Numerical modelling of dynamic consolidation on granular soils

The application of Pastor–Zienkiewicz constitutive model for sands to dynamic consolidation problems is presented in this paper. This model is implemented in a coupled code formulated in terms of displacements for both solid and fluid phases (u?w formulation), which is firstly compared with u?p_w formulation for some simple examples. Its range of validity, previously established for elastic problems and harmonic loading, is explored. Once the suitability of the u?w formulation has been ascertained for this kind of dynamic problems in soils, one‐ and two‐dimensional (plane strain) dynamic consolidation numerical examples are provided, aiming to give some light into the physics of this ground improvement technique. A ‘wave of dryness’, observed at the soil surface during the impact in field cases, is numerically reproduced and justified. Some hints on the influence of the loading zone size are also given. Copyright © 2007 John Wiley & Sons, Ltd.     

Numerical modelling of fluid flow in microscopic images of granular materials

A program for the simulation of two‐dimensional (2‐D) fluid flow at the microstructural level of a saturated anisotropic granular medium is presented. The program provides a numerical solution to the complete set of Navier–Stokes equations without a priori assumptions on the viscous or convection components. This is especially suited for the simulation of the flow of fluids with different density and viscosity values and for a wide range of granular material porosity. The analytical solution for fluid flow in a simple microstructure of porous medium is used to verify the computer program. Subsequently, the flow field is computed within microscopic images of granular material that differ in porosity, particle size and particle shape. The computed flow fields are shown to follow certain paths depending on air void size and connectivity. The permeability tensor coefficients are derived from the flow fields, and their values are shown to compare well with laboratory experimental data on glass beads, Ottawa sand and silica sands. The directional distribution of permeability is expressed in a functional form and its anisotropy is quantified. Permeability anisotropy is found to be more pronounced in the silica sand medium that consists of elongated particles. Copyright © 2001 John Wiley & Sons, Ltd.     

混凝土面板堆石坝蠕变沉降经验模型及其应用

根据岩土材料Merchant、Bergers和Butterfield蠕变模型的基本形式,建立3种预测混凝土面板堆石坝蠕变沉降经验模型。以蒲石河混凝土面板堆石坝为例,采用非线性有限元方法数值模拟堆石坝的顺序填筑过程。根据堆石坝沉降现场观测数据,采用回归分析方法确定3种沉降经验模型中的待定参数,确定堆石坝的蠕变沉降随时间的变化关系。工程实践应用结果表明,所提出的堆石坝蠕变沉降经验模型具有较高的拟合和预测精度,堆石坝的蠕变沉降除与时间有关之外,还与观测点的应力状态和位置以及堆石料的变形模量有关。     

天津地铁既有线改造工程中的控制差异沉降研究

天津地铁一号线改造工程是在原有地铁线路的基础上对车站和部分隧道进行重建，为了列车的运行安全，必须严格控制新旧隧道结构交接处的差异沉降。通过有限元数值分析，对所要采用的高压旋喷加固地基方案进行数值模拟，确定了加固方式和厚度。现场实测结果表明，此次数值模拟是符合实际的，所选取的方案是经济合理的。     

Numerical modelling of the effect of curing time on the creep behaviour of a chemically stabilised soft soil

This work studies the effect of the curing time on the creep behaviour of a stabilised soft soil, using volumetric and deviatoric creep laws associated with constitutive models. Results of unconfined compressive strength tests for several curing times are used to define the time evolution of the mechanical and creep properties. The models/creep laws are validated by oedometer and triaxial creep tests, for 28 and 90 days of curing. The long-term behaviour of an embankment built on a soft soil reinforced with deep mixing columns shows that the effect of curing time decreases the settlement and increases the improvement factor.     

Functioning and precipitation-displacement modelling of rainfall-induced deep-seated landslides subject to creep deformation

We propose an approach to study the hydro-mechanical behaviour and evolution of rainfall-induced deep-seated landslides subjected to creep deformation by combining signal processing and modelling. The method is applied to the Séchilienne landslide in the French Alps, where precipitation and displacement have been monitored for 20 years. Wavelet analysis is first applied on precipitation and recharge as inputs and then on displacement time-series decomposed into trend and detrended signals as outputs. Results show that the detrended displacement is better linked to the recharge signal than to the total precipitation signal. The infra-annual detrended displacement is generated by high precipitation events, whereas annual and multi-annual variations are rather linked to recharge variations and thus to groundwater processes. This leads to conceptualise the system into a two-layer aquifer constituted of a perched aquifer (reactive aquifer responsible of high-frequency displacements) and a deep aquifer (inertial aquifer responsible of low-frequency displacements). In a second step, a new lumped model (GLIDE) coupling groundwater and a creep deformation model is applied to simulate displacement on three extensometer stations. The application of the GLIDE model gives good performance, validating most of the preliminary functioning hypotheses. Our results show that groundwater fluctuations can explain the displacement periodic variations as well as the long-term creep exponential trend. In the case of deep-seated landslides, this displacement trend is interpreted as the consequence of the weakening of the rock mechanical properties due to repeated actions of the groundwater pressure.     

A displacement discontinuity approach to modelling the creep behaviour of rock and its discontinuities

Using the correspondence principle of viscoelasticity the Displacement Discontinuity Method has been extended to model the rate-dependent behaviour of a jointed rock mass. The intact rock is assumed to behave as a linear viscoelastic material while, in general, a non-linear rate-dependent behaviour of the rock joints can be accounted for. A number of numerical examples are presented which illustrate the accuracy and potential of the method for analysing complex engineering problems.     

Numerical analysis of foundation columns to support widening of embankments

Increased traffic volume has made it necessary to increase highway capacities by widening embankments and pavements. Adding a new embankment to an existing embankment induces additional stresses and deformations beneath the widened and existing portions of the embankment. Differential settlement may develop between and within the new and existing portions of the embankment, especially over soft soils. This differential settlement often causes pavement distress, such as longitudinal cracks or the drop-off (or sinking) of pavement sections. Different techniques have been adopted to remedy these problems, including the use of foundation columns, such as deep mixed columns, vibro-concrete columns, stone columns, and aggregate piers. However, design procedures for foundation columns constructed for this purpose are not well developed. The analyses of eight cases of column-supported widened embankments and two untreated foundations are presented in this paper. The factors considered include the consolidation of foundation soils under existing embankments and the spacing, region, and modulus of foundation columns. Two-dimensional finite difference software was used after the calibration of the model against a field case study and numerical analyses were conducted to investigate stresses and deformations of the widened embankments over soft soil with or without the remediation of foundation columns. The results presented in this paper include the vertical and the horizontal displacements, the maximum settlements, the transverse gradient change, and the distribution of the additional stresses induced by the widening. Recommendations are made for the design of foundation columns to remedy roadway pavement failure due to widening of embankments.     

Numerical analysis of time dependent settlements and pore water pressures by example of the plate loading test

Possibilities of modelling the deformation behaviour of water-saturated cohesive soils are discussed. it is concluded that a non-linear constitutive lawa should be used in connection with Biot's consolation theory. A hypoelastic law is considered in detail following a brief introduction to the theory and the numerical solution scheme. Starting from an “incremental Hookien” model extension is made by a dilatancy term in order to obtain pore pressure distribution in line with experimental results. The well known Skempton approach is included as a special case. Results of an earlier research project run in the Stuttgart University Insitute, namely stress and strain controlled plate loading tests, are chosen to compare calculation and measurements. Satisfactory results could be achieved to a large extent after the variation of two significant parameters - the cohesiion and the permeability - within their range of scatter. The importance of a time dependent coupled analysis is shown by comparison to an uncoupled analysis for the limiting cases “undrainded” and “ideally drained”.     

Numerical modelling of fluid-induced soil erosion in granular filters using a coupled bonded particle lattice Boltzmann method

This paper presents a three-dimensional coupled bonded particle and lattice Boltzmann method (BPLBM) with an immersed moving boundary scheme for the fluid-solid interaction. It is then applied to investigate the erosion process of soil particles in granular filters placed within earth dams. The microscopic migration of soil particles can be clearly visualised as the movement of particles can be directly recorded. Three granular filters with different representative size ratios are simulated and the numerical results are seen to match the empirical criteria. In addition, the effect of the representative size ratio of granular filters, hydraulic loading and erosion time are discussed.     

未打穿散体材料桩复合地基固结简化解研究

利用平均固结度普遍解,将未打穿散体材料桩复合地基转化为等效双层地基,推导出未打穿散体材料桩复合地基固结简化解,给出其平均固结度计算简化公式。根据简化解、现有解和有限单元法编制程序,绘制考虑贯入比、桩体渗透系数、土体扰动区渗透系数和土体扰动区影响因子等因素对固结的影响曲线图,分析了各因素对未打穿散体材料桩复合地基固结性状的影响,并对3种计算方法所求出的平均固结度进行了比较研究,进而探讨了简化解的适用性。研究结果表明,无论是在单面排水还是在双面排水条件下,简化解求出的平均固结度总体上是介于现有解和有限单元法二者之间,尤其在固结过程的“中后期”（即固结度≥50%时）。从实用角度来看,简化解的计算结果具有足够精度,且简便、实用;在各种影响因素中,散体材料桩长度和桩体渗透系数对固结过程的影响最大。     

基于离散元数值方法的砂土小应变弹性特性探讨

土体在非常小应变（小于10?6）时表现为线弹性。以往大量试验研究表明,土体的弹性特性主要受到土体有效围压、孔隙比、颗粒级配和试样制备方法等影响。通过离散元方法模拟Duffy和Mindlin中的物理试验,建立球形颗粒规则排列的三维模型,继而进行小变形条件下的数值加载试验来确定弹性特性,并从细观层面揭示决定土体弹性特性的内在因素。模拟结果表明：粒状土的弹性模量取决于土体的颗粒接触配位数、法向接触力的大小和分布,跟宏观土体孔隙比、有效围压作用相对应。弹性模量应力指数n高于Hertz-Mindlin接触法则对应的1/3,主要是由于应力增加过程中配位数的增加和接触力趋于更均匀导致。同时,泊松比也随着配位数的增加而减小,并非保持不变,跟试验结果趋势一致。     

考虑散体桩变形时复合地基径竖向固结解析解

现有散体桩复合地基固结问题的解析解大都在传统砂井地基等竖向应变假定条件下建立的,而且,在建立求解初始条件时,不能满足复合地基"应力集中"这一受力特点;另外,现有解答的固结微分方程为近似径竖向固结微分方程。为此,针对散体桩复合地基固结问题,抛弃桩周流量连续假定,采用桩土等竖向应变假设,根据复合地基"应力集中"引入新的初始条件,考虑涂抹以及平均附加应力沿深度任意分布,推导了瞬时加载条件下复合地基径竖向同时固结下的解答,给出了超静孔隙水压力和地基整体平均固结度的显示表达式,分析了平均附加应力分布形式,桩径比、置换率和涂抹参数对固结的影响。计算结果表明,平均附加应力沿深度倒三角衰减分布时复合地基固结最快,梯形衰减分布次之,矩形分布最慢;当桩径比较小时,不考虑桩体径向渗流会低估复合地基的固结速率;地基固结速率随置换率增大而加快,随涂抹参数kh/ksh比值增大而减慢,且两者对固结速率的影响均显著。     

Stress–dilatancy based modelling of granular materials and extensions to soils with crushable grains

Stress–dilatancy relations have played a crucial role in the understanding of the mechanical behaviour of soils and in the development of realistic constitutive models for their response. Recent investigations on the mechanical behaviour of materials with crushable grains have called into question the validity of classical relations such as those used in critical state soil mechanics. In this paper, a method to construct thermodynamically consistent (isotropic, three‐invariant) elasto‐plastic models based on a given stress–dilatancy relation is discussed. Extensions to cover the case of granular materials with crushable grains are also presented, based on the interpretation of some classical model parameters (e.g. the stress ratio at critical state) as internal variables that evolve according to suitable hardening laws. Copyright © 2004 John Wiley & Sons, Ltd.     

Numerical analysis of the installation effects on the behaviour of soft clay improved by stone columns

This paper addresses the installation effects of stone columns in soft soils. Focus is made on the lateral expansion of stone material using the vibro displacement and substitution techniques by means of numerical simulations. The behaviour of reinforced soil after stone column installation is investigated to show how the properties of soft soils can be improved prior to final loading. The effect of such an improvement on the prediction of reinforced soil settlement is evaluated. The axisymmetric unit cell model (UCM) served for the comparison between numerical predictions made by the Mohr-Coulomb and hardening soil constitutive laws adopted for the soft soil. An equivalent group of end bearing columns model was investigated in the axisymmetric condition to predict the settlement of reinforced soil by adopting the Mohr-Coulomb constitutive model for soft clay. The reduction of settlements predicted by the unit cell and group of columns models, due the improvement of the Young’s modulus of soft clay, were compared. It is concluded that a significant reduction of settlement is expected when the group of columns model is considered.