桩-网加固拓宽路堤土拱效应试验研究

采用桩-网加固拓宽路堤时,土拱效应对于分析新老路堤应力分布和差异沉降有至关重要的作用。依托某高速公路路堤拓宽项目对土拱效应进行研究,对试验段新路堤填筑过程及运营时基底桩及桩间土不同位置处土压力、加筋层拉应变进行监测,得到二维平面土拱效应的变化规律,并利用已有土拱效应计算方法对现场实测结果进行对比验证。结果表明,平面土拱作用范围在一定高度范围内,试验段约为2.0 m,即拱高、拱跨之比约为1.4,与英国规范BS8006[1]相近;按Guido法[2]与BS8006法[3]进行土工格栅的设计均过于保守,除Guido法以外,几种方法均低估桩间地基土的承担荷载贡献。因此,需在理论上作进一步研究。     

水泥土桩与土工格栅联合加固沟谷软基机理研究

针对山区沟谷软基的特殊土质,并结合夯实水泥土桩和土工格栅在地基加固中的作用特点,提出用夯实水泥土桩与土工格栅联合加固的方案来处理沟谷软基,并利用平面应变弹塑性有限元数值分析方法,对这种地基的受力性状、作用机理进行了多方面研究。结果显示：土工格栅在其中起到类似抗拉膜的作用,以控制地基的不均匀沉降,可以减小路堤坡脚附近的侧向位移,增加地基的极限承载能力。桩间土对夯实水泥土桩有负摩擦力作用,使得桩身的最大平均有效应力位置处在桩体的中下部。格栅对路基沉降量的影响则较小。     

拓宽方式对软土路基工程特性影响的离心模型试验

如何保持新旧路基间的变形协调是拓宽工程中普遍关注的问题,目前尚缺乏对于拓宽方式对软土路基工程特性影响的直接对比分析。本文开展离心模型试验,采用普通填料或气泡轻质土进行放坡或挡墙拓宽,分析了新旧路基变形、地基土中孔压和土压力在路基拓宽后的变化规律。试验结果表明,气泡轻质土显著减小拓宽过程中产生的孔隙水压力增量和附加应力增量。相较于边坡拓宽,挡墙拓宽方式对地基影响更小。轻质土路堤采用挡墙拓宽方式引起的挡墙倾角和墙背土压力均较小。采用Boussinesq公式计算得到的拓宽路堤引起的地基中附加应力分布与实测值基本吻合,且偏于保守。本文的研究成果对指导路基拓宽工程具有借鉴意义。     

防波堤土工织物加筋地基离心模型试验及数值模拟

以黄骅港北防波堤工程为依托,对土工织物加筋软粘土地基及斜坡式防波堤体系的固结过程进行了离心模型试验和有限元数值模拟,通过分析地基土体固结过程中防波堤-加筋垫层-基体系的位移场和应力场的发展及织物拉应力分布和发展,验证离心试验及数值模拟方法的合理性,并得出以下几点主要结论:(1)土工织物加筋垫层的作用机理为其抗拉性限制自身横向变形,通过与附近土体的摩擦作用限制其侧向变形;(2)加筋减小防波堤-垫层-地基体系的高应力水平区,避免堤身与地基高应力水平区域的连通,增强系统稳定性;(3)在离心试验采用的分层地基的条件下,加筋减小了浅层软基和堤身的侧向位移量,对系统总位移有抑制作用;(4)试验中织物替代材料的极限拉伸强度发挥程度较低,其与砂垫层间摩擦强度的发挥水平约为23.8％。     

桩承式加筋路堤的现场试验及数值分析

对一桩体面积置换率为8.7 %的低置换率桩承式加筋路堤进行了现场试验及三维有限元分析。现场主要进行了桩、土荷载分担,孔压、沉降及侧向水平变形等内容的观测。将观测数据与常规设计方法及三维有限元分析结果进行了对比研究,在此基础上对设计方法的适用性进行了分析。研究结果表明,路堤填土的土拱效应造成荷载向桩体转移,这种荷载转移大幅度减小了在软土层中产生的超孔隙水压力。当填土高度大于2.5 m时,土拱效应的应力折减系数可用Russell和Pierpoint或Hewlett和Randolph提出的土拱效应分析方法进行计算,其结果与三维有限元分析也较相符,但在路堤高度较小时,只有Russell和Pierpoint方法与实测结果相接近。路堤施工过程中,实测的水平变形与沉降之比仅为0.2左右,这表明采用桩承式加筋路堤不仅可减小沉降,而且可减小水平向的变形,提高路堤的稳定性。     

Analytical modeling on consolidation of stiffened deep mixed column-reinforced soft soil under embankment

Stiffened deep mixed (SDM) column is a new ground improvement technique to improve soft soil, which can be used to increase bearing capacity, reduce deformation, and enhance stability of soft soil. This technique has been successfully adopted to support the highway and railway embankments over soft soils in China and other countries. However, there have been limited investigations on its consolidation under embankment loading. This paper developed an analytical solution for the consolidation of embankment over soft soil with SDM column in which core pile is equal to or shorter than outer DM column. The consolidation problem was simplified as a consolidation of composite soil considering the load shear effect of core pile. The developed solution was verified by a comparison with the results computed by three-dimensional (3-D) finite element analysis. A parametric study based on the derived solution was conducted to investigate influence factors—length of core pile, diameter of core pile, diameter of SDM column, modulus of DM column, and permeability coefficient of DM column—on the consolidation behavior of SDM column-supported embankment over soft soil. The developed solution was applied to a case history of SDM column-supported embankment, and a good agreement was found between the predictions and the field measurements.     

格形地连墙与软土相互作用的离心试验研究

格形地下连续墙（GCRW）是一种常用于软土地区的基坑开挖的新型支护结构,该结构与软土的相互作用是必须深入研究的关键问题。结合背景工程,首先进行了格形地连墙模型的设计和试验方案的制定,通过离心模型试验模拟了分步开挖基坑时格形地连墙和软土的相互作用,并把测定结果与朗肯土压力进行了比较。试验结果表明,土压力基本上呈现线性变化特征,格形地连墙因基坑开挖引起的前墙内侧土压力而产生变形和位移;格形地连墙和格子内的土体作为一个整体在土压力的作用下保持平衡,受力特征与重力式支护结构相似。支护结构处于最不利状态时,主动区土压力介于朗肯静止土压力和主动土压力之间,而被动区土压力大于朗肯被动土压力,前墙没有倾覆是因为受到了隔墙拉力     

Nonlinear analysis of multilayer extensible geosynthetic-reinforced granular bed on soft soil

The paper presents a model for the analysis of granular foundation beds reinforced with several geosynthetic layers. Such reinforced granular beds are often placed on soft soil strata for an efficient and economical transfer of superstructure load. The granular bed is modeled by the Pasternak shear layer and the geosynthetic reinforcement layers by stretched rough elastic membranes. The soft soil is represented by a series of nonlinear springs. The reinforcement has been considered to be extensible and it is assumed that the deformation at the interface of the reinforcements and soil are same. The nonlinear behavior of the granular bed and the soft soil is considered. 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. The results from the proposed model are compared to the results obtained for multilayer inextensible geosynthetic reinforcement system. Significant reduction in the settlement has been observed when the number of reinforcement layer is increased. In case of inextensible reinforcements as the number of reinforcement layer is increased the settlement is decreased with a decreasing rate, but in case of extensible reinforcement the reduction rate is almost constant. Nonlinear behavior of the soft soil decreases as number of reinforcement layer is increased. The effect of the stiffness of the geosynthetic layer on the settlement response becomes insignificant for multilayer reinforced system, but the mobilized tension in the reinforcement layers increases as the stiffness of the geosynthetic layers increases.     

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.     

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     

桩承加筋路堤中路堤与褥垫层共同作用理论分析

从桩承加筋路堤中加筋褥垫层与路堤填土共同作用的实际情况出发,利用弹性力学对路堤填土单元进行了力学平衡分析,同时考虑到加筋褥垫层的工作状态类似于连续梁板的特性,根据PCC桩承加筋路堤的设计参数及现场测试结果,将现浇混凝土薄壁管桩（PCC）桩承加筋路堤中的加筋褥垫层视为弹性薄板,再根据薄板小挠度理论,推导出了桩土差异沉降等计算公式。又根据填土与褥垫层变形协调、应力连续的条件,在忽略褥垫层材料本身压缩量的前提下将加筋褥垫层变形分析与填土变形分析相结合,提出了求解填土等沉面高度、桩土应力、桩土差异沉降的方法,最后通过与现场实测结果对比,验证了方法的正确性。     

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.     

软土地基上土工织物加筋堤的离心模型试验研究

以黄骅港防波堤工程为依托,对有、无土工织物加筋垫层及不同织物加筋条件下堤坝和软土地基的变形与稳定进行了离心模型试验对比研究。得到了地基土体的竖向与水平位移分布规律以及孔隙水压力的消散特性。研究表明：土工织物加筋垫层的加筋机理源于其对土体的侧限效果,铺设土工织物后能有效地减小地基的沉降,并能够限制浅层土体的侧向变形;为使土工织物的加筋效果能够得以充分发挥,要求织物的强度及模量要与垫层的界面强度相匹配。     

桩承式路堤土拱效应的试验和数值研究

对桩承式路堤中的土拱效应进行了三维模型试验研究。根据试验结果,分析了应力折减系数大小和填土中的竖向应力分布特点。对比分析了实测值与Terzaghi方法和Hewlett & Randolph方法计算值之间的差异,验证了各方法的适用性。采用三维有限元对模型试验进行了数值模拟,重点分析了路堤填土的大主应力方向、应力水平和沉降模式等几个关键问题。试验和数值结果表明,Terzaghi方法与Hewlett、Randolph方法给出的应力折减系数数值接近,但填土中竖向应力分布模式截然不同,且与实测值有所区别。若在Terzaghi方法中采用合适的滑动面形状,得到的计算结果与实测值较吻合,能够反映竖向应力沿深度分布的特点     

水泥土桩联合土工格栅复合地基的离心模型试验研究

在软土地基上修筑路堤需要兼顾承载力和变形两方面的要求。针对山区沟谷软基的特点,采用了夯实水泥土桩与土工格栅联合加固的方法,是一种创新,选择合适的试验手段来研究该种型式的复合地基对于工程设计和施工具有重要意义。通过离心模型试验,模拟夯实水泥土桩联合土工格栅复合地基上路堤的填筑进程,水泥土桩采用石膏柱模拟,土工格栅采用预拉伸加筋格网模拟,试验得出了该种复合地基的工程特性。夯实水泥土桩具有较好的加固基础作用,格栅的主要作用是均衡上部荷载和减小差异沉降,采用夯实水泥土桩联合土工格栅的复合地基形式可以满足沟谷软基的沉降控制要求,其最终结论可以为设计和施工提供参考。     

软土地基路堤施工控制的离心模拟试验研究

软土路基上快速填筑路堤时的稳定控制是非常重要的。为此在现行规范中,采用了一些位移或位移速率的控制标准。但实践表明,采用现行的标准仍然出现一些地基失稳的事例,说明这些标准需要进一步求证和改进。除了继续收集、分析失效事例之外,通过离心模型试验也可以进一步理解堤基失稳的机制。离心模型试验结果表明：路堤快速填筑使得地基破坏时,地基变形略呈马鞍形,坡肩处沉降比道中处沉降略大,坡脚水平位移增加较快;破坏时的位移速率与现有规范建议的控制标准基本符合,但地基内的孔隙水压力是在地基进入破坏状态并发生较大变形之后才有突然增加的趋势;此外,坡脚水平位移和道中沉降的速率比,可能是一个较好的稳定性控制的指标。     

山区公路拓宽路基与衡重式挡土墙动态相互作用的数值模拟

针对山区公路路基拓宽改建的特殊性,运用非线性有限元法,开展衡重式挡土墙修建、新路基逐层分步填筑的动态施工力学行为数值模拟。针对新路基是否加筋、加筋层位以及衡重台上下方新路基填土压实状况的差异性,对新旧路基稳定安全性、滑动面形态、沉降、挡土墙变位等力学响应的影响开展参数敏感性分析。结合室内模拟墙体不同主动位移模式下拓宽路基土工离心模型试验成果,考察新旧路基顶面沉降与挡土墙变位的动态耦合关系。在山区公路路基拓宽改建动态施工中,挡土墙的变位和新旧路基顶面沉降、差异沉降相互耦合,呈正相关关系。衡重台下方新路基填土压实不足会导致新旧路基顶面差异沉降增大。挡土墙主动外倾诱发新旧路基顶面产生较大附加（差异）沉降,宜对新路基自重压密作用造成的差异沉降予以适当修正放大后进行上承路面结构设计。     

动荷载下桩承式路堤的承载特性及机制研究

对于桩承式路堤作用效应的研究目前主要侧重于对静荷载作用下桩土应力比和土拱效应等,较少考虑动荷载的影响,而车辆运行产生的动应力会对路堤中的土拱产生一定的影响,进而影响桩承式路堤的整体性能。为了分析静、动荷载作用下桩承式加筋路堤的性能变化,采用可视化模型试验和颗粒流数值模拟相结合的方法,对桩承式路堤在静载和动载下的应力传递和变形性状进行研究,分析动载作用下填土高度、桩帽、桩距、加筋形式、荷载频率的影响。试验结果表明,在动载下无筋路堤的桩顶的应力减小,而桩间的应力和位移增大,并且变化的幅度均比加筋路堤大,加筋材料的设置有利于减小动载的影响效应,但不同加筋形式下桩承式路堤的工作性状有所不同,受动载影响程度的大小主要与土拱效应的强弱有关。设置双层加筋时,因加筋材料与周围砂土形成半刚性平台,土拱效应减弱,故受动载影响的程度最小,单层加筋时,格栅设于桩顶上方10 cm比格栅置于桩顶受动载影响的程度明显减小,颗粒流的模拟结果验证了以上结果,并且进一步得出随荷载频率的增加、填土高度与桩净距的减小,动载的影响效应增大的结论。     

Numerical analysis of an embankment on soft soils considering large displacements

The behaviour of an embankment built on a Portuguese soft soil is analysed considering the material and geometric non-linearity associated with a coupled soil–water formulation. The numerical predictions are compared with the field data in terms of settlements, horizontal displacements and excess pore water pressures. The repercussions of including the large displacements formulation are also studied. It is found that the analysis considering large displacements results in a decrease in settlements and an increase in the rate of excess pore pressure dissipation, both of which are related to the reduction of the thickness of a deformable layer.     

A discrete numerical approach for modeling face stability in slurry shield tunnelling in soft soils

In this paper, a numerical simulation method for evaluating tunnelling-induced ground movement is presented. The method involves discrete element simulation of TBM slurry shield advancement and considers explicitly soil excavation from the face, effects of varying face support pressure, and the influence of tunnel cover depth. For the cases studied, it is found that for tunnel cover depths (C/D) between 0.7 and 2.1, ground deformations inducing by the tunnelling can be controlled within a certain extent and tunnel face stability can ensured, provided the support pressure ratio (N) lies between 0.8 and 1.5. The proposed method is reasonably benefited to modeling the face stability in shield-driven tunnels in soft soils.