Response of multilayer geosynthetic-reinforced bed resting on soft soil with stone columns

In the present study, a mechanical model has been developed to study the behavior of multilayer geosynthetic-reinforced granular fill over stone column-reinforced soft soil. The granular fill and geosynthetic reinforcement layers have been idealized by Pasternak shear layer and rough elastic membranes, respectively. The Kelvin–Voight model has been used to represent the time-dependent behavior of saturated soft soil. The stone columns are idealized by stiffer springs and assumed to be linearly elastic. The nonlinear behavior of the soft soil and granular fill is considered. The effect of consolidation of soft soil due to inclusion of the stone columns on settlement response has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution and results are presented in nondimensional form. It has been observed that if the soft soil is improved with stone columns, the multilayer reinforcement system is less effective as compared to single layer reinforcement to reduce the total settlement as there is considerable reduction in the total settlement due to stone column itself. Multilayer reinforcement system is effective for reducing the total settlement when stone columns are not used. However, multilayer reinforcement system is effective to transfer the stress from soil to stone column. The differential settlement is also slightly reduced due to application of multiple geosynthetic layers as compared to the single layer reinforcement system.     

Performance of geosynthetic-reinforced granular piles in soft clays: Model tests and numerical analysis

The laboratory model tests and numerical analyses have been performed on reinforced granular piles installed in very soft clay. The granular piles were reinforced with geosynthetic in the form of vertical encasement, horizontal strips and combined vertical-horizontal reinforcement. The short term-displacement control model tests were carried out either only a granular pile loaded or with an entire area loaded. The laboratory results in the form of vertical load intensity-settlement behaviour were compared with that obtained from FEM software, PLAXIS 3D. The results indicated significant improvement in ultimate load intensity and stiffness of treated ground due to inclusion of geosynthetic.     

Geosynthetic-reinforced and jet grout column-supported embankments on soft soils: Numerical analysis and parametric study

Using a computer code based on the finite element method, a study is conducted to analyse the time-dependent behaviour of a geosynthetic-reinforced and jet grout column-supported embankment on soft soils, as well as the influence of three factors: the embankment height, the elastic modulus of column and the column spacing. The cylindrical unit cell formulation is used. The numerical model incorporates the Biot consolidation theory with soil constitutive relations simulated by the p–q–θ critical state model. Special emphasis is given to the analysis of several parameters: settlement, excess pore pressure, effective stress, stress level, tension in the geosynthetic, soil arching effect and overall efficiency coefficient.     

A procedure for the design and analysis of geosynthetic reinforced soil slopes

Summary A procedure for the stability analysis and design of geosynthetic reinforced soil slopes over a firm foundation is described. Firstly the unreinforced slope is analysed, and for this a circular failure method is used which allows a surcharge load to be taken into account. Any method of slip circle analysis could be used to identify the coordinates of the centre of the slip circle, its radius and the minimum factor of safety. In this study, both internal and external stability analysis of the reinforced slope is presented. Internal stability deals with the resistance to pullout failure within the reinforced soil zone resulting from the soil/reinforcement interaction. The external stability is considered by an extension of the bilinear wedge method which allows a slip plane to propagate horizontally along a reinforcing sheet. The results for total tensile force, internal and external stability are presented in the form of charts.For given properties of soil and slope geometry, the required strength of the geosynthetic and the length of reinforcement at the top and bottom of the slope can be determined using these charts. The results are compared with the published design charts by Schmertmannet al. (1987).     

重力式加筋土挡墙的工作性能和土压力计算

“重力式加筋土挡墙”在实际工程中已有不少应用,但由于缺乏对其工作性能的系统研究,目前我国相关规范尚无其设计方法,实际工程中一般根据经验设计,缺乏理论依据。采用FLAC2D程序,模拟一个施工顺序为“先筑墙,后填加筋土”的重力式加筋土挡墙,分墙后填土到墙顶和墙顶堆载完成两个阶段,分析加筋土工格栅对挡墙的水平位移和墙后填土的水平位移、水平应力的影响;土工格栅拉力及拉应变随墙后填土和墙顶堆载的变化。在此基础上,对采用“先筑墙,后填加筋土”工序的重力式加筋土挡墙墙后土压力的计算方法提出了初步建议     

Nonlinear analysis of laterally loaded rigid piles in cohesionless soil

In this paper, a method is developed for nonlinear analysis of laterally loaded rigid piles in cohesionless soil. The method assumes that both the ultimate soil resistance and the modulus of horizontal subgrade reaction increase linearly with depth. By considering the force and moment equilibrium, the system equations are derived for a rigid pile under a lateral eccentric load. An iteration scheme containing three main steps is then proposed to solve the system equations to obtain the response of the pile. To determine the ultimate soil resistance and the modulus of horizontal subgrade reaction required in the analysis, related expressions are selected by reviewing and assessing the existing methods. The degradation of the modulus of horizontal subgrade reaction with pile displacement at ground surface is also considered. The developed method is validated by comparing its results with those of centrifugal tests and three-dimensional finite element analysis. Applications of the developed method to laboratory model and field test piles also show good agreement between the predictions and the experimental results.     

连云港某软土基坑滑坡分析与整治方法研究

针对连云港某软土基坑围护不当导致的基坑滑坡事故,采用钻探、取样、室内试验、水平位移监测等手段,查明基坑侧壁软土的物理、力学性质,分析滑坡产生的原因,通过多种方法综合确定基坑侧壁软土滑动带的位置,提出滑坡整治措施,并对整治结果进行分析验证。     

软土路基沉降与软土层厚度及填土高度的关系分析

本文就路基沉降观测资料进行深入分析,总结出路基沉降与软土层厚度及填土高度的关系和规律,并分析了原因,推导出计算路基沉降的公式,提出了判断填土速率快慢和填土高度变化大小的新方法。     

软土地层桥梁群桩基础桩土共同作用性状的非线性有限元分析

通过对某高速铁路特大桥群桩基础进行三维非线性有限元分析,并结合现场试验得出的规律进行相应的对比分析,研究了软土地层桥梁群桩基础桩身轴力、桩侧摩阻力、基底土体附加应力、孔隙水压力分布、超孔隙水压力消散和群桩基础荷载沉降规律。计算结果表明,基桩所承受的轴力,角桩＞边桩＞中心桩,角桩和边桩的轴力沿桩身减小的幅度较大,而中心桩的轴力沿桩身减小的幅度稍小;各基桩桩侧摩阻力的发挥情况,侧摩阻力值总体上呈角桩＞边桩＞中心桩,相对滑移量基本呈上大下小的形态,即桩身上部桩-土之间产生的相对滑移量较中下部要大;外荷载作用下产生的土体附加应力和超孔隙水压力主要集中在承台底以下土体的一定范围内,其衰减梯度沿深度方向逐渐降低,随着固结时间的延长,群桩基础沉降达到稳定。     

Modeling of granular bed‐stone column‐improved soft soil

The present study pertains to the development of a mechanical model for predicting the behavior of granular bed‐stone column‐reinforced soft ground. The granular layer that has been placed over the stone column‐reinforced soft soil has been idealized by the Pasternak shear layer. The saturated soft soil has been idealized by the Kelvin–Voigt model to represent its time‐dependent behavior and the stone columns are idealized by stiffer Winkler springs. The nonlinear behavior of the granular fill has been incorporated in this study by assuming a hyperbolic variation of shear stress with shear strain as in one reported literature. Similarly, for soft soil it has also been assumed that load‐settlement variation is hyperbolic in nature. The effect of consolidation of the soft soil due to inclusion of the stone columns has also been included in the model. Plane‐strain conditions are considered for the loading and foundation soil system. The numerical solutions are obtained by a finite difference scheme and the results are presented in a non‐dimensional form. Parametric studies for a uniformly loaded strip footing have been carried out to show the effects of various parameters on the total as well as differential settlement and stress concentration ratio. It has been observed that the presence of granular bed on the top of the stone columns helps to transfer stress from soil to stone columns and reduces maximum as well as differential settlement. Copyright © 2007 John Wiley & Sons, Ltd.     

土工格栅加筋垫层加固软土地基模型试验分析

基于压缩、剪切、拉拔、抗拉试验结果分析了土工格栅加筋土的力学特性。通过室内模型试验研究了加土工格栅和不加土工格栅地基土的侧向位移和竖向位移随深度的变化规律、位移场、应力场、应变场。结果表明,土工格栅加筋垫层能有效地改变和阻止塑性区的形成和发展,对下卧软土地基起到扩散应力、均化应力的作用,控制软土地基塑性变形,增强地基延性和刚度,有效地控制地基竖向位移和侧向位移,增强地基的抗剪强度,增强整个地基的稳定性,提高了加筋土体的承载能力,为实际工程应用提供了可靠的理论依据。     

软土深基坑变形与控制技术初探

通过对软土深基坑变形机理与影响因素的分析,提出了基坑工程变形全过程控制的理念,探讨了控制变形的设计与施工技术方法及针对变形异常应采取的应急措施。结合工程实践,分析了软土深基坑变形的特征,应用地层损失法原理计算了墙后地表沉降量,与实测值比较,得出了该方法可以应用于工程实践的认识。     

现代多功能孔压静力触探及在软土地区测试要点

简要分析了我国CPT技术与国际CPT技术的差距,介绍了现代多功能CPTU测试系统、应用于CPT的传感器技术的发展、现代多功能CPTU技术的优势及SCPTU、RCPTU、VisCPT技术的新应用。同时,分析了软土地区多功能CPTU测试技术要点,包括测试计划的制定、孔压系统的饱和与标定、测试精度的保证、负值的分析及探头的保养等问题。     

Probabilistical distribution statistical analysis of Tianjin soft soil indices

In the design of soft ground treatment, the soft soil indices are the basic design parameter, and their changes have a strong geographical feature and significant uncertainty. Today, the soft soil indices statistical analysis content is mainly in the correlation and distribution regularity of physico-mechanical indices, which lack systematic study in different regions for different types of soft soils, and the statistical samples are from indoor tests and in-situ tests, without considering the impact of different test methods. In this paper, taking the soft soil in Tianjin as an independent statistical unit body, based on studying the change confines, variability, correlation, probabilistical distribution model of soil indices, some statistical relationship empirical formulas and probabilistical distribution models for physical indices, compression index, strength index were obtained; moreover, the correlation of shear strength results by straight shear test, triaxial test, vane shear test are analyzed; and a method for using random factor to evaluate the uncertainty involved in different test methods of soft soil parameters is proposed. The study results provide a basis and reference for defining the values of geotechnical parameters for soft soil.     

天津软土土性指标概率分布统计分析

软土土性指标作为软基处理设计的基本参数,其变化具有很强的地域特征和显著的不确定性。目前,对软土土性指标统计分析集中在软土物理力学指标的相互关系和分布规律等方面,采用的样本为室内试验和原位测试数据,没有考虑不同试验方法对土性指标不确定性分析的影响。将天津软土作为一个独立的统计单元体,研究了该地区软土物理力学指标的变化范围、变异性、指标间的相互关系和各指标的概率分布模型,得到了天津软土物理指标与压缩指标、强度指标间的统计关系经验公式以及土性指标的概率分布模型;并分析了直剪试验、三轴试验、十字板剪切试验3种不同方法抗剪强度试验结果的相互关系,提出了利用随机因子分析法对试验方法不确定性进行概率评定。研究成果为软土地区工程土工参数的取值提供了依据和参考。     

基于颗粒介质的桩承式路堤土拱效应试验分析

土拱效应的形成和荷载传递机制是进行桩承式路堤中桩土相互作用分析的关键性技术问题。为了探究宏观土拱效应形态的演化过程,借助于传统的光弹试验技术,研制出一种直径为3 mm的聚碳酸酯颗粒,通过自制的加载装置和光测力学图像处理系统,实现了桩承式路堤内部应力分布的可视化,直接观测到模型内部力链网格的产生、分布及变化,求得不同高跨比下桩土应力比变化,并与数值模拟的对比分析。结果表明,随着高跨比的减小,土拱形状由三角形变为半圆形,桩土应力比逐渐减小,当填土高度与桩间距的比值 时,可以形成完整的土拱;当 时,土拱效应开始消失。     

Numerical Analysis of Multi Layer Geosynthetic-Reinforced Granular Bed over Soft Fill

In this paper, considering the plain strain conditions, a numerical study has been conducted to investigate the behavior of multi layer geosynthetic-reinforced granular bed overlying a soft soil using the Fast Lagrangian Analysis of Continua (FLAC) program. The granular fill, soft soil, and geosynthetic reinforcements are considered as linear elastic materials. The geosynthetic reinforcements are modeled as cable elements fully bonded with the surrounding soil, thus neglecting any slip. The results obtained from the present investigation showed very close agreement when compared with the results of finite element analysis and lumped parameter modeling. The distribution of vertical, lateral and shear stresses in the soil are greatly affected as the number of reinforcement layers is increased. If the tensile stiffness of geosynthetic layers increases and its value is no more than 4,000–5,000 kN/m, the settlement of the reinforced foundation decreases significantly. The reduction in settlement is insignificant when the tensile strength of the geosynthetics exceed the above value.     

仙岭水库土坝渗流稳定分析及除险加固措施

根据湖南省仙岭水库的地质勘探资料,利用有限元进行计算分析,弄清了大坝的渗流情况,分析了大坝存在的渗漏和渗透变形问题,并提出了除险加固的措施,经过计算分析说明：所提出的措施是合理可靠的,所得出的结论可为同类土坝的安全鉴定、加固以及今后土坝设计、施工和管理提供借鉴。     

Numerical prediction of the creep behaviour of an unstabilised and a chemically stabilised soft soil

This paper examines the ability of volumetric and deviatoric creep laws associated with constitutive models to simulate the creep behaviour of a soft soil in its natural state or chemically stabilised state. Initially, the models/laws are validated by oedometer and triaxial creep tests, for the stabilised and unstabilised soils. Finally, the long-term behaviour of an embankment built on a soft soil reinforced with deep mixing columns is predicted based on the properties for a curing time of 28 days. The results show that the creep phenomenon should be considered in a long-term analysis of deep mixing columns.