is based on Field Programmable Gate Array (FPGA) logic. It utilises an array of 32 × 32 cells. Parameters that construct the local CA rule constitute the input data. The initial seed, which to some extent corresponds to the seismic features of the area under test, is loaded in a semi-parallel way and the process is completed in a certain number of time steps. The automatic response of the processor provides the corresponding GR scaling law of the area under study. The hardware implementation of the CA-based earthquake simulation model is advantageous in terms of low-cost, high-speed, compactness and portability features. It can operate as a preliminary data-acquisition filter that accelerates the evaluation of recorded data as far as its origin time, spatial and magnitude completeness and quality are concerned. Software that performs reliable automatic phase picking, as well as data elaboration, can be assembled next to the earthquake recording instruments (the whole network) output to assure a quick and reliable iteration of the focal parameters of a recorded earthquake (epicentre coordinates, focal depth and magnitude). The dedicated processor can be accommodated right after this stage (before any manual elaboration) focusing on the near real-time development of a reliable qualitative dynamical seismic record and a mapping of the seismic characteristics of the area.     

A hypoplastic model for clay and sand

The theory of hypoplasticity was developed initially for non-cohesive soils. However, sand and clay have many common properties; therefore arose the idea to extend the hypoplastic model to clay. The proposed model is able to describe the behaviour of cohesive soils with the incorporation of an appropriate structure tensor into the constitutive equation. This tensor is a stress-like internal parameter, also called back stress. This enables us to describe the behaviour of cohesive soils with the same material parameters for several states of consolidation and also to model barotropy and pycnotropy of sand. Numerical simulations of element tests are performed in order to check the performance of this hypoplastic model. Experimental data obtained with normally and overconsolidated clay and sand specimens with various densities are taken for comparison, and it is shown that the model is capable of describing the material behaviour of clay and sand. The determination of the material constants, the calibration method, is also presented in this paper.     

A simple hypoplastic constitutive model for sand

The paper presents a hypoplastic constitutive model for the three-dimensional non-linear stress–strain and dilatant volume change behaviour of sand. The model is developed without recourse to the concept in elastoplasticity theory such as yield surface, plastic potential and decomposition into elastic and plastic parts. Benefited from the non-linear tensorial functions available from the representation theorem the model possesses simple mathematical formulation and contains only four material parameters, which can be easily identified with triaxial compression tests. Comparison of the predictions with the experimental results shows that the model is capable of capturing the salient behaviour of sand under monotonic loading and is applicable to both drained and undrained conditions.     

A basic hypoplastic constitutive model for sand

Hypoplastic constitutive models are based on nonlinear tensor functions and are characterized by simple formulation and few parameters. In its early stage, mainly basic hypoplastic constitutive equations were concerned, where the stress tensor is assumed as the only state variable. There followed some enhanced models based on the basic constitutive equation by including void ratio as an additional state variable. In this paper, we first show that the widely used hypoplastic model by Wolffersdorff is seriously flawed because the underlying basic equation does not perform properly. We proceed to develop a basic hypoplastic constitutive equation by introducing a new tensorial term, which preserves the critical state at large strain. The model performance is demonstrated by parameter study for some element tests. This simple and robust basic equation is well suited to build more sophisticated models.     

三峡库区泄滩滑坡渗流场与应力场耦合分析

随着大规模梯级水电工程的不断发展,库岸滑坡的稳定性及潜在的涌浪灾害将是影响水电工程(群)全生命周期安全运行的关键因素。本文分别从库岸边坡稳定性及滑坡涌浪研究两个方面,对目前国内外的相关研究成果进行了系统总结和分析。在库岸边坡稳定性研究方面,越来越多的研究集中于降雨、库水位变动等水动力环境下的库岸边坡的水-岩耦合作用及稳定性演化的分析方法方面。通过对滑坡涌浪的物理模型试验法、解析法及数值模拟法3大类方法的优缺点分析,随着数值计算技术的飞速发展其在滑坡涌浪链生灾害动力学分析方面体现出其强大的生命力。库岸滑坡从渐进破坏→启动→高速运动、涌浪→堆积、涌浪传播与爬坡等构成了滑坡涌浪灾害链的不同阶段,也是一个连续的灾害过程,而不同阶段灾害特点又有很大的差异。然而,目前通常将库岸滑坡稳定性分析和涌浪分析分割开来,从而不能很好地反应这一灾害链过程。基于高性能数值计算技术,从全过程的物理力学机制出发构建灾害链的模拟和分析方法,将是目前的发展趋势,也是从解决库岸滑坡涌浪链生灾害动力学分析的关键。     

单调荷载下砂土变形过程数值模拟及细观机制研究

采用离散单元法的颗粒流理论,模拟了松砂和密砂在单调荷载作用下的变形过程,研究了砂土渐进破坏过程中的宏观力学行为和细观组构参量的演化规律。采用PFC的FISH语言开发了细观组构统计程序,通过记录加载不同时刻试样的细观参量,如配位数、接触法向分布、粒间法向接触力、切向接触力等的演化,分析了砂土变形过程中细观组构变化与宏观力学响应之间的内在联系。应用表征上述量的组构参数研究了砂土的诱发各向异性,探讨了松砂剪缩、密砂剪胀的细观机制。研究成果对于揭示砂土变形的细观机制以及建立砂土的细观力学模型具有重要意义。     

Simple shear in sand with an anisotropic hypoplastic model

The paper deals with the anisotropic extension of an inherently isotropic hypoplastic constitutive model. Inherent transverse isotropy is incorporated into the model with the help of an operator of anisotropy which is determined by the orientation of the bedding plane and three constitutive parameters. The approach is rather general and is not connected with any specific type of constitutive model. In the present study, two possible extensions of the hypoplastic equation are considered: the application of the anisotropy operator to the nonlinear part of the equation and to the whole equation. The two modifications are exemplified by numerical calculations of simple shear.     

Numerical integration and FE implementation of a hypoplastic constitutive model

Hypoplastic constitutive equation based on nonlinear tensor functions possesses a failure surface but no yield surface. In this paper, we consider the numerical integration and FE implementation of a simple hypoplastic constitutive equation. The accuracy of several integration methods, including implicit and explicit methods, is examined by performing a set of triaxial compression tests. Adaptive explicit schemes show the best performance. In addition, the stress drift away from the failure surface is corrected with a predictor-corrector scheme, which is verified by two boundary value problems, i.e. rigid footing tests and slope stability.     

Numerical simulation of evapotranspiration using a root water uptake model

The abstraction of ground water by vegetation and its influence on the behaviour of geotechnical structures such as pavements, foundations and slopes has been researched for many years. In recent years, the use of numerical methods in geotechnical engineering has rapidly increased. A major challenge for numerical modelers involves the specification of the surface flow boundary condition to model root water uptake by vegetation. The abstraction of ground water by roots is a complex process which involves the interaction of the atmosphere, vegetation and the ground. Accurate prediction of the pore water pressure changes induced by vegetation requires the development of algorithms which can mimic the process of transpiration by vegetation in the continuity equation of fluid flow. This paper presents a root water uptake model which has been coded into a finite element program that can perform coupled (mechanical and fluid flow behaviour) analyses. The input data includes rainfall, potential evapotranspiration and maximum root depth. The key features of the model are discussed first, followed by a presentation of results from a series of parametric studies.     

Modelling of sand behaviour under earthquake excitation

In this paper, liquefaction potential of loose sand deposit subjected to an earthquake loading is evaluated. The analysis is performed by using a finite element technique incorporating the equations of dynamics of saturated porous elastoplastic media. The soil response is modelled by an anisotropic hardening rule, similar to that as proposed by Poorooshasb and Pietruszczak.¹ The concept is based on the theory of bounding surface plasticity incorporating a non-associated flow rule and the idea of reflected plastic potential. The present paper provides a modified formulation to that discussed in Reference 1. Modifications are aimed at simplifying the concept for numerical implementations.     

基于Gudehus-Bauer模型的砂土中灌注桩护壁套管高频振动贯入速率

随着全套管护壁振动取土灌注桩施工工艺的发展,灌注桩在工程中的应用越来越广泛,但是关于灌注桩护壁套管高频振动贯入速率的研究较少。通过假定套管为刚体,将套管周围土体划分为同心轴的圆环柱体,利用Gudehus-Bauer亚塑性本构模型计算套管外侧各土体单元接触面间的剪应力和套管端部土体竖向应力,并考虑套管振动贯入过程中土塞效应,建立灌注桩套管高频振动贯入砂土中贯入速率的计算模型。将所提出的贯入速率计算结果和物理模型试验结果与有限元结果进行对比分析,验证了计算模型的合理性。通过参数分析,得到地基土体孔隙比、振动频率和套管直径对贯入速率的影响规律,为实际工程中快速、准确地预测套管振动贯入速率提供了可靠方法。     

建筑物下砂土地震液化分区特征数值模拟研究

基于动力有限元反应分析,对存在于建筑物下的非自由场地的液化特性进行了研究。通过多种工况的计算,探讨了建筑物刚度、宽度和荷载大小对液化区分布的影响。以往一般认为建筑物下的砂土难于液化,而由计算结果及分析,可先实际情况并非这样简单。建筑物基底以下存在易于液化的“V”字型区域,该区域形态与建筑物的刚度、宽度和荷载大小都有密切关系。     

Numerical simulation of fabric anisotropy and strain localization of sand under simple shear

This paper studies the effects of initial fabric anisotropy of dry sand in simple shear deformation. The effects of anisotropy are taken into consideration through the modification of the mobilized friction in the Mohr–Coulomb‐type yield surface as a function of a fabric parameter. In addition, the constitutive model uses a gradient term that directly incorporates the effects of material length scale. The constitutive formulation is implemented into ABAQUS finite element code and used to simulate shearing of the dry sand under various conditions of simple shear. The numerical simulations show that while the shear stress response is affected by fabric anisotropy, its effects on strain localization in simple shear are minimal. This is in contrast to other devices such as the biaxial shear. The strain localization in simple shear is controlled more by the imposed boundary conditions. The use of material length scale is shown to remove the effects of strain localization in the shearing response. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical simulation of a stratigraphic model

A multi-lithology diffusive stratigraphic model is considered, which simulates at large scales in space and time the infill of sedimentary basins governed by the interaction between tectonics displacements, eustatic variations, sediment supply, and sediment transport laws. The model accounts for the mass conservation of each sediment lithology resulting in a mixed parabolic, hyperbolic system of partial differential equations (PDEs) for the lithology concentrations and the sediment thickness. It also takes into account a limit on the rock alteration velocity modeled as a unilaterality constraint. To obtain a robust, fast, and accurate simulation, fully and semi-implicit finite volume discre tization schemes are derived for which the existence of stable solutions is proved. Then, the set of nonlinear equations is solved using a Newton algorithm adapted to the unilaterality constraint, and preconditioning strategies are defined for the solution of the linear system at each Newton iteration. They are based on an algebraic approximate decoupling of the sediment thickness and the concentration variables as well as on a proper preconditioning of each variable. These algorithms are studied and compared in terms of robustness, scalability, and efficiency on two real basin test cases.     

深埋隧道锚杆支护作用的数值模拟与模型试验研究

以重庆至长沙公路共和隧道为研究对象,研制出环氧-硅橡胶的相似模型材料,在大型真三轴模型试验机上进行了隧道在毛洞和锚杆支护条件下的超载试验,采用应变测量技术对隧道关键部位的应变值和锚杆的应变值进行了测量,据此分析了围岩体在锚杆支护后的应力变化规律和锚杆的轴力变化规律。采用与模型试验相同的边界条件进行了隧道在锚杆支护条件下的数值模拟工作,对围岩体的塑性区、应力场和锚杆轴力随荷载的变化过程进行了研究。结果表明,两种手段反映的围岩体应力变化规律和锚杆轴力随荷载的变化规律是一致的     

阪神地震中大开地铁车站震害机制数值仿真分析

1995年阪神地震中,神户市的地下结构遭受了严重的破坏,尤其是地铁车站的破坏最为严重,其中大开地铁车站有一半以上的中柱发生坍塌,导致地面坍塌近2.5 m。对大开地铁车站的震害进行了全面地描述,基于大型商用有限元软件ABAQUS,采用在该软件上二次开发的土体非线性黏弹性动力本构模型模拟土在循环荷载作用下的动力特性,采用黏塑性动力损伤模型模拟混凝土在循环荷载作用下的拉、压应力-应变关系,对大开地铁车站的非线性地震反应进行了数值仿真分析,给出了破坏演化过程,把有限元分析的结果与大开地铁站的实际震害现象进行了对比,结果表明：数值仿真分析结果能够合理地解释大开地铁车站的主要震害现象,建模方法能够被用来进行地下结构的非线性地震反应分析。