Numerical simulation and sensitivity analyses of full-scale test embankment with reinforced lightweight geomaterials on soft Bangkok clay

A full-scale test embankment was constructed on soft Bangkok clay using rubber tire chip–sand mixture as a lightweight geomaterial reinforced with geogrid under working stress conditions. The facing of the embankment was made of segmental concrete blocks with rock filled gabion boxes as the facing to the sloping sides. This paper attempts to simulate the behavior of the full-scale test embankment using PLAXIS finite element 2D program by means of undrained analysis in the construction stage and thereafter consolidation analysis was performed during the service stage. The settlement predictions of the soft clay foundation mostly depended on the assumed thickness of the weathered crust and the OCR values of the soft clay layer. The predicted excess pore water pressures were sensitive to the OCR values of the soft clay layer. The lateral wall movements were overpredicted by the analysis due to the partially drained consolidation process at the early stage of the construction. The FEM computed geogrid movements were smaller than the observed field data due to the use of lightweight tire chips-sand backfill. The maximum tension line agreed reasonably well with the coherent gravity bilinear failure plane. The sensitivity analyses of settlements, excess pore water pressures, lateral wall movements, geogrid movements and tensions in geogrid were performed by varying the weathered crust thickness, the OCR values of soft clay, the permeability values of the soft clay and the interface coefficient of the geogrid. The settlements and the excess pore water pressures changed significantly when the OCR and the permeability values of soft clay were varied. The interface coefficient of the geogrid reinforcements affected the lateral wall movements, geogrid movements and tensions in the geogrids. The higher interface coefficient yielded less wall/geogrid movement and resulted in higher tensions in geogrids as expected. The results of analyses show that the FEM analysis using 2D plane strain conditions provided satisfactory predictions for the field performance.     

刚性桩复合地基支承路堤稳定破坏机理的离心模型试验

对上软下硬成层土地基中刚性桩复合地基支承路堤,进行了单排桩和群桩条件下路堤稳定破坏机理的离心模型试验,研究了不同桩体抗弯刚度和强度,不同桩体加固位置,不同桩间距和桩端嵌入硬土层深度条件下桩体的受力与变形性状,破坏模式以及路堤的失稳破坏机理.离心机试验结果表明,不论是在单排桩还是群桩条件下,桩身最大弯矩作用点均产生于软硬土层交界面附近;在群桩条件下,桩体越靠近路堤中心桩身弯矩越小;当桩身抗弯刚度与强度较高,桩距较大且桩下端嵌入较硬土层足够深度时,可产生桩间土体沿桩的绕流甚至因产生绕流而导致路堤失稳破坏;当桩身抗弯刚度与强度较低时,在单排桩条件下,桩体会首先在软硬土层交界面处发生弯曲破坏,并可在路堤失稳前在桩身上部发生第二次弯曲破坏,而在群桩条件下,坡脚附近部分桩体首先在软硬土层交界面附近发生弯曲破坏,并可能伴随桩体受拉破坏而使桩断为上下两段,最后由于部分桩体发生整体倾覆破坏或者再次发生弯曲破坏而导致路堤发生失稳破坏;针对路堤具体的破坏情况有针对性地增大桩身抗弯强度,减小桩间距或增加桩端嵌入硬土层深度均可提高路堤的稳定性.     

A case study of geotextile-reinforced embankment on soft ground

Full-scale test embankments, with and without geotextile reinforcement, were constructed on soft Bangkok clay. The performances of these embankments are evaluated and compared with each other on the basis of field measurements and FEM analysis. The analyses of failure mechanisms and the investigations on the embankment stability using undrained conditions were also done to determine the critical embankment height and the corresponding geotextile strain. The high-strength geotextile can reduce the plastic deformation in the underlying foundation soil, increase the collapse height of the embankment on soft ground, and produce a two-step failure mechanism. In this case study, the critical strain in the geotextile corresponding to the primary failure of foundation soils may be taken as 2.5–3% irrespective of the geotextile reinforcement stiffness.     

Instability of a geogrid reinforced soil wall on thick soft Shanghai clay with prefabricated vertical drains: A case study

A 7.6 m high geogrid reinforced soil retaining wall (RSW) was constructed at the end of an embankment on very thick, soft Shanghai clay with 12 m deep prefabricated vertical drains (PVDs). The settlement of the ground, the wall movement and pore water pressure were monitored during the construction. From day 118, halfway through the construction, unexpected pore water pressure increment was recorded from the pore water pressure meters installed in the PVD drained zone indicating a possible malfunction of the PVDs due to large deformation in the ground. After the last loading stage, on day 190, a sudden horizontal movement at the toe was observed, followed by an arc shaped crack on the embankment surface at the end of the reinforced backfill zones. The wall was analyzed with a coupled mechanical and hydraulic finite element (FE) model. The analysis considered two scenarios: one with PVDs fully functional, and the second one with PVD failure after day 118 by manually deactivating the PVDs in the FE model. The comparison between the measured and simulated ground settlement, toe movement, and pore water pressure supported the assumption on the malfunction of the PVDs. It is believed that the general sliding failure in the wall was caused by the increase of pore water pressure in the foundation soil and soils in front of the toe. It is suggested that possible failure of PVDs should be considered in the design of such structures, and the discharge rate of the PVDs and the pore water pressure should be closely monitored during the construction of high soil walls on soft soils to update the stability of the structures, especially for grounds where large deformations are expected which may cause the failure of the PVDs.     

Performance of a road embankment on soft clay supported on a Geocell mattress foundation

A five kilometre-long road was constructed in the New Territories of Hong Kong between 1989 and 1992. Part of the road crossed an alluvial plain on an embankment, and at two locations on the alluvial plain this embankment was underlain by soft clay deposits. A geocell mattress was used as the means of supporting the embankment on these soft clay deposits. Wick drains were installed beneath the embankment to speed up consolidation of the soft clays, and using staged construction the embankment was built up to a maximum height of 10 m.

The embankment was fully instrumented with pneumatic piezometers, inclinometers, hydrostatic profile gauges, settlement plates, surface settlement markers, and lateral movement blocks. This paper presents the results of the instrumentation monitoring and the performance of the geocell mattress foundation during the construction of the embankment. At one section, unusually high excess pore water pressures and a slight heave of the toes of the embankment were recorded. The accompanying small lateral extension and the deflected shape of the geocell mattress indicated that it had behaved as a raft foundation to the embankment.     

高原斜坡软土路基施工试验研究

高原斜坡软土是一种罕见的地质现象 ,是在特定的地质条件和气候条件下形成的具有特殊性质的软土 ,在高原斜坡软土地基上修筑公路、铁路都将面临路基稳定这一课题。通过内昆铁路老锅厂-李子沟区间斜坡路基施工试验研究 ,提出了“强基固本”、“治软抗滑”确保斜坡路基稳定性的技术措施 ,所得出的研究结论对类似工程具有一定的参考价值。     

Prediction of embankment behavior of regulating reservoir with foundation improved by vacuum consolidation method

The present paper addresses the numerical prediction of the behavior of a ground and a reservoir dyke with a retaining wall at the site of a regulating reservoir whose soft soil foundation is improved by using both the usual embankment preloading and vacuum consolidation. To evaluate the settlement, the lateral deformation, and the dissipation of pore pressure during vacuum preloading, a numerical analysis using an elasto-plastic FEM for soil–water coupled problems, incorporating the SYS Cam-clay model, is carried out in two dimensions. However, a change in the soil parameters during the vacuum preloading leads to a less accurate computation. To account for the uncertainties in the input parameters of the constitutive model for the improved ground, an inverse analysis approach is adopted. The particle filter is used to identify the compression index of the clay layers and the coefficient of permeability of the organic soil layer based on the measured settlement at the bottom of the preloading embankment during the vacuum consolidation. The reservoir dyke with a retaining wall is constructed on an improved foundation after removing the preloading embankment, and an attempt is made to predict its performance after construction by an elasto-plastic FEM for soil–water coupled problems using the identified parameters.     

软基上加筋防波堤的离心模型试验

通过离心模型试验，研究在有无土工织物加筋垫层条件下防波堤和软基的变形性状，得到了地基沉降、隆起及水平位移的分布规律。提出了一种量测筋材张力的新方法，对土工织物在离心试验过程中的应力状态进行了测试。研究结果表明：土工织物加筋垫层能有效地减小地基的侧向位移，有一定的加筋效果：土工织物张力的发挥水平与堤坝的沉降量密切相关，工作状态下其最大强度发挥水平约为52％。     

Field study and numerical modelling for a road embankment built on soft soil improved with concrete injected columns and geosynthetics reinforced platform

Generally numerical modelling can provide an accurate and cost-effective approach to understand the behaviour of geosynthetic-reinforced column-supported embankment. When the problem geometry cannot be simplified to the two-dimensional plane-strain or axisymmetric, a full three-dimensional solution is required to obtain sensible results. This study presents a modelling of the geosynthetic-reinforced composite ground supporting a road embankment. Response of soft soil is captured by adopting Modified Cam-Clay model. In addition, Hoek-Brown constitutive model is considered to simulate non-linear stress-dependent yield criterion for Concrete Injected Columns (CIC) that describes shear failure and tensile failure by a continuous function. To assess whether the proposed numerical model can capture real behaviour of composite ground, field monitoring data of deep soft clay deposit improved by CIC from Gerringong Upgrade is used to validate the model. The settlement and lateral displacements of ground, stress transferred to column, and pore water pressure results for the embankment during and after the construction, measured using the field instrumentations including settlement plates, inclinometers, earth pressure cells on CIC, and pore pressure transducers, are compared with numerical predictions. In addition, the numerical results provide insights to investigate load transfer mechanism in the composite ground, capturing response of soil – column - embankment system.     

成层软土地基上土堤填筑稳定性的塑性极限分析

针对成层软土地基上土堤填筑的稳定性分析问题,考虑地基土体的不排水抗剪强度既随土层变化又随深度任意线性变化,根据工程实践中揭示的堤基深层破坏模式,基于塑性极限分析的上限理论,构建满足运动许可的对数螺旋线与圆弧组合的转动破坏机制,建立了便于快速评估成层软土地基上土堤填筑稳定性的临界高度、无量纲化的稳定数、稳定图、最小安全系数和临界滑动面的分析计算方法,并将计算结果与2个土堤失稳现场试验工程案例的实测结果进行了对比。结果表明,采用本文方法计算得到的土堤最小安全系数接近于土堤失稳时的最小安全系数的理论值1.0,计算确定的临界滑动面也与实测的临界滑动面比较接近,从而验证了本文方法的适用性。     

Bearing capacity of composite ground with soil-cement columns under earth fills: Physical and numerical modeling

Soil-cement columns are widely used to improve soft ground, and the bearing capacity of the formed composite ground is a key design parameter. The currently employed design method was developed for composite grounds under rigid footings, whilst the bearing capacity behavior of composite grounds under earth fills with different degrees of stiffness has rarely been investigated. Hence, the present study attempts to fill this gap. In this investigation, 1-g laboratory model tests are conducted to compare the bearing capacity behavior of composite grounds under a rigid footing and under embankment fill, based on which a numerical model that can capture the strain-softening behavior of soil-cement columns is established. The calibrated numerical model is further employed to perform 144 analyses. The results indicate that the failure mode of composite grounds differs for different types of earth fills: soil failure occurs prior to column failure under soft clay and dredged slurry, whereas column failure is the primary failure mode for composite grounds under embankment fill. This difference in failure mode of composite grounds can be explained using soil arching theories. For different failure modes, different bearing capacity efficiency factors should be used in design.     

软土地区基坑坑底隆起稳定破坏滑动半径研究

为了研究软土地区基坑坑底隆起的破坏模式并确定合理的隆起破坏滑动面大小,采用不连续布局优化法对不同围护结构插入比条件下的基坑坑底隆起失稳滑动面进行分析; 通过大量计算得到可用于判断滑动面是否通过围护结构底端的不同基坑开挖深度下土体强度和围护结构临界插入比的幂函数关系表达式; 通过分析围护结构插入比较小时归一化的坑底隆起破坏滑动半径和围护结构插入比的关系,得到修正传统圆弧滑动法滑动半径的对数表达式及其系数。最后,将修正滑动半径后计算的安全系数与数值模拟结果进行了对比,验证了滑动半径公式的可靠性。结果表明:在插入比较大(大于临界插入比)情况下,滑动面通过围护结构底端; 在插入比较小情况下,滑动面不通过围护结构底端,需对传统圆弧滑动法进行修正; 提出的修正方法可为软黏土层较厚的基坑坑底隆起稳定分析时滑动半径的选取提供参考,同时可以解决传统圆弧滑动法在插入比较小时安全系数变化不合理的问题。     

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.     

汶川地震中道路边坡工程震害分析

 发生在山区的地震对边坡工程所造成的危害,在致灾机制和破坏形式方面具有鲜明的特征。结合汶川地震灾区道路边坡工程震害实例,分析路堑、路堤以及与桥隧相连的各类边坡及相应支挡结构的震害机制和破坏形式。锚索(杆)地梁或预应力锚索抗滑桩加固的边坡具有较好的抗震性能,其原因是这些结构已与坡体联接在一起而形成一个整体,在地震波作用下结构与坡体的位移和变形能够很好地协调一致。铺设土工格栅或施加加筋材料的路堤边坡工程具有较好的抗震性能,一般填筑路堤特别是高路堤,其抗震性能较差。根据沙土液化和软弱黏性土层震陷造成的震害实例,提出含水沙质地层路堤边坡应注意坡脚沙土液化造成的震害,应采取措施防止软弱黏性土层地基震陷造成路面破坏以及坡脚震陷造成的边坡失稳。山区隧道洞口边、仰坡的抗震设计应重视支挡结构的耐震性。建于坡体上的桥台、桥基和桥路过渡段的安全性与坡体稳定性直接相关,应切实加强这些结构所在边坡的抗震设计。对于依山傍水而建的顺河桥,相关边坡的失稳危害桥梁时,应对其采取抗震措施。目前公路、铁路工程抗震规范涉及边坡工程及支挡结构的内容极少,研究成果可为规范的修改和补充提供有益的参考。     

台华高速公路路堤失稳原因分析与对策

台华高速公路采用干振碎石桩进行软土地基处理,在路堤填筑过程中发生了地基失稳滑移.通过数值方法对路堤填筑过程中地基沉降、超静孔隙水压力及地基塑性区的发展变化进行了模拟计算,并结合现场补勘数据,分析了引起路堤失稳滑移的原因.在此基础上,提出刚性桩地基处理加固方案.数值计算及现场勘查结果表明,在强度较低的软土中采用干振碎石桩进行地基处理是不成功的,施工不仅没有形成完整的碎石桩体,还因扰动降低了原状地基土强度,路堤填筑速度过快则直接导致了地基滑移破坏;当采用刚性桩加固软基时,桩间土仅承担小部分路堤荷载,而大部分路堤荷载由桩承担并传递给深层承载力较高的土层,因此路堤的稳定性高、沉降小,可实现快速填筑.数值分析及现场刚性桩试验段测试结果均表明：刚性桩方案可以满足路堤填筑要求.     

Centrifuge modeling of geosynthetic-encased stone column-supported embankment over soft clay

Geosynthetic-encased stone column (GESC) has been proven as an effective alternative to reinforcing soft soils. In this paper, a series of centrifuge model tests were conducted to investigate the performance of GESC-supported embankment over soft clay by varying the stiffness of encasement material. The enhancement in the performance of stone columns encased with geosynthetic materials was quantified by comparing the test with ordinary stone columns (OSCs) under identical test conditions. The test results reveal that by encasing stone columns with geosynthetic material, a significant reduction in the ground settlement, relatively faster dissipation of excess pore pressure and enhanced stress concentration ratio was noticed. Moreover, with the increase in the encasement stiffness from 450 kN/m to 3300 kN/m, the stress concentration ratio increased from 4 to 6.5, which signifies the importance of encasement stiffness. In addition, a relatively lower value of soil arching ratio observed for GESCs compared to OSCs indicate the formation of a relatively strong soil arch in the GESC-supported embankment. Interestingly, under embankment loading, GESCs fail by bending while OSCs fail by bulging. The stress reduction method can be used to calculate the settlement of GESC-supported embankment with larger stress reduction factor than that in the OSC-supported embankment. Finally, the limitation of the construction of the embankment at 1 g was addressed.     

基于软土地基最终沉降量的推算方法分析

在曾国熙固指数曲线结度理论以及多种软土地基沉降推算方法的基础上,以某高速铁路软土试验路堤的实测沉降曲线为依据,用多种方法分别推算软土地基的最终沉降量并进行了分析比较,在不同情况下得出了适应各种情况的方法。     

路堤加筋土挡墙变形分析与研究

近年来随着高速公路建设的发展,加筋挡墙在碎石高填方路堤中的应用逐渐增多,同时也表现出不少路基病害,根据挡墙的变形特征对其破坏机理进行了研究。     

软土桩基上拓宽路堤的变形破坏离心模型试验

采用离心模型试验手段,以广州高速公路改扩建工程为背景建立模型,设计相应尺寸的模型试验方案,模拟软土桩基上的新路堤拓宽过程,研究软土桩基的变形破坏规律与机理。结果表明:旧路堤主要发生竖向位移,新路堤发生沉降和一定量水平位移; 新路堤坡肩区域出现向坡脚滑动趋势,导致新路堤施工过程中路堤和软基产生滑动破坏; 新路堤中的滑裂面首先出现,从新路堤坡肩到短桩顶端自上而下逐渐形成; 在新路堤滑裂面形成过程中,软基中的滑裂面从旧路堤下的地基表面至新路堤坡脚下地基部分自内向外逐渐形成,2条滑裂面几乎同时达到贯通状态; 管桩发生一定程度的倾斜,在滑裂面出现之前的某一时刻倾斜角度迅速增加; 在局部滑裂面附近位移出现较快增长,变形较为集中; 变形局部化导致了局部发生破坏,而局部破坏促进了变形局部化的进一步发展,二者的耦合发展最终导致土体发生破坏。