Influence of length effect on embankment slope reliability in 3D

Embankment slopes composed of spatially variable soils have a variety of different failure modes that are affected by the correlation distances of the material properties and the geometry and total length of the slope. This paper examines the reliability of soil slopes for embankments of different length and uses parallel computing to analyse very long embankments (up to 100 times the embankment height) for a clay soil characterised by a spatially varying undrained shear strength. Based on a series of analyses using the 3D random finite element method (RFEM), it is first shown that the reliability of slopes of various length can be efficiently computed by combining simple probability theory with a detailed 3D RFEM analysis of a representative shorter slope of length 10 times the slope height. RFEM predictions of reliability indices for longer slopes are then compared with results obtained using Vanmarcke's (1977a) simplified 3D method and Calle's (1985) extended 2D approach. It is shown that these methods can give significantly different results, depending on the horizontal scale of fluctuation relative to the slope length, with RFEM predicting a lower slope reliability than the Vanmarcke and Calle solutions in all cases. The differences in the solutions are evaluated and attributed to differences in the assumed and computed failure surface geometries.     

Pseudo Static Seismic Stability Analysis of Reinforced Soil Structures

The paper pertains to the pseudo-static seismic stability analysis of reinforced soil structures. Using limit equilibrium method and assuming the failure surface to be logarithmic spiral, analysis has been conducted to maintain internal stability against both tensile and pullout failure of the reinforcements. The external stability of the reinforced earth wall is also assessed in terms of its sliding, overturning, eccentricity and bearing modes of failure. The influence of the intensity of the surcharge load placed on the backfill is also considered in the analysis. The obtained results are validated by comparing the same with those reported in literature. Studies have also been made regarding the influence of backfill soil friction angle, horizontal and vertical seismic accelerations, surcharge load, the tensile strength of reinforcement, pullout length of the reinforcement and number of reinforcement layers on the seismic stability against various failure modes as mentioned earlier.     

Reliability bearing capacity analysis of footings on cohesive soil slopes using RFEM

Reliability analysis of bearing capacity of a strip footing at the crest of a simple slope with cohesive soil was carried out using the random finite element method (RFEM). Analyses showed that the coefficient of variation and the spatial correlation length of soil cohesion can have a large influence on footing bearing capacity, particularly for slopes with large height to footing width ratios. The paper demonstrates cases where a footing satisfies a deterministic design factor of safety of 3 but the probability of design failure is unacceptably high. Isotropic and anisotropic spatial variability of the soil strength was also considered.     

Reliability Based Earthquake Resistant Design for Internal Stability of Reinforced Soil Structures

This paper presents a method to evaluate reliability for internal stability of reinforced soil structures using reliability based design optimization. Using limit equilibrium method and assuming the failure surface to be logarithmic spiral, analysis is conducted to maintain internal stability against both tensile and pullout failure of the reinforcements. Properties of backfill soil and strength of the geosynthetic reinforcement are considered as random variables. For the seismic conditions, reliability indices of all the geosynthetic layers in relation to tension and pullout failure modes are determined for different magnitudes of seismic accelerations both in the horizontal and vertical directions, surcharge load and design strength of the reinforcement. The efforts have been made to obtain the number of layers, pullout length and total length of the reinforcement at each level for the desired target reliability index values against tension and pullout modes of failure. The influence of horizontal and vertical earthquake acceleration, surcharge load, design strength of the reinforcement, coefficient of variation of soil friction angle and design strength of the reinforcement on number of layers, pullout length and total length of the reinforcement needed for the stability at each level is discussed.     

Experimental and numerical analyses of circular footing on geogrid-reinforced granular fill underlain by soft clay

Experimental and numerical investigations into the bearing capacity of circular footing on geogrid-reinforced compacted granular fill layer overlying on natural clay deposit have been conducted in this study. A total of 8 field tests were carried out using circular model rigid footing with a diameter of 0.30 m. 3D numerical analyses were performed to simulate soil behavior using finite element program Plaxis 3D Foundation. The results from the FE analysis are in very good agreement with the experimental observations. It is shown that the degree of improvement depends on thickness of granular fill layer and properties and configuration of geogrid layers. Parameters of the experimental and numerical analyses include depth of first reinforcement, vertical spacing of reinforcement layers. The results indicate that the use of geogrid-reinforced granular fill layers over natural clay soils has considerable effects on the bearing capacity and significantly reduces the lateral displacement and vertical displacement of the footing.     

快速加固吹填土的室内模拟试验

吹填造陆是解决沿海地区建设用地不足的主要方法之一。为了使吹填土在较短的时间内获得较好的加固效果,结合吹填土特殊工程地质性质和加固过程中强度增长具有阶段性的特点,提出了边吹填边加固的加固思路。在该思路的指导下进行室内模拟试验。试验发现不同加固阶段吹填土强度增长表现出明显的分带性;与竖向排水体距离小于25 cm的土体强度增长较快;随着加固时间的延长,待加固土体强度整体增长;当持续真空加荷一个月时,部分待加固土体承载力已达到80 kPa。可以看出,强度增长与外部荷载的大小、渗透路径的长短、附加应力的大小密切相关。分阶段加固的方法在一定程度上抑制了泥皮的形成,缩短了真空加荷时间,节约工期,有利于吹填土的加固。     

土工织物与吹填土界面作用特性试验研究

筋-土界面强度参数是加筋结构设计和稳定分析的关键技术指标。拉拔试验能较好模拟现场加筋行为而得到广泛的应用。基于拉拔和直剪试验研究了拉拔（剪切）速率对筋-土界面特性和吹填砂强度特性的影响规律及机制,同时探讨了不同填料界面、筋材类型的加筋效果。结果表明：随着拉拔（剪切）速率的增大,吹填砂-筋材-吹填砂界面的抗剪强度下降明显,而软土-筋材-吹填砂界面以及砂本身强度则变化不大。从强度指标来看,拉拔速率增大,筋-砂界面强度的降低主要表现为似黏聚力的降低,筋-软土界面抗剪强度的增加表现为内摩擦角增大,剪切速率对吹填砂则基本无影响。筋-土界面特性受拉拔速率和正应力的共同影响,与筋材类型和填料特性有关。筋-土界面内摩擦角小于填料摩擦角,但在一定正应力下低速剪切时（如<0.53 mm/min）可获得高于填料的抗剪强度。宜根据似黏聚力大小合理选择摩擦参数进行加筋结构的设计与评价。     

三维加筋边坡地震稳定性上限分析

基于极限分析上限定理,考虑均匀加筋和三角形加筋两种加筋模式,采用拟静力分析方法推导了一定边坡高度条件下的三维加筋边坡临界加筋强度的计算公式。通过与已有文献结果的对比,验证了所提方法的正确性,并讨论了边坡宽高比、水平和竖向地震力系数对三维边坡稳定性的影响。结果表明:地震力作用下边坡临界加筋强度随边坡宽高比的增大而增加,但其变化速率逐渐减小,当边坡宽高比大于10时,三维边坡加筋强度与二维情况相近;随着水平地震作用的增大,三维边坡临界加筋强度呈非线性增大;随着竖向地震作用的增大,临界加筋强度大致呈线性增加,且随着坡角的增大,竖向地震作用对临界加筋强度的影响更加显著;两种加筋模式下的边坡临界加筋强度值变化规律一致,且三角形加筋模式所需的加筋强度较小,效果较优。最后,针对实际工程提出了一些工程建议。     

Bearing capacity of square footings on reinforced layered soil

In the present study, an approximate method has been suggested to calculate the ultimate bearing capacity of a square footing resting on reinforced layered soil. The soil is reinforced with horizontal layers of reinforcement in the top layer of soil only. The pre requisite to the method is the ultimate bearing capacity of unreinforced layered soil, which can be determined from the methods already available in literature. The results have been validated with the model tests conducted on two layered soil compacted at different densities and the top layer reinforced with horizontal layers of geogrid reinforcement.     

Behaviour of Cellular Reinforced Sand Under Triaxial Loading Conditions

Cellular reinforcement is a three dimensional reinforcement used for reinforced soil structures. Behaviour of such reinforcement is important for its use in actual practice. Present paper focuses on the behavior of cellular reinforcement in sand under the triaxial loading conditions. Series of triaxial tests are performed on unreinforced and reinforced sand with single layer as well as double layers of cellular reinforcements with 75 mm sample diameter. Six different reinforcement heights of cellular reinforcements (varying from 3 to 50 mm) are used along with one sheet reinforcement of thickness 1 mm. From the experimental failure patterns of the triaxial samples, multiple zones of failure are observed as an effect of cellular reinforcement. Deviator stress–strain curves are studied for single and double layers of cellular reinforcement under three different confining pressures. Peak deviator stress is found increasing with increasing height of cellular reinforcement, which shows the confining effect of cellular reinforcement. Shear strength parameters are evaluated and are found increasing with increase in height of cellular reinforcement, also cellular reinforcement with heights 10 mm and more have showed increased shear strength parameters, as compared to 1 mm thick sheet reinforcement. This assures better behavior performance of cellular reinforcement over the planar one. Failure patterns are also visualized by finite element analysis and found in accord with experimental observations Horizontal displacement for reinforced samples visualized multi-zoned failure pattern. Finite element results for deviator stress–strain relationship are found in reasonably good accord with experimental results.     

约束随机场下的边坡可靠度随机有限元分析方法

目前边坡可靠度中常用的简化分析方法,不考虑边坡土体的空间变异性,每次计算整个边坡都取用相同的强度参数,由离散点试样试验得到的土体参数统计特性只能反映点特性,而边坡的稳定性受滑面上平均抗剪强度特性控制,因此,需要考虑空间范围内的平均特性。描述空间变异性的随机场理论对变异性较高的土体,实际上高估了其空间变异性。把随机场理论和地质统计中的区域化变量理论结合起来,建立约束随机场,并在此基础上进行Monte-Carlo随机有限元分析。计算实例表明,在高变异性条件下约束随机场能有效降低完全随机场的模拟方差,得到更低的破坏概率。对比了随机有限元和简化法的计算结果表明,简化法在土体强度变异性很高时其结果并非偏于保守。另外也指出了可靠度分析中存在的边坡尺度效应和简化法的适用条件。     

草根加筋土的室内三轴试验研究

用室内三轴试验方法研究了草根加筋土的应力-应变及强度特性,探讨了在不同加筋情况下草根加筋土抗剪强度指标的变化规律,分析了素土和草根加筋土的变形破坏模式以及筋材在土体剪切过程中的阻抗机理。试验结果表明：（1）草根加筋土的强度和抵抗变形的能力较素土有显著的增强;（2）对于抗剪强度指标,与素土相比草根加筋土的内摩擦角变化较小（相对变化率绝对值在11 %以内）,但黏聚力增长较大（最大可达9倍）;（3）在草根加筋层数一定的情况下,随着加筋量的增加,草根加筋土的主应力差值、抗剪强度值以及黏聚力值呈现先增加后降低的趋势,亦即加筋层数相同时存在最佳含根量。本研究结果对于深入认识植被护坡机理和合理选择植被密度具有指导意义。     

填土斜坡的排水条件对斜坡稳定性的影响

降雨是导致斜坡破坏的主要原因。地下水位上升和在土中渗流引起土体应力状态的变化是导致斜坡在降雨过程中或降雨后变形与破坏的关键因素。填土斜坡的稳定性不仅取决于填土的强度,还与填土斜坡的排水条件密切相关。适当的排水条件不仅可以限制斜坡中地下水位上升的高度,而且可以及时消散因材料剪切收缩而生成的超孔隙水压力,防止土体液化,避免斜坡整体流动破坏。本文采用完全耦合有效应力分析程序和与状态相关的剪胀性砂土模型模拟地下水位上升过程中填土排水条件对压实填土斜坡稳定性的影响。完全耦合有效应力分析方法可以模拟孔隙水和土骨架间包括浮力和渗流在内的相互作用。分析结果表明斜坡排水条件是影响斜坡因地下水位上升而变形和破坏的重要因素,加固松散填土斜坡时必须设置适当的排水与泄水装置。     

Experimental Study of Bearing Capacity of Granular Soils,Reinforced with Innovative Grid-Anchor System

The pull-out resistance of reinforcing elements is one of the most significant factors in increasing the bearing capacity of geosynthetic reinforced soils. In this research a new reinforcing element that includes elements (anchors) attached to ordinary geogrid for increasing the pull-out resistance of reinforcements is introduced. Reinforcement therefore consists of geogrid and anchors with cubic elements that attached to the geogrid, named (by the authors) Grid-Anchor. A total of 45 load tests were performed to investigate the bearing capacity of square footing on sand reinforced with this system. The effect of depth of the first reinforcement layer, the vertical spacing, the number and width of reinforcement layers, the distance that anchors are effective, effect of relative density, low strain stiffness and stiffness after local shear were investigated. Laboratory tests showed that when a single layer of reinforcement is used there is an optimum reinforcement embedment depth for which the bearing capacity is the greatest. There also appeared to be an optimum vertical spacing of reinforcing layers for multi-layer reinforced sand. The bearing capacity was also found to increase with increasing number of reinforcement layer, if the reinforcement were placed within a range of effective depth. The effect of soil density also is investigated. Finally the results were compared with the bearing capacity of footings on non-reinforced sand and sand reinforced with ordinary geogrid and the advantages of the Grid-Anchor were highlighted. Test results indicated that the use of Grid-Anchor to reinforce the sand increased the ultimate bearing capacity of shallow square footing by a factor of 3.0 and 1.8 times compared to that for un-reinforced soil and soil reinforced with ordinary geogrid, respectively.     

循环荷载及静载下土工格室加筋路堤模型试验研究

为探究土工格室加筋路堤在循环荷载及静载下的各种性能,利用美国GCTS公司的USTX-2000加载装置进行加载,通过改变加筋层数、格室高度,格室焊距对土工格室加筋路堤进行一系列模型试验。对各种工况下加筋路堤极限承载力、长期循环荷载及固定振次循环荷载后极限承载力的变化进行研究。试验表明,土工格室加筋能显著提高地基极限承载力并能显著减小坡顶和坡中临界破坏时的法向累积变形,在加筋间距一定的情况下,加筋层数增加和格室高度增大均可不同程度提高极限承载力并减小临界破坏时坡顶法向累积变形,格室焊距的减小也可在一定程度提高极限承载力,格室焊距对边坡法向变形影响不大;长期循环荷载下固定间距加筋层数对路堤竖向累积沉降量影响不大,而对边坡坡顶法向累积变形有一定影响,格室高度增大和格室焊距减小均可不同程度减小路堤竖向累积沉降量和坡面法向累积变形;越靠近加载点处,路堤土压力值受加筋影响越显著,加筋提高了土体刚度和密实度,使加筋路堤土压力值较无筋路堤明显增大;对于无筋路堤,改变动载幅值和振次均导致振后极限承载力有不同程度的降低,而对于加筋路堤,当动载幅值≥30 kPa或动载振次≥1 000时,振后极限承载力均有不同程度的提高。     

稻壳灰加筋土力学性能研究

稻壳灰与土体混合应用,一方面废弃资源再利用,环保,又可增强土体强度。通过三轴试验,研究不同比例稻壳灰混合黏土及其加筋土应力?应变性能、强度特性以及不同应变水平下模量、偏应力及加筋强度比等土体变化特征。试验结果表明,随稻壳灰比例增加,混合土最大干密度显著减小,最优含水率显著增加。添加不同比例稻壳灰对加筋土抗剪强度有较大影响,10%～15%稻壳灰比例下,加筋稻壳灰混合土初始切线模量和应力峰值达到最大,抗剪强度较优。与土工织物加筋稻壳灰混合土相比,土工格栅加筋稻壳灰混合土偏应力及抗剪强度更大,土工格栅层数对土体抗剪强度增大效果更明显,对应的应力?应变曲线拐点也更突出。试样弹性模量与稻壳灰比例及筋材种类、层数有关,加入稻壳灰后,土体弹性模量增长显著,土工格栅加筋稻壳灰混合土较优比例下可增加1.5倍多,稻壳灰及筋材均能有效提高土体强度。随加筋层数增加,稻壳灰混合土加筋强度比明显增大,与围压关系较小。     

FE analysis of grid reinforced embankment system on soft Bangkok clay

The behavior of a reinforced embankment on soft Bangkok clay has been analyzed by plane strain finite element method. The finite element analysis considers the selection of proper soil/reinforcement properties according to the relative displacement pattern of upper and lower interface elements. The large deformation phenomenon is simulated by updating the node coordinates, including those of the embankment elements above the current construction level, which ensures that the applied fill thickness simulates the actual field value. A full scale test reinforced embankment with a vertical face (wall) on Bangkok clay has been analyzed by the proposed finite element method, and the numerical results are compared with the field data. The response of a reinforced embankment on soft ground is principally controlled by the interaction between the reinforced soil mass and soft ground and the interaction between the grid reinforcement and the backfill soil. The tension in reinforcement and lateral displacement of the wall face varied during consolidation of foundation soil. The maximum tension force occurred in the reinforcement layer placed at the base of reinforced mass, due to bending of the reinforced mass resulting from differential settlements. It is considered necessary to account for the permeability variation of the soft ground foundation in the finite element analysis.     

加筋陡坡临界高度的影响因素敏感性研究

基于塑性极限分析的上限定理,利用自编程序,分别计算了不同坡脚、顶面倾角、内摩擦角、黏聚力、筋带抗拉强度、加筋间距、格栅布置方式、填土重度、地震烈度、顶部荷载共10种影响因素作用下加筋陡坡的临界极限高度。根据正交分析方法给出了各个因素的敏感性顺序。结果显示,土的内摩擦角和地震作用对陡坡临界高度的影响最为显著,黏聚力敏感性则较缓坡有所下降。因此,对陡坡应重视对土的内摩擦角的选取,并在设计中对地震作用予以重视。     

双向增强体复合地基桩土应力比分析

针对双向增强体复合地基,由于桩体与桩间土变形不协调,导致路堤填土内部垂直变形有差异,应力状态发生变化。引入剪切位移法来分析路基填土中水平加筋体的变形和应力传递规律,在综合考虑填土力学性质（c, ）和水平加筋体作用的基础上,通过微元体平衡分析推导得到了桩土应力比、桩顶刺入量等计算公式。通过工程实例验证公式的可靠性,并综合分析了相关因素对桩土应力比的影响,得到了一些有益的结论。     

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.