Centrifuge modeling of geotextile-reinforced slopes subjected to differential settlements

Today, geosynthetic-reinforced soil structures are widely used to support bridge abutments and approach roads in place of traditional pile supports and techniques. In such situations, foundation conditions have been shown to adversely affect the stability and deformation behaviour of overlying geosynthetic-reinforced slopes and walls. This paper addresses the response of geotextile-reinforced slopes subjected to differential settlements in a geotechnical centrifuge. Centrifuge model tests were carried out on model geotextile-reinforced sand slopes with two different types of reinforcement. A wrap-around technique was used to represent a flexible facing. In order to initiate failure in the reinforcement layers, the ratio of length of reinforcement to height of the slope was maintained as 0.85. One of the objectives of this paper is to present about a special device developed for inducing differential settlements during centrifuge test at 40g for a reinforced soil structure. A digital image analysis technique was employed to arrive at displacement vectors of markers glued to the reinforcement layers. The displacements were used to compute and analyze the strain distribution along the reinforcement layers during different settlement stages. Results of the centrifuge test indicate that even after inducing a differential settlement equivalent to 1.0 m in prototype dimensions, the geotextile-reinforced soil structure with a flexible facing was not found to experience a collapse failure. Analysis of geotextile strain results shows that the location of the maximum peak reinforcement strain occurs along the bottom-most reinforcement layer at the onset of differential settlements, at the point directly below the crest of the slope.     

Centrifuge modeling of the geotextile reinforced slope subject to drawdown

Geotextile is an effective reinforcement approach of slopes that experiences various loads such as drawdown. The geotextile reinforcement mechanism is essential to effectively evaluate the safety of geotextile-reinforced slopes under drawdown conditions. A series of drawdown centrifuge model tests were performed to investigate the deformation and failure behaviors of slopes reinforced with different geotextile layouts. The deformation and failure of unreinforced and reinforced slopes were compared and the geotextile reinforcement was indicated to significantly increase the safety limit and the ductility, reduce the displacement, and change the failure feature of slopes under drawdown conditions. The slopes exhibited remarkable progressive failure, downward from the slope top, under drawdown conditions. The progressive failure was induced by coupling of deformation localization and local failure based on full-field measurements of displacement of slopes subjected to drawdown. The geotextile reinforced the slope by decreasing and uniformizing the slope deformation by the soil-geotextile interaction. Through geotextile displacement analysis, the geotextile-reinforced slope was divided into the anchoring zone and the restricting zone by a boundary that was independent of the decrease of water level. The geotextile restrained the soil in the anchoring zone and the soil restrained the geotextile in the restricting zone. The reinforcement effect was distinct only when the geotextile was long enough to cross the slip surface of the unreinforced slope under drawdown conditions.     

Behaviour of footings on reinforced sand subjected to repeated loading – Comparing use of 3D and planar geotextile

This paper describes a series of laboratory model tests performed on strip footings supported on 3D and planar geotextile-reinforced sand beds under a combination of static and repeated loads. Footing settlement due to initial static applied load and up to 20,000 subsequent load repetitions was recorded, until its value becomes stable or failure occurred due to excessive settlement. The response under the first few cycles was found to be a significant behavioral characteristic of footings under repeated loads. The influence of various amplitudes of repeated load on foundations containing different numbers of planar geotextile layers and different heights of the 3D geotextile reinforcement were investigated. Most of the observed responses show plastic shakedown developing – that is a stable, resilient response is observed once incremental plastic strains under each load repetition have ceased to accumulate. The results show that the maximum footing settlement due to repeated loading is comparable for either planar- or 3D-reinforced sand and much improved over the settlement of unreinforced sand. The efficiency of reinforcement in reducing the maximum footing settlement was decreased by increasing the mass of reinforcement in the sand. On the whole, the results indicate that, for the same mass of geotextile material used in the tests, the 3D geotextile reinforcement system behaves more effectively than planar reinforcement as a retardant for the effects of dynamic loading. Thus, a specific improvement in footing settlement can be achieved using a lesser quantity of 3D geotextile material compared to planar geotextile.     

An experimental evaluation of the behavior of footings on geosynthetic-reinforced sand

This research was performed to investigate the behavior of geosynthetic-reinforced sandy soil foundations and to study the effect of different parameters contributing to their performance using laboratory model tests. The parameters investigated in this study included top layer spacing, number of reinforcement layers, vertical spacing between layers, tensile modulus and type of geosynthetic reinforcement, embedment depth, and shape of footing. The effect of geosynthetic reinforcement on the vertical stress distribution in the sand and the strain distribution along the reinforcement were also investigated. The test results demonstrated the potential benefit of using geosynthetic-reinforced sand foundations. The test results also showed that the reinforcement configuration/layout has a very significant effect on the behavior of reinforced sand foundation. With two or more layers of reinforcement, the settlement can be reduced by 20% at all footing pressure levels. Sand reinforced by the composite of geogrid and geotextile performed better than those reinforced by geogrid or geotextile alone. The inclusion of reinforcement can redistribute the applied footing load to a more uniform pattern, hence reducing the stress concentration, which will result reduced settlement. Finally, the results of model tests were compared with the analytical solution developed by the authors in previous studies; and the analytical solution gave a good predication of the experimental results of footing on geosynthetic reinforced sand.     

Centrifuge modeling of geotextile-reinforced cohesive slopes

Geosynthetics are widely used to reinforce slopes due to their successful performance and economical efficiency. A series of centrifuge model tests was conducted in order to investigate the behavior of the geotextile-reinforced cohesive slopes and to compare their behavior to unreinforced slopes. The displacement history of the slopes was measured using an image analysis system. The failure process of an unreinforced slope can be categorized into three stages: (1) uniform deformation stage; (2) strain localization stage; and (3) post-failure stage. The geotextile has a significant effect on the deformation of the slope and increases the stability level while affecting the failure modes. On a reinforced slope, two surfaces can result from the distribution of the displacement difference between the unreinforced and the corresponding reinforced slopes; thus, the slope can be categorized into three zones. The front zone is characterized as a restricted region that is subjected to a backward tension via the geotextile while the middle zone is mainly subjected to a forward tension (like a support body). The back zone is unaffected by the geotextile. The reinforcement can take effect when its length is longer than the effective reinforcement length. The effective reinforcement length usually increases with increasing elevation and is significantly affected by the inclination of the slope.     

Centrifuge model study on low permeable slope reinforced by hybrid geosynthetics

The objective of this paper is to study the performance of hybrid geosynthetic reinforced slopes, with permeable geosynthetic as one of its components, for low permeable backfill slopes subjected to seepage. Four centrifuge tests have been performed to study the behavior of hybrid geosynthetic reinforced slopes subjected to seepage, keeping the model slope height and vertical spacing of geosynthetic reinforcement layers constant. Centrifuge model tests were performed on 2V:1H slopes at 30 gravities. One unreinforced, one model geogrid reinforced and two hybrid geosynthetic reinforced slope models with varying number of hybrid geosynthetic layers were tested. The effect of raising ground water table was simulated by using a seepage flow simulator during the flight. Surface movements and pore water pressure profiles for the slope models were monitored using displacement transducers and pore pressure transducers during centrifuge tests. Markers glued on to geosynthetic layers were digitized to arrive at displacement vectors at the onset of raising ground water table. Further, strain distribution along the geosynthetic reinforcement layers and reinforcement peak strain distribution have been determined using digital image analysis technique. The discharge for the performed model tests is determined by performing seepage analysis. It was confirmed by the centrifuge tests that the hybrid geosynthetics increases the stability of low permeable slope subjected to water table rise. The hybrid geosynthetic layers in the bottom half of the slope height play a major role in the dissipation of pore water pressure.     

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。     

Finite element limit analysis of load-bearing performance of reinforced slopes using a non-associated flow rule

This paper presents a numerical study on the load-bearing performance of reinforced slopes under footing load using a finite element limit analysis (FELA) method where a non-associated flow rule is assumed in the analysis. The method was validated against results from full-scale model tests and a limit equilibrium (LE) analytical method. A series of parametric analyses was subsequently carried out to examine the influences that the soil dilation angle, footing location, and reinforcement design (i.e. length, tensile strength, and vertical spacing) could have on the load-bearing performance of reinforced slopes. Results indicate that dilation angle has a significant influence on the predicted magnitudes of bearing capacity, slope deformation, and mobilized reinforcement load. The predicted values of bearing capacity using the FELA are smaller than those from the Meyerhof's analytical method for unreinforced semi-infinite foundation, especially for larger friction angle values. Additionally, the ultimate bearing capacity of the slope and its corresponding horizontal deformation increase with the reinforcement tensile strength. Finally, the slip planes under the applied footing load are found to be y-shaped and primarily occur in the upper half of the slope.     

土工格栅加筋边坡坡顶条基极限荷载的预测

通过土工合成材料加固的边坡,承载能力显著提高,因而获得广泛应用。为了合理的评价加筋边坡的坡顶条形基础的极限荷载,制作了足尺寸模型并进行了试验,采用延性较好但强度较低的聚丙烯(PP)土工格栅对边坡进行了加固,在坡顶通过条形基础(钢梁)施加荷载直至边坡破坏,获得了极限荷载以及边坡的变形和破坏规律,通过细致的测试手段,详细地捕捉到模型的力学响应。在此基础上,通过校验的FLAC数值模型,对土工格栅加筋边坡的承载能力进行了预测,得到了满意的结果。     

Shaking table tests on gravel slopes reinforced by concrete canvas

The behaviour and performance of different reinforced slopes during earthquake loading were investigated through a series of shaking table tests. Concrete-canvas and composite reinforcement (geogrid attached to concrete-canvas) were proposed for reinforcing slopes. By considering the effects of different reinforcement methods, the seismic responses of the reinforced slopes were analysed, along with the accelerations, crest settlements, and lateral displacements. The failure patterns of different model slopes were compared using white coral sand marks placed at designated elevations to monitor the internal slide of the reinforced slopes. Both the concrete-canvas and composite reinforcement could increase the safety distance, which ranged from the slide-out point to the back of the model box. The composite reinforcement decreased the volume of the landslide and increased the failure surface angle as a result of the larger global stiffness in the reinforced zone. These results indicate that the recently developed concrete canvas has a better effect on restricting the slope deformation during seismic loading than the nonwoven geotextile reinforcement, and that the use of composite reinforcement could improve the seismic resistance of slopes.     

Visualisation and quantification of geogrid reinforcing effects under strip footing loads using discrete element method

Geogrids have been commonly used in reinforced soil structures to improve their performance. To investigate the geogrid reinforcement mechanisms, discrete element modelling of unreinforced and geogrid reinforced soil foundations and slopes was conducted under surface strip footing loads in this study. For unreinforced and reinforced soil foundations, the numerically obtained footing pressure-settlement relationships were validated by experimental results from the literature. In the numerical modelling of unreinforced and reinforced soil slopes, identical models and micro input parameters to those used in the numerical modelling of unreinforced and reinforced soil foundations were used. The geogrid reinforcing effects under strip footing loads were visualised by the qualitative contact force distributions in the soil structures, as well as the qualitative and quantitative tensile force distributions along the geogrids. In addition, the qualitative displacement distributions of soil particles in the soil structures and the quantitative vertical displacement distributions along soil layers/geogrids also indicated the geogrid reinforcing effects in such practical reinforced soil structures. The discrete element modelling results visualise and quantify the load transfer and spreading behavior in geogrid reinforced soil structures, and it provides researchers with an improved understanding of geogrid reinforcing effects at microscopic scale under strip footing loads.     

Comparison of bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement

Comprehensive results from laboratory model tests on strip footings supported on the geocell and planar reinforced sand beds with the same characteristics of geotextile are presented. The various parameters studied in this testing program include the reinforcement width, the number of planar layers of geotextile and height of the geocell below the footing base. Contrary to other researches, the performance of the geocell and planar reinforcement is investigated at the range of low to medium settlement level, similar to those of interest in practice. The results show that the efficiency of reinforcement was decreased by increasing the number of the planar reinforcement layers, the height of the geocell reinforcement and the reinforcement width. For the same mass of geotextile material used in the tests at the settlement level of 4%, the maximum improvement in bearing capacity (IF) and percentage reduction in footing settlement (PRS) were obtained as 2.73 and 63% with the provision of geocell, respectively, while these values compare with 1.88 and 47% for the equivalent planar reinforcement. On the whole, the results indicate that, for the same quantity of geotextile material, the geocell reinforcement system behaves much stiffer and carries greater loading and settles less than does the equivalent planar reinforcement system. Therefore, a specified improvement in bearing pressure and footing settlement can be achieved using a lesser quantity of geocell material compared to planar geotextile.     

Behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing

In the past, the beneficial effects of prestressing the geosynthetic in reinforced soil foundations have been studied mathematically. It is timely to experimentally investigate the degree of improvement generated by prestressing the geosynthetic layer for several embedment depths of a footing resting on a reinforced sand bed. Therefore, laboratory physical model tests and finite element analyses were conducted to study the behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing. The addition of prestress to the geotextile reinforcement results in significant improvement to the settlement response and the load-bearing capacity of the foundation. For a surface footing, the load-carrying capacity at 5 mm settlement for the prestressed case (with prestress equal to 2% of the allowable tensile strength of the geotextile) is approximately double that of the geotextile-reinforced sand without prestress. The beneficial effects of the prestressed geotextile configuration were evident for greater footing depths, in comparison with unreinforced and reinforced (without prestress) counterparts. Experimental and numerical results were also used to validate a few empirical relationships, which are commonly used for solving soil-structure interaction problems. The results obtained from finite element analysis using the program, PLAXIS are generally found to be in reasonabaly good agreement with experimental results.     

Centrifuge model tests of geotextile-reinforced soil embankments during an earthquake

The behavior of geotextile-reinforced embankments during an earthquake was investigated using centrifuge model tests, considering a variety of factors such as gradient of slope, water content of soil, geotextile spacing, and input shaking wave. The geotextile-reinforcement mechanism was revealed on the basis of the observations with comparison of the unreinforced embankment. The geotextile significantly decreases the deformation of the embankment and restricts sliding failure that occurs in the unreinforced embankment during an earthquake. The displacement exhibits an evidently irreversible accumulation with a fluctuation during the earthquake which is significantly dependent on the magnitude of input shaking. The peak strain of the geotextile exhibits a nearly triangular distribution in the vertical direction. The embankment can be divided into two zones, a restricting zone and restricted zone, where the soil and geotextile, respectively, play an active restriction role in the soil-geotextile interaction. The soil restricts the geotextile in the restricting zone, and this restriction is transferred to the restricted zone through the geotextile. The strain magnitude of the geotextile and the horizontal displacement of the geotextile-reinforced embankment decrease with increasing geotextile layers, with decreasing water content of the soil, with decreasing gradient of the slope, and with decreasing amplitude of the earthquake wave.     

A reinforcing method for steep clay slopes using a non-woven geotextile

The effect of non-woven geotextile reinforcement on the stability and deformation of two clay test embankments is examined based on their performance for about 3 years for the first embankment and about years for the other. Horizontal planar sheets of a non-woven geotextile are expected to work in three ways: for compaction control; for drainage; for tensile reinforcement. The degree of stability of the steep slopes of the test embankments decreased during heavy rainfall. It is found that the use of non-woven geotextile reinforcement may effectively improve embankment performance. Only the stability analysis in terms of effective stresses can explain the performance of the test embankments. The horizontal creep deformation of the embankments during 2–3 years, which is partly attributed to the creep deformation of the non-woven geotextile, was found to be small. The results of both laboratory bearing capacity tests of a strip footing on a model sand ground reinforced with the non-woven geotextile and plane strain compression tests on sand specimens reinforced with the non-woven geotextile show that the non-woven geotextile gives tensile reinforcement to soils.     

砂土地基中加筋深度效果研究

利用非线性弹塑性有限元对具有不同加筋层数砂土地基的一系列模型试验结果进行了较为全面的计算与分析。有限元解析中所采用的砂土本构模型以修正塑性应变能量为硬软化基本参量,它可以较为精确地模拟砂土的应力路径效果。作为解析对象的模型地基由干燥的日本丰浦砂组成,不同层数的加筋材平铺在条形基础下方。结果表明,利用这种精度较高的有限元解析方法对加筋砂土地基进行解析,可以较好地再现由于加筋层数(加筋深度)变化而带来的对承载力与变形的影响。尽管加筋宽度与基础宽度相同,但是随着加筋层数的增多,加筋砂土地基的承载力明显增大,加筋深度效果明显优于加筋宽板效果。另外,利用以上的有限元分析,也能合理地模拟砂土地基渐进性剪切破坏的现象以及加筋材的诱发引张内力,可以更加合理地认识和理解砂土地基中加筋深度补强机理。     

Investigation of geotextile–soil interaction under a cyclic vertical load using the discrete element method

Geotextiles are often used in roadway construction as separation, filtration, and reinforcement. Their performance as reinforcement in geotextile-reinforced bases depends on geotextile–soil interaction. This paper investigates the geotextile–soil interaction under a cyclic wheel load using the Discrete Element Method (DEM). In this study, soil was modeled as unbonded particles using the linear contact stiffness model, and the geotextile was modeled as bonded particles. The micro-parameters of the soil and the geotextile were determined using biaxial tests and a tensile test, respectively. The influence of the placement depth and the stiffness of the geotextile on the performance of the reinforced base was investigated. The DEM results show that the depth of the geotextile significantly affected the degree of interaction between the geotextile and the soil. Under the applied cyclic vertical load, the geotextile developed a low tensile strain. The effect of the stiffness of the geotextile on the deformation was more significant when the geotextile was placed at a shallower location than when placed at a deeper location.     

Clay—Geotextile interaction in large retaining wall models

Large geotextile reinforced clay wall models were built to investigate the mechanism of clay—geotextile interaction and the effects of the geotextile reinforcement on the load-bearing capacity of the clay. A silty clay soil (CL) with an undrained strength of 25 kPa was used as backfill and a low-cost, non-woven, needle-punched geotextile as the reinforcement. No face panels were used. The wrapped back geotextile reinforcement provided the face of the wall. The wall models were tested under uniformly distributed and discrete strip loads. Vertical and horizontal displacements as well as geotextile strains were monitored. The load-bearing capacity of the clay was increased nearly two times with the geotextile reinforcement. For the interpretation of the test results total stress analysis was carried out on the active failure plane taking into account the tensile forces acting in the geotextiles reinforcing layers intersecting the failure plane. Good agreement was found between the measured and the calculated failure loads. The results of the testing programme are promising and encourage further research into the applicability of cohesive soils in geotextile-reinforced soil structures which might have great economic significance in areas where good-quality granular backfill is not readily available.     

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。     

加筋土坡的可能滑移模式和基于库仑理论的稳定分析方法

加筋土坡因其填方量少、工期短、经济安全等优势在国内外已得到广泛应用,故其稳定性分析也显得尤为重要。目前,已有多位学者将极限分析上 限定理运用于加筋土坡的稳定性分析中,并假定水平条块速度间断面。然而在其分析过程中,构造的速度场并不符合位移协调条件。提出了由于筋材的隔断 作用形成的斜向界面破坏模式,并计算相应速度场式,分别提出主动、库仑、被动三种滑移模式。通过计算比较发现,在加筋间距较密时,库仑模式总是相 应安全系数最小的控制工况。实际工作中,可以只使用这一概念清晰、计算简便的方法分析加筋土坡的稳定性。为方便工程设计快速获取安全系数或筋材间 距,将土坡各参数进行无量纲化,绘制了安全系数图,并与Michalowski设计图表进行对比,验证了该算法的有效性。且针对多个实际工程算例进行验算, 验证了库仑模式上限法的可行性。