A comparative study for the performance of encased granular columns

This paper describes a three-dimensional (3D) numerical analysis of a test embankment on geotextile-encased columns (GECs), in comparison with two-dimensional (2D) axisymmetric and plane strain analyses. The 3D numerical analysis was performed considering a rectangular strip under the embankment centerline. The 2D analysis was also carried out using axisymmetric unit cell and plane strain approaches. Numerical results indicated that the adopted 3D strip model represented well the measured deformations and pore pressure evolution during embankment construction and post-contraction periods. Unlike the unit cell model, both plane strain and 3D analyses could properly determine the settlement profile along the embankment base as well as the profile of the horizontal soil deformation beneath the embankment toes. The plane strain analysis, however, was not able to compute the geotextile ring force which might be simply calculated using the axisymmetric unit cell approach. The paper also showed that, due to horizontal boundary fixities applied to the embankment borders, the unit cell model clearly underestimated the tension developed in the basal geogrid.     

长板–短桩工法处理高速公路软土地基的数值分析

长板–短桩工法综合了排水固结法和水泥土搅拌桩法在加固软土地基工程中的优点,适用于处理高速公路深厚软土地基。以江苏省淮安–盐城高速公路盐城试验段为原型和依据,利用Plaxis软件,针对长板–短桩工法、排水固结法和水泥土搅拌桩法三种工况下的路堤和地基性状进行了二维有限元计算分析。从数值分析的角度研究了长板–短桩复合地基的加固机理和特性,进一步认识了总沉降、分层沉降、侧向位移、潜在滑动面及超孔隙水压力等指标的特征,得到了一些有益的结论。     

基于FLAC3D的加筋水泥土锚桩深基坑围护结构稳定性分析

介绍了加筋水泥土锚桩新型深基坑围护结构的形成过程和工作原理,并采用大型有限差分软件FLAC3D对加筋水泥土锚桩深基坑支护全过程进行了动态施工仿真模拟。研究了在基坑分步开挖过程中,围护结构的侧向位移、基坑周围的地表沉降、坑底隆起,并采用有限差分强度折减法计算出经桩锚加固后基坑的安全系数,得到了一些有益于基坑工程设计和施工的结论。     

Displacements of column-supported embankments over soft clay after widening considering soil consolidation and column layout: Numerical analysis

The common challenges for constructing embankments on soft clay include low bearing capacity, large total and differential settlements, and slope instability. Different techniques have been adopted to improve soft clay, such as the use of foundation columns including stone columns, deep mixed columns, and vibro-concrete columns, etc. Due to increased traffic volume, column-supported embankments may be widened to accommodate the traffic capacity need. Adding a new embankment to an existing embankment generates additional stresses and deformations under not only the widened portion but also the existing embankment. Differential settlements between and within the existing embankment and the widened portion may cause pavement distresses. Limited research has been conducted so far to investigate widening of column-supported embankments. In this study, a two-dimensional finite difference numerical method was adopted. This numerical method was first verified against field data and then used for the analysis of widened column-supported embankments over soft clay. The modified Cam-Clay model was used to model the soil under the existing embankment and the widened portion. Mechanically and hydraulically coupled numerical models were created to consider the consolidation of the foundation soil under the existing embankment and the widened portion. Different layouts of foundation columns under the existing embankment and the widened portion were investigated. The numerical results presented in this paper include the vertical and horizontal displacements, the maximum settlements, the transverse gradient changes, and the stress concentration ratios, which depended on column spacing. The columns installed under the connection side slope were most effective in reducing the total and differential settlements, horizontal displacement, and transverse gradient change of the widened embankment.     

Three-dimensional numerical analysis of individual geotextile-encased sand columns with surrounding loose sand

Use of geotextile-encased sand columns (GESC) to improve weak soils is an emerging technology that has great promise for field applications. This paper contains the results of a numerical study with the goal of quantifying the benefits of geotextile encasement under different conditions. A three-dimensional finite difference method implemented in FLAC3D 5.01 was used to evaluate the performance of a vertically loaded individual GESC installed in loose sand. The numerical model was first verified using the results of experimental tests performed on 150-mm diameter GESC installed in loose sand. The influence of various parameters was investigated in this study, including GESC diameter and length, soil thickness, geotextile encasement length, geotextile stiffness, and friction angle and dilation angle of the infill material. The results of the numerical model showed that vertically loaded GESC of smaller diameter experienced less settlement and lateral expansion than those of larger diameter. The geotextile material with higher stiffness had a substantial influence on the performance of GESC. The maximum effective geotextile encasement length depended on the load on the column head or the compressibility of the column.     

某地铁车站软土深基坑加固效果研究

采用水泥土搅拌加固软土深基坑土体是工程中常用的有效方法。本文采用有限差分软件FLAC3D建立某地铁车站深基坑的数值计算模型,并对未加固和加固两种设计方案进行了施工全过程数值模拟,对两种方案的土压力、地下连续墙水平位移以及邻近铁路路堤沉降的模拟计算结果进行了对比分析。结果表明:三轴水泥土搅拌桩对于软土深基坑加固效果显著,能够有效的减少围护结构的侧向位移和地表沉降;当周边环境对基坑变形有严格要求时,对土体进行加固,提高土体强度是十分有效的措施。     

带桩帽桩网加固路堤的模型试验及分析比较

采用三维模型试验,考虑路堤高度、桩帽宽度、桩净间距、工后固结沉降、桩间土压缩模量、水平加筋体等影响因素,对桩网结构加固路堤的沉降变形和荷载传递规律进行系统的研究,得到一些有益的结论,并且将试验的部分结果与理论解析解进行比较,结果发现:对于三维空间土拱,BS 8006法、改进的太沙基法、Hewlett和Randolph法所计算的结果与试验结果较为吻合,并且随着桩帽尺寸的增加,它们之间的吻合程度越来越好。而改进的Guido法和Carlsson法计算的结果与试验结果差异较大。该研究成果可为桩网结构加固路堤设计提供有益的参考。     

刚性桩加固软弱地基上路堤的稳定性问题(Ⅰ)——存在问题及单桩条件下的分析

目前各类刚性桩在软土路基加固中已有较多应用。首先对复合抗剪强度极限平衡法存在的问题进行了分析。在此基础上,基于刚性桩的刚度及强度特征,采用三维有限差分方法和二维强度折减有限差分数值方法,针对刚性桩复合地基支承路堤的整体稳定分析,对单桩位于路堤下不同位置时的路堤稳定性进行了研究,并与传统复合抗剪强度极限平衡法稳定分析结果进行对比。结果表明,当路堤下单桩位于不同位置时,路堤填筑过程中桩土相互作用机理、桩的破坏形式及桩对路堤稳定的贡献机理不同,桩的刚度对路堤稳定存在显著影响。路堤趋于稳定破坏时,位于路堤下不同位置的刚性桩的弯曲破坏比剪切破坏更易于发生。对刚性桩加固路堤,复合抗剪强度极限平衡法不能反映不同位置单桩的破坏机理,将显著高估路堤稳定性。     

大型疏浚土充填袋筑堤技术研究

为缓解砂石资源紧缺、保护生态环境、降低工程造价和高效利用航道疏浚土,以大型疏浚土充填袋筑堤技术应用于连云港港徐圩港区围堤建设为工程实例,通过稳定性分析计算、离心模型试验开展了大型疏浚土充填袋筑堤技术的理论可行性研究,并结合原型试验提出了新型配套施工工艺,形成了相关成套技术。研究表明:袋体土工织物加筋作用良好,堤体及地基稳定性能够满足安全设计;控制疏浚土充填料初始含水率、采用泥浆泵与浓浆泵串联接力的施工工艺,能够提升大型疏浚土充填袋筑堤施工效率,满足正常围堤建设强度需要;低含水率疏浚土充填料排水较快、袋体实际有效厚度较为理想、堤体地基较为稳定,大型疏浚土充填袋筑堤具有其技术可行性。     

Centrifuge tests on geosythetic-encased stone column supported embankment on seasonal frozen soil

Geosynthetic-encased stone columns (GESCs) have been widely applied into soft foundation. This paper aimed to evaluate the availability and efficiency of GESCs in seasonal frozen ground. Four centrifuge tests were conducted on GESCs supported embankment on seasonal frozen soil under embankment load and thawing, where the foundation was frozen before the construction of embankment. The effects of encasement stiffness and lateral reinforced cushion were investigated. Three phases could be distinguished in the tests by two timing nodes due to the complete thawing of columns and soil, and analysis were made based on the three phases. It is found that high-stiffness encasement can effectively reduce the differential settlement between soil and columns before complete thawing of soil. The GESCs appeared a deformation mode of inward bending, which is in inverse to that in common composite foundation. The reinforced cushion rearranged stress on columns and soil, and influenced the development of pore pressure. The stress concentration ratio (SCR) first decreased to less than 1 due to column thawing, and experienced a steady stage until soil was completely thawed. The SCR rapidly increased after complete thawing of soil, and decreased to a constant value due to soil consolidation.     

Model Test and Consolidation Analysis of Failure of a Loose Sandy Embankment Dam During Seepage

Failures of earth dam embankments and river dikes, which are constructed of sandy soils with low dry densities, have been observed to occur during rising of water levels. In this paper, a large-scale physical model test was conducted in order to investigate the behavior of a small dam embankment as water levels rose. The test results were simulated by use of a consolidation analysis method coupled with an elastoplastic model for unsaturated soils. All parameters used in the simulation were obtained from element tests; oedometer, triaxial compression, soil water retention, and permeability tests. First, in order to verify the parameters identified, direct shear tests were simulated using the consolidation analysis method; results of the simulation and the tests were consistent. From the embankment model test it was found that the crest of the embankment moved upstream at the first stage of impounding and then moved back. After a seepage surface appeared on the downstream slope, tension cracks occurred on the downstream slope, and sliding occurred through the crack and the downstream toe. The results of the simulation were consistent with those of the model test. This consolidation analysis method could be used to simulate the complex deformations induced by saturation collapse and shear strains and even failure behavior. Old embankments constructed with loose densities might have histories in which cracks occurred on the downstream slopes when the reservoir water level rose, and their stabilities might have decreased.     

Centrifuge model tests of geotextile-reinforced soil embankments during an earthquake

The behavior of geotextile-reinforced embankments during an earthquake was investigated using centrifuge model tests, considering a variety of factors such as gradient of slope, water content of soil, geotextile spacing, and input shaking wave. The geotextile-reinforcement mechanism was revealed on the basis of the observations with comparison of the unreinforced embankment. The geotextile significantly decreases the deformation of the embankment and restricts sliding failure that occurs in the unreinforced embankment during an earthquake. The displacement exhibits an evidently irreversible accumulation with a fluctuation during the earthquake which is significantly dependent on the magnitude of input shaking. The peak strain of the geotextile exhibits a nearly triangular distribution in the vertical direction. The embankment can be divided into two zones, a restricting zone and restricted zone, where the soil and geotextile, respectively, play an active restriction role in the soil-geotextile interaction. The soil restricts the geotextile in the restricting zone, and this restriction is transferred to the restricted zone through the geotextile. The strain magnitude of the geotextile and the horizontal displacement of the geotextile-reinforced embankment decrease with increasing geotextile layers, with decreasing water content of the soil, with decreasing gradient of the slope, and with decreasing amplitude of the earthquake wave.     

Numerical modelling and validation of geosynthetic encased columns in soft soils with installation effect

If the bearing capacity of the soil is not sufficient an improvement method has to be considered. In case of soft and cohesive soils the vibroreplacement technique can be used. This paper describes the numerical simulation of a group of encased granular columns under an embankment based on a real life project situated to the north of Hamburg, Germany. The soft soil creep model and the hardening soil model were used to model the behaviour of the soft clay and granular material respectively. The material parameters were determined based on laboratory tests conducted on test samples from the field. The installation effect of columns in numerically modelled based on the cavity expansion method in a 2D axis symmetric model. The results of the installation effect in terms of stress state changes in the soft soil after complete consolidation are then imported to the 3D model involving group of columns. The results of the numerical simulations are validated against field measurement data in form of vertical settlement of the ground at various locations with respect to time and horizontal deformations in the encased columns with depth.     

铁路施工、运营对下穿地铁区间的计算分析

大连地铁2号线姚家站～南关岭站区间结构下穿铁路路基,本文利用FLAC3D有限元计算软件,采用三维地层一结构模型对高铁的施工、运营过程进行了模拟计算,并根据分析结果对地铁区间结构及铁路路基建设提出设计和施工方面的技术要求和建议.     

Geotextile strain in a full scale reinforced test embankment

A geotextile reinforced test embankment was constructed on a soft organic clayey silt deposit at Sackville, New Brunswick, Canada in September/ October 1989. A relatively high-strength polyester woven geotextile (ultimate strength of 216 kNm⁻¹) was used as reinforcement. The reinforcement was instrumented with a number of electrical resistance, electromechanical and mechanical gauges. The details of this instrumentation and field performance of the geotextile reinforcement during the construction of this test embankment are described in this paper.

The field data indicated that the strain in the geotextile was comparatively small (typically less than about 0·7%) up to an embankment thickness of 3·4 m. The strain increased to a maximum of about 2% when the embankment thickness was increased above 4·1 m, suggesting the initiation of movement (or yielding) of the foundation soil. A large increase of strain was evident for thicknesses exceeding 5·7 m and the available evidence indicates that the soil approached failure at a fill thickness of about 5·7 m. The strain increased to over 8·5% when the embankment was first constructed to 8·2 m thickness and then failed as the soil continued to deform at constant fill thickness and the geotextile strain increased until failure (inferred tearing) of the geotextile occurred. After the embankment failed at a thickness of 8.2 m, the addition of more fill did not result in any     

A case study on utilizing geotextile tubes for tailings dams construction in China

This research presents a successful case, in which geotextile tubes have been used to construct a tailings dam for fine tailings disposal since 2008. Up to the end of 2016, the tailings dam raise has reached 9?m in height. The total height of the dam including the starter dike was 61?m. In order to ensure that the tailings reservoir is safe in its current condition and during future construction, the comprehensive geotechnical investigation and stability analyses of the tailings embankment are conducted in this study. The results show that the application of geotextile tubes in the construction of tailings dam is a good alternative for fine tailings disposal.     

Measured and simulated results of a Kenaf Limited Life Geosynthetics (LLGs) reinforced test embankment on soft clay

For the first time, woven Kenaf Limited Life Geosynthetics (LLGs) were used for short term reinforcement of full scale embankment constructed on soft clay and their behavior is presented. The observed data in terms of settlements, excess pore water pressures and deformations or stresses in the reinforcements were compared with the simulated data. Two types of Kenaf LLGs were utilized, namely: coated and not coated with polyurethane. The coating can reduce water absorption and increase their life time. Subsequently, numerical simulations were performed on the behavior of Kenaf LLGs reinforced embankment using 2D and 3D finite element software. The rates of settlement from FEM 2D method overestimated the observed settlements data while the FEM 3D predictions agreed with observed settlements due to the three-dimensional geometrical loading of the embankment with length to width ratio (L/B) of 1.0. Regarding the maximum excess pore-water pressures at the locations of 3 m and 6 m depth, the FEM 2D analyses overestimated while the FEM 3D simulation yielded satisfactory agreement with the observed data. The reinforcement deformations and stresses in both coated and non-coated Kenaf LLGs reinforcement have higher values at the middle portions of the embankment and the predicted results from FEM 3D simulation yielded closer deformations of Kenaf LLGs reinforced than the FEM 2D simulation. Consequently, FEM 3D simulation captured the overall behavior of the Kenaf LLGs reinforced embankment with more reasonable agreement between the field observations and the predicted values compared to the FEM 2D simulation. The behavior of the sections on coated and non-coated LLGs were similar. The Kenaf LLGs can be applied for short term embankment reinforcement in order to improve the stability of embankment on soft clay.     

双向搅拌桩加固高速公路软土地基现场对比试验研究

 针对国内水泥土搅拌桩施工工艺存在的问题,提出了双向水泥土搅拌桩技术。为了分析双向搅拌桩加固高速公路软土地基的处理效果,在同一场地进行了双向搅拌桩和常规搅拌桩试验段工程,对比分析了双向搅拌桩和常规搅拌桩的桩身质量、施工扰动、复合地基承载力和路堤荷载下的工作性状特点。现场试验结果表明,双向搅拌技术能有效地提高搅拌均匀性、消除冒浆现象,并减小搅拌桩施工对桩周土体的扰动;双向搅拌桩桩身质量均匀,桩身强度沿深度变化较小,复合地基承载力高于常规搅拌桩复合地基;填土高度相近时,双向搅拌桩荷载分担比高于常规搅拌桩,地表沉降和坡角处最大水平位移小于常规搅拌桩复合地基,而地基固结速率快于常规搅拌桩复合地基。研究结果表明,双向搅拌桩在高速公路软土地基处理工程中具有很好的应用前景。     

路堤下钉形搅拌桩复合地基沉降计算方法——广义桩体法

针对钉形搅拌桩这种变截面搅拌桩,通过一定的假设,将其简化为上、下直径相等而模量不等的常规等截面桩,提出了钉形搅拌桩复合地基沉降计算方法--广义桩体法。根据位移、应力连续条件,将路堤填土、加固区和下卧层作为统一的整体,提出了考虑桩顶上刺、桩土相互作用和桩端下刺的加固区计算模型,并建立了模型的有限差分解法。基于三维数值模拟结果,提出考虑路堤下钉形搅拌桩复合地基下卧层应力集中的附加应力计算方法,从而建立路堤荷载下钉形搅拌桩复合地基的沉降计算方法。计算实例表明:复合模量法计算结果偏小,而应力修正法偏大,本文方法最接近实测结果,说明其具有一定的适用性。     

基于ANSYS平台复杂地质体FLAC3D模型的自动生成

由于 FLAC3D软件建模难度大,提出了一种快速建模方法,即以 ANSYS 有限元程序完成的复杂地质体建模、网格划分为基础,采用 Visual Basic 语言编写了 FLAC3D-ANSYS 接口程序,实现了 FLAC3D软件建模的直观、快速和自动化。通过新疆下坂地水库坝址区和北京地铁四号线车站三维建模实例检验了该方法的有效性和可行性。该方法是运用不同程序优点解决复杂工程地质问题的典型范例。