条形荷载下加筋土边坡破坏机制的数值模拟

为了克服模型的尺寸效用,获得加筋与不加筋边坡在条形荷载下的各种性状参数和边坡的破坏机制,建立用于分析和模拟3个大型室内足尺加筋与不加筋边坡破坏机制的数值计算模型。边坡回填材料采用级配较差的粗砂,土体的非线性弹性响应采用Duncan-Chang双曲线模型E-B模式加以描述,破坏准则采用Mohr-Coulomb屈服准则,并采用与屈服条件不相关联的流动法则。加筋材料采用两节点的弹塑性锚索结构单元进行模拟,并采用无厚度的弹簧-滑动系统来模拟筋土之间的相互作用和相对运动。数值计算采用基于有限差分的连续介质快速拉格朗日分析方法(FLAC),分别对与破坏面位置和形态密切相关的节点位移速度向量、塑性区和剪应变速率分布3个参数进行了计算,获得了3个边坡在条形极限荷载下的双楔体破坏机制和极限承载力,与试验结果吻合较好,验证了模型的可行性。在此基础上,对不同的条形荷载位置及不同填土材料强度下边坡破坏机制进行了数值模拟和分析。研究结果表明,无论加筋与不加筋边坡,当条形荷载位置距坡肩的距离减小时,边坡破坏面形态由双楔体过渡到圆弧形;当填土材料强度降低时,破裂面形态转化为圆弧形或对数螺线形。     

条形荷载作用下加筋土边坡稳定性分析

建立了用于模拟和分析3个大型室内足尺加筋与不加筋边坡稳定性的数值计算模型。数值计算采用基于强度折减技术的连续介质快速拉格朗日分析方法,分别对条形荷载下的位移响应、节点位移速度向量、塑性区和剪应变速率分布进行计算,获得3个边坡在条形荷载下的极限承载力和双楔体破坏机制,计算结果与试验结果吻合较好,验证了模型的可行性。在此基础上,对影响边坡稳定性的各主要因素进行分析。研究结果表明,经过格栅加固的边坡承载能力和稳定性明显提高,且随加筋层数、格栅刚度和强度的增加而增大;条形荷载越大或荷载位置离坡顶越近,边坡的稳定性越低;土体强度增大,边坡的稳定性明显增加,但土体摩擦角对安全系数的影响比黏聚力更为敏感;此外,顶层筋材埋深与条基荷载宽度比值大小与边坡的安全性密切相关,其最佳比值随加筋层数不同而改变。     

加筋边坡在坡顶荷载作用下的极限承载能力

采用大型室内试验的方法,研究了两个土工格栅加固的土坡和一个未加固边坡在坡顶荷载作用下的变形与破坏规律。本文重点介绍大型模型的实验设计、测试技术和研究方法。实验结果表明,土工格栅加固边坡的承载能力为相同条件下未加固边坡的1.6-2倍。     

Physical and numerical modelling of strip footing on geogrid reinforced transparent sand

This paper presents the results of laboratory scale plate load tests on transparent soils reinforced with biaxial polypropylene geogrids. The influence of reinforcement length and number of reinforcement layers on the load-settlement response of the reinforced soil foundation was assessed by varying the reinforcement length and the number of geogrid layers, each spaced at 25% of footing width. The deformations of the reinforcement layers and soil under strip loading were examined with the aid of laser transmitters (to illuminate the geogrid reinforcement) and digital camera. A two-dimensional finite difference program was used to study the fracture of geogrid under strip loading considering the geometry of the model tests. The bearing capacity and stiffness of the reinforced soil foundation has increased with the increase in the reinforcement length and number of reinforcement layers, but the increase is more prominent by increasing number of reinforcement layers. The results from the physical and numerical modelling on reinforced soil foundation reveal that fracture of geogrid could initiate in the bottom layer of reinforcement and progress to subsequent upper layers. The displacement and stress contours along with the mobilized tensile force distribution obtained from the numerical simulations have complimented the observations made from the experiments.     

Laboratory investigation of bearing capacity behaviour of strip footing on reinforced flyash slope

The paper presents the results of laboratory model tests on bearing capacity behaviour of a strip footing resting on the top of a geogrid reinforced flyash slope. A series of model footing tests covering a wide range of boundary conditions, including unreinforced cases were conducted by varying parameters such as location and depth of embedment of single geogrid layer, number of geogrid layers, location of footing relative to the slope crest, slope angles and width of footing. The results of the investigation indicate that both the pressure–settlement behaviour and the ultimate bearing capacity of footing resting on the top of a flyash slope can be enhanced by the presence of reinforcing layers. However the efficiency of flyash geogrid system increases with the increasing number of geogrid layers and edge distance of footing from the slope. Based on experimental results critical values of geogrid parameters for maximum reinforcing effects are established. Experimental results obtained from a series of model tests have been presented and discussed in the paper.     

Cyclic response of footing on geogrid-reinforced sand with void

This paper describes laboratory tests on footing constructed on unreinforced and geogrid-reinforced sand with circular a void subjected to a combination of static and repeated loads. The settlement of the footing was measured for up to 5000 cycles of loading and unloading. The variables examined in the testing program include the number of geogrid layers, the location of the void within the soil, the amplitude of cyclic load, and the number of load cycles. The results show that the footing performance due to cyclic loading is better for thicker geogrid reinforced sand with a void than for unreinforced sand with no void. In addition, a critical region was found to exist under the footing, under which a void results in increased footing settlement. Overall, the results indicate that the reinforced soil-footing systems with sufficient geogrid-reinforcement and sufficient void embedment depth behave much more stiffly and are thus capable of handling greater loads with lower settlement than those in unreinforced soil without a void. The undesirable effect of the void on the footing behavior can be eliminated. In addition, the results show that the values of footing settlement increase rapidly during the initial loading cycles; thereafter the rate of settlement is reduced significantly as the number of loading cycles increases.     

Effects of transverse members on geogrid pullout behavior considering rigid and flexible top boundaries

Geogrid pullout tests have been regarded as the most direct and effective way to describe the interfacial behavior between geogrid and soil. To investigate the coupled effects of geogrid transverse members and top-loading boundaries on the geogrid-soil interaction, numerical simulations of geogrid pullout tests using the Discrete Element Method (DEM) were carried out in this study. The rigid top boundary was simulated by a rigid wall, while the flexible top boundary was modeled with a string of bonded particles that could rotate and move up and down freely. The coupled effects of geogrid transverse members and top boundary conditions on the geogrid-soil interaction under pullout loads were visualized not only by the force distributions along the geogrids and in the specimens but also by the displacements of soil particles and geogrids. Additionally, the quantitative geogrid force and strain distributions along the geogrids, the lateral force distributions on the front walls, and the vertical displacements of top boundaries also showed the influence of transverse members on the geogrid pullout behavior considering the rigid and flexible top boundaries. The DEM investigation results of this study may provide helpful guidelines for regulating the geogrid pullout test apparatus and methods.     

Modeling of the cyclic behavior of shallow foundations resting on geomesh and grid-anchor reinforced sand

Storage tank foundations with frequent discharges and filling or road embankments under repeated traffic loads are examples of foundations subjected to the cyclic loading with the amplitude well below their allowable bearing capacity. The concern exists for the amount of uniform and non-uniform settlement of such structures. The soil under such foundations may be reinforced with geosynthetics to improve their engineering properties.This paper deals with the effects of using the new generation of reinforcement, grid-anchor, for the purpose of reducing the permanent settlement of these foundations under the influence of proportion of the ultimate load. Unloading-reloading field tests were performed to investigate the behavior of a square footing on the sand reinforced with this system under such loads. The effects of footing size and reinforcement types on the cyclic behavior of the reinforced sand were studied experimentally and numerically by the aid of computer code. The large-scale results show that by using the grid-anchors, the amount of permanent settlement decreases to 30%, as compared with the unreinforced condition. Furthermore, the number of loading cycles reaching the constant dimensionless settlement value decreases to 31%, compared with the unreinforced condition. Another goal of this paper is to present the equations for reinforced soil under cyclic loading to prevent such complicated calculation involved in deformation analysis. According to these equations, calculation of the permanent settlement and the number of load cycles to reach this amount for each foundation with a given size on the geomesh and grid-anchor reinforced sand, without further need to carry out the large-scale test, is supposed to perform easily.     

Mehrlagig mit Geogittern bewehrte Erdkörper über pfahlartigen Gründungselementen – Numerische Simulation des Verformungsverhaltens unter statischer und zyklischer Einwirkung

Numerical simulation of the deformation behaviour of multi‐layered geogrid‐reinforced embankments on pile foundations under static and cyclic loading. Embankments for traffic constructions above soft soil are often founded on piles and geogrids are inserted at the bottom of the embankment. In the framework of present design procedures the cyclic (dynamic) traffic loads are considered in a very simplified manner. They are replaced by a static load with a magnification factor. The established model perception for static loading is a redistribution of stress due to arches in the embankment and tensile stress in the geogrids. However it has to be expected that the load bearing and deformation behaviour of such soil structures will change during the life time of the structure (millions of cycles). The cycles cause an accumulation of deformations and changes of stresses in the soil. This may cause a large destruction of the arches and may lead to unexpected settlements. Numerical strategies and constitutive models for the investigation of the behaviour of soils under high‐cyclic loading using finite element method were recently developed. This paper presents the results of such calculations of multi‐layered geogrid‐reinforced embankments on soft soil for the 2D case. The results show that, depending on the position of the geogrids in the embankment, their contribution is unequally to the bearing behaviour and that the stress arches will actually be destroyed under cyclic loading.     

A shakedown limit calculation method for geogrid reinforced soils under moving loads

A method to calculate the elastic shakedown limit of transportation systems (e.g. pavements and railways) supported by geogrid reinforced soils is presented. For the first time, lower-bound shakedown theory is combined with a strength-based geogrid simulation approach, resulting in a rapid method to quantify the benefit of geogrids on the elastic shakedown limit. It allows decoupling of elastic stress generation and shakedown calculations, meaning it is straightforward to implement, and requires minimal computational effort. Therefore it presents a useful tool to optimise geogrid design for transportation structures such as highway pavements and railways. To show the capability of the method, shakedown limits are calculated for a variety of geogrid configurations using elastic stresses induced by a moving Hertz load. The effect of geogrid depth, soil cohesion, soil friction angle and loading type (normal versus tangential) are investigated for reinforced and non-reinforced soils. It is found that the optimum depth is sensitive to the soil strength properties. Regarding loading, it is shown that for highly tangential loads, shallower geogrids are effective, while for loads with a minimal tangential component, deeper geogrids are effective.     

Bearing capacity of strip footings on sand slopes reinforced with geogrid and grid-anchor

This paper presents the effect of a new type of geogrid inclusion on the bearing capacity of a rigid strip footing constructed on a sand slope. A broad series of conditions, including unreinforced cases, was tested by varying parameters such as geogrid type, number of geogrid layers, vertical spacing and depth to topmost layer of geogrid. The results were then analyzed to find both qualitative and quantitative relationships between the bearing capacity and the geogrid parameters. A series of finite element analyses was additionally carried out on a prototype slope and the results were compared with the findings from the laboratory model tests and to complete the results of the model tests. The results show that the bearing capacity of rigid strip footings on sloping ground can be intensively increased by the inclusion of grid-anchor layers in the ground, and that the magnitude of bearing capacity increase depends greatly on the geogrid distribution. It is also shown that the load-settlement behavior and bearing capacity of the rigid footing can be considerably improved by the inclusion of a reinforcing layer at the appropriate location in the fill slope. The agreement between observed and computed results is found to be reasonably good in terms of load-settlement behavior and optimum parameters.     

土工格栅加筋拓宽路堤有限元分析

采用二维非线性有限元法对软土地基上的拓宽路堤土工格栅加筋作用和效果进行模拟分析。计算中采用一维抗拉单元来模拟土工格栅,采用接触面单元考虑筋土界面的状态非线性。研究土工格栅嵌入老路基长度、格栅设置层数对格栅拉力、拓宽路堤不均匀沉降和水平位移的影响。计算结果表明,格栅嵌入老路基长度越长,拓宽路堤侧向位移和新老路基差异沉降越小,增加格栅层数并不会显著减小拓宽路堤的差异沉降。     

Post-construction performance of a two-tiered geogrid reinforced soil wall backfilled with soil-rock mixture

There have been very few studies on the application of soil-rock mixtures as the backfills of geogrid reinforced soil retaining walls with due concern for their long-term performance and safety. In this study, a 17-m high two-tiered reinforced soil wall backfilled with soil-rock mixture was instrumented for its performance under gravity load after construction. The instrumentation continued for 15 months. It is found that soil-rock mixtures with small rock content (<30%) have the potential to be used as the backfill materials of geogrid-reinforced retaining walls, but special attentions should be given to compaction quality, backfill–geogrid interaction, and installation damage to geogrids. Reinforcement slippage is possible because of the large particles, but it was small in this case and ceased to develop nine months after the end of construction. Compressibility difference between reinforced and unreinforced backfill might led to rotation of the upper tier. Using the estimated soil strength, the predictions of reinforcement loads by the FHWA methods were 100% higher than the estimated ones from measured strains.     

Experimental study on uplift behavior of shallow anchor plates in geogrid-reinforced soil

Geogrid reinforcement can significantly improve the uplift bearing capacity of anchor plates. However, the failure mechanism of anchor plates in reinforced soil and the contribution of geogrids need further investigation. This paper presents an experimental study on the anchor uplift behavior in geogrid-reinforced soil using particle image velocimetry (PIV) and the high-resolution optical frequency domain reflectometry (OFDR). A series of model tests were performed to identify the relationship between the failure mechanism and various factors, such as anchor embedment ratio, number of geogrid layers, and their location. The test results indicate that soil deformation and the uplift resistance of anchor plates are substantially influenced by anchor embedment ratio and location of geogrids, whereas the number of geogrid layers has limited influence. In reinforced soil, increasing the embedment ratio greatly improves the ultimate bearing capacities of anchor plates and affects the interlock between the soil and geogrids. As the embedment depth increases, the failure surfaces gradually change from a vertical slip surface to a bulb-shaped surface that is limited within the soil. The strain monitoring data shows that the deformations of geogrids are symmetrical, and the peak strains of geogrids can characterize the reinforcing effects.     

静动载下筋材变化对加筋土挡墙性能影响分析

为了研究动静荷载下,加筋长度及筋材类型变化对加筋土挡墙工作性能的影响,进行了7种工况下的加筋土挡墙模型试验,对比分析了加筋土挡墙的水平土压力、水平土压力系数、墙面水平位移和加载板竖向沉降及筋材应变等参数的发展规律。试验结果表明:动载下加筋土挡墙筋材应变随着加载时间的增长、加筋长度的减小、位置高度的增加而增大,且顶层筋材应变远远大于其他层;加筋长度及筋材横肋的减少明显降低挡墙的承载性能,格栅横肋减少导致挡墙极限承载力降低18% ,加筋长度减少使面板水平位移最大增大了2.2倍;与静载作用下相比,动载下土工格栅的侧向约束作用及网兜效应能够得到更好地发挥。     

多层加筋垫层刚性桩网复合地基的承载特性

为研究多层加筋垫层刚性桩网复合地基的承载特性,将设置有多层土工格栅的加筋垫层视为大挠度薄板进行分析,运用层合板理论,模拟多层土工格栅与碎石垫层之间的相互作用,建立加筋垫层抗弯刚度矩阵的计算方法。考虑刚性桩网复合地基的三维应力和位移边界条件,根据静力平衡条件,建立加筋垫层应力函数和挠度微分控制方程,并利用伽辽金方法进行求解。在此基础上,利用Winkler地基梁理论和大挠度薄板理论对桩土应力比和格栅拉力进行计算。最后,运用实际工程对计算方法进行验证,并综合分析格栅总层数、铺设间隔和位置等因素对桩土应力比及格栅拉力的影响。研究结果表明:理论计算结果与实测结果较为吻合;随着格栅总层数的增大,桩土应力比增大而格栅拉力降低,铺设2～3层格栅效率最高;随着铺设格栅间隔和底层格栅距桩帽距离的增大,桩土应力比降低,而格栅拉力增大。     

Load-settlement response of shallow square footings on geogrid-reinforced sand under cyclic loading

To study the settlement and dynamic response characteristics of shallow square footings on geogrid-reinforced sand under cyclic loading, 7 sets of large scale laboratory tests are performed on a 0.5?m wide square footing resting on unreinforced and geogrid reinforced sand contained in a 3?m?×?1.6?m?×?2?m (length?×?width?×?height) steel tank. Different reinforcing schemes are considered in the tests: one layer of reinforcement at the depth of 0.3B, 0.6B and 0.9B, where B is the width of the footing; two and three layers of reinforcement at the depth and spacing both at 0.3B. In one of the two double layered reinforcing systems, the reinforcements are wrapped around at the ends. The footings are loaded to 160?kPa under static loading before applying cyclic loading. The cyclic loadings are applied at 40?kPa amplitude increments. Each loading stage lasts for 10?min at the frequency of 2?Hz, or until failure, whichever occurs first. The settlement of the footing, strain in the reinforcement and acceleration rate in the soil have been monitored during the tests. The results showed that the ultimate bearing capacity of the footings was affected by the number and layout of the reinforcements, and the increment of bearing capacity does not always increase with the number of reinforcement layers. The layout of the reinforcement layers affected the failure mechanisms of the footings. Including more layers of reinforcement could greatly reduce the dynamic response of the foundations under cyclic loading. In terms of bearing capacity improvement, including one layer of reinforcement at the depth of 0.6B was the optimum based on the test results. It is found that fracture of geogrid could occur under cyclic loading if the reinforcement is too shallow, i.e. for the cases with the first layer of reinforcement at 0.3B depth.     

Load eccentricity effects on behavior of circular footings reinforced with geogrid sheets

In this paper,an experimental study for an eccentrically loaded circular footing,resting on a geogrid reinforced sand bed,is performed.To achieve this aim,the steel model footing of 120 mm in diameter and sand in relative density of 60%are used.Also,the effects of depth of first and second geogrid layers and number of reinforcement layers(1-4) on the settlement-load response and tilt of footing under various load eccentricities(0 cm,0.75 cm,1.5 cm,2.25 cm and 3 cm) are investigated.Test results indicate that ultimate bearing capacity increases in comparison with unreinforced condition.It is observed that when the reinforcements are placed in the optimum embedment depth(u/D = 0.42 and h/D = 0.42),the bearing capacity ratio(BCR) increases with increasing load eccentricity to the core boundary of footing,and that with further increase of load eccentricity,the BCR decreases.Besides,the tilt of footing increases linearly with increasing settlement.Finally,by reinforcing the sand bed,the tilt of footing decreases at 2layers of reinforcement and then increases by increasing the number of reinforcement layers.     

Influence of geosynthetic stiffness on bearing capacity of strip footings seated on thin reinforced granular layers over undrained soft clay

Thin granular fill layers are routinely used to aid the construction of shallow footings seated over undrained soft clay foundations and to increase their load capacity. The influence of time- and strain-dependent reduction in reinforcement stiffness on the bearing capacity and load-settlement response of a footing seated on a thin reinforced granular fill layer over undrained soft clay foundations is examined in this paper using finite-difference method (FDM) numerical models. The time- and strain-dependent stiffness of the reinforcement described by a two-component hyperbolic isochronous tensile load-strain model is shown to influence the bearing capacity and load-settlement response of the reinforced granular base scenario. The additional benefit of a reinforced granular layer diminishes as the time-dependent stiffness of the geosynthetic reinforcement increases. An analytical solution for the ultimate bearing capacity of strip footings seated on thin unreinforced and reinforced granular layers over undrained clay is proposed in this study. The main practical outcome from this study are tables of bearing capacity factors to be used with the analytical solution. The bearing capacity factors were back-calculated from the numerical analyses and account for the influence of rate-dependent properties of geogrid reinforcement materials and clay foundations with soft to very soft undrained shear strength.