Numerical analysis of the response of reinforced soils to direct shearing: Part 2

A soil-reinforcement load transfer model was developed by the authors¹ to simulate the response of the reinforced soil material to triaxial compression and direct shearing. This paper presents the application of the proposed model for the numerical analysis of direct shear tests on sand samples reinforced with different types of tension resisting reinforcements. A parametric study is conducted to evaluate the effect of the mechanical characteristics and dilatancy properties of the soil, extensibility (elastic modulus) of the reinforcements, and their inclination with respect to the failure surface on the response of the reinforced soil material to direct shearing. An attempt is made to verify the proposed model by comparing numerical test simulations with experimental results reported by Jewell,² and Gray and Ohashi.³ Comparisons of predicted and experimental results illustrate that the model can provide adequate simulations of the response of the reinforced soil material to shearing. In particular, it allows an evaluation of the effect of soil dilatancy (or contractancy), and extensibility of the reinforcement on tension forces generated in inclusions during shearing.     

H-V加筋黏性土的强度与变形特性

在提出非满布的H-V（水平－竖向）加筋的基础上,设计了非满布多层H-V加筋黏土的试验方案,以镀锌铁皮和有机玻璃为加筋材料进行了44组固结不排水三轴剪切试验。通过不同竖筋布置的加筋黏土三轴试验研究了加筋黏性土的应力-应变关系、强度特性及破坏形态,探讨了不同加筋高度、不同围压及不同筋材对加筋黏土强度的影响。试验结果表明：相对于无筋土及传统的水平加筋土而言,其峰值偏应力和抗剪强度均有大幅度提高。     

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.     

Constitutive modelling of a reinforced soil using hierarchical model

Drained triaxial tests are conducted on natural and reinforced sand under various stress paths. Direct shear tests and pull-out tests are conducted on soil–reinforcement interface and on reinforcement, respectively. The effects of two types of reinforcement, viz, woven and non-woven geotextile and number of layers of reinforcement are investigated. Hierarchical single surface model is used to depict the behaviour of natural and reinforced soil by treating the soil as a single composite material and by considering soil, reinforcement and interface as independent elements. It is shown that the material parameters are very much affected by the type and the number of layers of reinforcement. The hierarchical model provides satisfactory prediction for both natural and reinforced soil. Copyright © 1999 John Wiley & Sons, Ltd.     

石灰改良膨胀土的应力-应变-强度特征与本构描述

为探讨石灰改良膨胀土的变形特征与破坏机制,以压实度为95 %的荆门石灰改良膨胀土为研究对象,开展了单轴、侧限、三轴压缩应力状态下的力学性质试验。试验结果表明：即使湿化饱和后石灰土也具有较高的刚度与强度;单轴压缩状态下,无论饱和还是非饱和状态,石灰土的破坏都为典型的脆性破坏;三轴压缩状态下石灰土破坏前剪缩,同时伴随应变强化,即将破坏时开始剪胀,随后表现为应变软化;围压对石灰土的脆性破坏与破坏后的剪胀有一定的抑制作用,但即使在200 kPa围压下,试样仍发生脆性破坏。在辨明石灰土应力、变形机制的基础上,选用Duncan模型描述其脆性破坏前表现出的压硬性、剪缩性与应变强化特性,标定了相应模型参数,通过数值模拟与平行试验的对比验证了模型的适用性与参数的可靠性。     

H-V加筋饱和砂土性状的三轴试验研究

进行了H-V（水平-竖向）加筋饱和砂土固结不排水三轴试验。加筋材料采用镀锌铁皮和有机玻璃两种不同材料,在不同竖筋高度、不同围压下共进行了44组试验,研究了加筋饱和砂土的应力-应变关系、孔压-应变关系、强度特性及破坏形态,探讨了不同加筋高度、不同围压及不同筋材对加筋饱和砂土强度的影响,同时比较了立体加筋与土工格室加筋的异同,对饱和紧砂的空化现象及其影响进行了分析。试验结果表明,相对于无筋土及传统的水平加筋土而言,其抗剪强度均有大幅度提高;尤其是镀锌铁皮加筋,不仅提高了饱和砂土的有效黏聚力,也提高了有效内摩擦角,同时改善了砂土的延性。     

横-竖立体加筋地基中附加应力的分析计算

建立了横-竖立体加筋（H-V筋）地基的有限元模型,通过分析地基中的竖向应力分布、水平向位移分布以及筋-土界面相互作用,发现横-竖立体加筋地基中的竖向应力在筋材下方出现扩散和重分布,并逐渐向土体下部传递,使得土体中整体的应力分布更加均匀;同时,横-竖筋材中的竖筋类似于一个侧壁,其提供的垂直侧向力约束了介于竖筋间的土体,限制了土体的侧向水平位移,使得地基中筋材上部土体的侧向水平位移变小。基于有限元模拟对横-竖立体加筋地基加固机制的认识,将横-竖立体筋视为作用在地基上的一维弹性地基梁,通过弹性地基梁理论,根据弗拉曼解推导求解了横-竖立体加筋地基中任意一点竖向附加应力的计算表达式。将模型计算结果与有限元模拟所得结果进行对比发现两者吻合良好。     

Capturing strain localization in reinforced soils

Lade’s single hardening soil model with Cosserat rotation embodied in the finite element method is employed to investigate the behavior of geosynthetic reinforced soils with special attention to the development of shear banding. The ability of the finite element model to detect shear banding in a reinforced soil is examined against three high quality small-scale laboratory plane strain tests on Toyoura sand with and without reinforcement. These three tests were chosen because of the clear failure surfaces that developed in the soil during loading. The FEM analyses were able to reasonably simulate the plane strain laboratory tests including both unreinforced and reinforced cases. The FEM analyses gave reasonably good agreement with the experimental results in terms of global stress–strain relationships and shear band occurrences. Furthermore, and based on FE analyses of a hypothetical geosynthetic reinforced soil (GRS) retaining wall, it is shown that the geosynthetic reinforcements are very effective in hindering the formation of shear bands in GRS retaining walls when small spacing between the reinforcement layers was used. When used properly, the geosynthetic reinforcements made the soil behave as a truly reinforced mass of considerable stiffness and strength.     

土工格栅加筋土地基平板载荷试验研究

在近年来的岩土工程实践中,土工合成材料加筋土技术得到越来越广泛的应用。采用平板载荷板试验方法,进行了多组加筋砂土地基模型试验,监测和分析了不同加筋材料（双向格栅与四向格栅）和加筋层数对土工格栅加筋土地基承载特性的影响。研究结果表明：土工格栅加筋土地基与无筋地基相比,承载性能得到改善,双层加筋明显优于单层加筋;土工格栅加筋限制了浅层地基的侧向变形,相同荷载下地基沉降减小,可恢复变形增大;模型试验中测得加筋材料应变和拉力很小,与土工格栅强度相比,拉伸模量对加筋土地基承载力的贡献更大。     

A bounding surface model for saturated and unsaturated soil-structure interfaces

In many geotechnical systems, such as reinforced slopes and embankments, soil-structure interfaces are often unsaturated. Shear behaviour of unsaturated interfaces is strongly dependent on their matric suctions, as revealed by the results of extensive laboratory tests. So far, constitutive models for unsaturated interfaces are very limited in the literature. This paper reports a new bounding surface model for saturated and unsaturated interfaces. New formulations were developed to incorporate suction effects on the flow rule and plastic modulus. To examine the capability of the proposed model, it was applied to simulate suction- and stress-controlled direct shear tests on unsaturated soil–cement, soil–steel and soil–geotextile interfaces. Measured and computed results are well matched, demonstrating that the proposed model can well capture key features of the shear behaviour of unsaturated interfaces, including suction-dependent dilatancy, stress–strain relation and peak and critical state shear strengths.     

A homogenization approach for evaluating the longitudinal shear stiffness of reinforced soils: column versus cross trench configuration

The macroscopic linear elastic behaviour of inclusion‐reinforced soils, regarded as periodic composite media, is investigated by means of the homogenization theory. Special attention is given here to the determination of their longitudinal shear stiffness properties, which strongly govern the reinforced ground response under lateral loading. Combining the use of analytical, variational and numerical methods, we thoroughly examined three particular engineering‐relevant configurations: single trench, column and cross trench reinforcements. Fairly accurate closed‐form expressions are thus obtained, giving the value of the reinforced soil longitudinal shear stiffness as a function of the individual components shear moduli and reinforcement volume fraction. It is shown in particular that adopting a cross trench reinforcement layout instead of the classical column configuration results in a much higher improvement of the longitudinal shear stiffness. The results are then applied to assessing the reduction of soil liquefaction risk, which can be attributed to the presence of the reinforcing inclusions. Again, they clearly demonstrate the excellent performance of the cross trench configuration as compared with the complete inefficiency of the column reinforcement technique. Copyright © 2013 John Wiley & Sons, Ltd.     

不同应力路径下剪切带的数值模拟

采用回映应力更新算法,编写了基于伏斯列夫面的超固结黏土本构关系模型子程序,嵌入非线性有限元软件ABAQUS。通过对单元试验进行三轴压缩、三轴伸长及平面应变等问题的模型预测,再现了超固结黏土在不同初始超固结比和应力路径时的变形和强度特性,从而验证了子程序的正确性。借助该本构模型,对三轴压缩、三轴伸长及平面应变应力路径下超固结黏土体变形局部化问题,进行了三维数值模拟。分析结果表明：超固结黏土在三轴压缩及伸长状态时,土体变形局部化在应力-应变关系软化时出现,而平面应变状态时,在应力-应变关系硬化阶段出现,其超固结黏土的剪胀特性在剪切带的形成过程中起重要作用。     

横-竖筋带布置对地基影响的试验研究

分别针对纯砂地基、水平加筋地基和新型三维立体加筋(简称横-竖加筋)地基进行了多组模型试验。主要研究了单层横-竖加筋深度和横-竖加筋层数对地基的影响,并通过与水平加筋地基的比较,结合横-竖地基砂土滑移面的形状,初步分析了横-竖加筋地基的加固机制。试验结果表明,同等试验条件下,横-竖筋的加筋效果较水平筋的好,对于单层横-竖加筋地基,加筋效果随加筋深度的增加而减弱,加筋深度超过一定范围后,加筋对地基受力性能的改善不明显。对于多层横-竖加筋地基,随加筋层数的增加,承载力增加,沉降减小。     

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.     

纤维加筋土抗液化性能的研究进展

根据加筋方式的不同,纤维加筋可分为定向分布的纤维加筋和随机分布的纤维加筋两大类。本文针对这两类加筋方式,分别综述了纤维加筋技术在土体抗液化方面的相关研究进展,同时对纤维加筋土抗液化性能的主要影响因素进行了分析。既有针对纤维加筋土进行的动三轴试验、扭剪试验及模型试验等均表明,纤维加筋技术能有效地增强土体的抗液化强度,同时减小液化所引起的变形,是一种具有广泛应用前景的抗液化措施。     

纤维加筋膨胀土的三轴蠕变特性试验研究

为了初步探究纤维加筋膨胀土的蠕变特性,采用室内非饱和三轴蠕变试验,分析了初始含水率和纤维掺量两个因素对玄武岩纤维加筋膨胀土蠕变特性的影响,通过等时应力-应变曲线得出加筋土的长期强度,并尝试建立了纤维加筋膨胀土的蠕变模型,得到的结论主要有以下几点:通过对比分级加载条件下的应变-时间曲线,发现纤维加筋对于减小膨胀土的蠕变变形有显著的作用,并存在最优纤维掺量,当纤维掺量超过最优纤维掺量,蠕变效应无显著改善;纤维加筋膨胀土的蠕变变形随着含水率的减小而减小,并存在最优含水率,当小于最优含水率时蠕变效应无明显改善;纤维加筋可以显著提高膨胀土的长期强度,纤维掺量分别为0.4%和0.6%的加筋土长期强度比相同条件下的素土分别提高了26.7%和23.3%;通过拟合,得到了Mesri蠕变模型参数,认为该模型从总体上可以反映纤维加筋膨胀土的三轴蠕变特性。     

土工格室加筋土的大尺寸直剪试验研究

采用自行研制的500 mm?500 mm?400 mm（长?宽?高）大尺寸直剪仪,对土工格室加筋土以及土工格室加筋水泥稳定土的剪切性能进行了试验研究。通过大尺寸直剪试验模拟土工格室加筋土的剪切作用过程,得出加筋土剪切应力与剪切应变关系为非线性确定了土工格室加筋土的抗剪强度指标以及土工格室对土的抗剪强度增强机理,土工格室加筋土的黏聚力提高较大,内摩擦角变化相对较小。通过对素土和掺入量为5 %的水泥稳定土进行常规直剪试验、大尺寸直剪试验和三轴压缩试验对比分析,探讨不同试验方法对抗剪强度指标的影响,得出3种试验方法对应的抗剪强度指标及其相对大小;即素土的摩擦角大小依次为：三轴试验小于大尺寸直剪试验小于常规直剪试验,素土的黏聚力大小依次为：大尺寸直剪试验小于三轴试验小于常规直剪试验;水泥稳定土的摩擦角大小为：三轴试验小于大尺寸直剪试验,黏聚力结果比较大小依次为：三轴试验小于大尺寸直剪试验。     

DEM simulation of the behaviour of geogrid stabilised ballast fouled with coal

Geogrids are commonly used in railway construction for reinforcement and stabilisation. When railway ballast becomes fouled due to ballast breakage, infiltration of coal fines, dust and subgrade soil pumping, the reinforcement effect of geogrids decreases significantly. This paper presents results obtained from Discrete Element Method (DEM) to study the interface behaviour of coal-fouled ballast reinforced by geogrid subjected to direct shear testing. In this study, irregularly-shaped aggregates (ballast) were modelled by clumping together 10–20 spheres in appropriate sizes and positions. The geogrid was modelled by bonding a large number of small spheres together to form the desired grid geometry and apertures. Fouled ballast with 40% Void Contaminant Index (VCI) was modelled by injecting a predetermined number of miniature spheres into the voids of fresh ballast. A series of direct shear tests for fresh and fouled ballast reinforced by the geogrid subjected to normal shear stresses varying from 15 kPa to 75 kPa were then simulated in the DEM. The numerical results showed a good agreement the laboratory data, indicating that the DEM model is able to capture the behaviour of both fresh and coal-fouled ballast reinforced by the geogrid. The advantages of the proposed DEM model in terms of capturing the correct stress–displacement and volumetric behaviour of ballast, as well as the contact forces and strains developed in the geogrids are discussed.     

桩承式加筋路堤张力膜效应模型试验研究

在桩承式加筋路堤设计计算及工程实践中,筋材的受力计算是具有争议且亟待解决的问题,然而,现有的研究很少关注桩承式加筋路堤中筋材应变的空间分布形式和三维模式下筋材的受力特征。采用自行研制的设备,以桩间距为变量,针对桩承式加筋路堤中筋材张力膜效应进行了物理模拟试验。试验中通过气压施加荷载,对筋材竖向变形和不同位置筋材应变进行观测。结果表明,张力膜效应下筋材应变和受力很不均匀。正方形布桩情形下,加筋材料的空间变形形态可采用空间抛物面与抛物柱面的组合来模拟。正方形布桩情形下假定筋材上所有荷载全部由相邻桩之间的4条筋带承担,得到的筋材受力计算值明显偏大。在模拟试验结果基础上,初步提出了考虑筋材三维空间变形的拉力计算方法。     

交通荷载下加筋土桥台工作性能试验研究

加筋土桥台因其具有良好的复合体特性,被广泛应用于各项工程建设,研究它在实际工程条件下的工作性能对其设计和推广具有重要意义。加筋土桥台在服役过程中主要承受交通荷载作用,通过室内大比例缩尺模型的加速应力试验,研究交通荷载条件下加筋土桥台的工作性能,考虑加筋间距与筋材刚度的影响,综合分析了不同交通荷载等级和循环次数下桥台变形与筋材应变的变化规律。研究结果表明,加筋土桥台在交通荷载作用下整体变形随循环次数趋于收敛,其顶部沉降、面板水平位移与筋材应变均远小于规范给出的阈值;各变形值随加载时间呈阶梯状发展,不同条件下变形收敛趋势有所差异;增大筋材刚度和减小加筋层间距有利于控制桥台的变形,筋材刚度对变形的控制作用要优于加筋间距;交通荷载作用下面板水平应变与顶部竖向应变并不满足2倍关系,二者之间的定量关系有待进一步研究。