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.     

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 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 model tests on the behavior of strip footing on geotextile-reinforced slopes

This paper examines the stability of geotextile-reinforced slopes when subjected to a vertical load applied to a strip footing positioned close to the slope crest. Vertical spacing between geotextile reinforcement was varied while maintaining a constant slope angle, load position, soil density and geotextile type. Small-scale physical tests were conducted using a large beam centrifuge to simulate field prototype conditions. After the model was accelerated to 40g, a load was applied to the strip footing until slope failure occurred. Digital image analysis was performed, using photographs taken in-flight, to obtain slope displacements and strain distribution along the reinforcement layers at different loading pressures during the test and at failure. Stability analysis was also conducted and compared with centrifuge model test results. The vertical spacing between reinforcement layers has a significant impact on the stability of a reinforced slope when subjected to a vertical load. Less vertical distance between reinforcement layers allows the slope to tolerate much greater loads than layers spaced further apart. Distributions of peak strains in reinforcement layers due to the strip footing placed on the surface of the reinforced slope were found to extend up to mid-height of the slope and thereafter they were found to be negligible. Stability analysis of the centrifuge models was found to be consistent with the observed performance of geotextile-reinforced slopes subjected to loading applied to a strip footing near the crest.     

土钉加固黏性土坡动力离心模型试验研究

 很多滑坡是由地震引发的,为了防止或减轻地震造成的边坡灾害,目前在边坡的加固治理方面已经发展并形成一些较好的方法,而土钉是边坡抗震加固的一种简便有效的方法。采用动力离心模型试验方法,再现地震条件下土钉加固黏性土坡和素土坡的响应;测量了试验过程中边坡的位移场和加速度响应的变化过程。基于试验结果,通过对比素土坡和土钉加固土坡的动力响应,探讨土钉加固土坡的变形规律和加固机制。试验结果表明,地震过程中土坡产生不可恢复的累积变形,其大小与输入的地震加速度峰值有关。通过比较土钉加固土坡和素土坡的位移分布,研究土钉加固土坡的机制。引入土单元应变进行分析,结果表明,土钉加固措施能显著地改变边坡的位移场分布,限制土坡的剪切变形,避免滑裂面的产生,从而提高了边坡的稳定性。     

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.     

Centrifuge model tests on pile-reinforced slopes subjected to drawdown

Piles are generally an effective way to reduce the risk of slope failure. However, previous approaches for slope stability analysis did not consider the effect of the piles coupled with the decrease of the water level (drawdown). In this study, a series of centrifuge model tests was performed to understand the deformation and failure characteristics of slopes reinforced with various pile layouts. In the centrifuge model tests, the pile-reinforced slopes exhibited two typical failure modes under drawdown conditions: across-pile failure and through-pile failure. In the through-pile slope failure, a discontinuous slip surface was observed, implying that the assumption of the slip surface in previous stability analysis methods was unreasonable. The test results showed that drawdown led to instability of the piles in cohesive soil, as the saturated cohesive soil failed to provide sufficient constraint for piles. The slope exhibited progressive failure, from top to bottom, during drawdown. The deformation of the piles was reduced by increasing the embedment depth and row number of piles. In addition, the deformation of soils outside the piles was influenced by the piles and showed a similar distribution shape as the piles, and the similarity degree weakened as the distance from the piles increased. This study also found that the failure mechanism of unreinforced and pile-reinforced slopes induced by drawdown could be described by coupling between the deformation localization and local failure, and it revealed that pile-reinforced slopes could reduce slope deformation localization during drawdown.     

水位下降条件下土坡破坏的离心模型试验研究

水位下降引起土坡破坏的规律机理对于发展稳定性分析方法具有重要意义。研制了离心模型试验中超重力场水位升降模拟设备,进行水位下降条件下黏性土坡变形破坏的离心模型试验。根据试验位移测量结果,基于变形与破坏过程集成分析的思路探讨了土坡破坏机理。水位下降导致土坡发生由坡顶向坡脚的渐进性的错动破坏。水位下降条件下土坡的变形和破坏过程是耦合的。变形局部化发展是导致滑裂面出现的根本原因。滑裂面出现后滑动体内部仍发生显著的变形,并与滑裂面上的错动变形相耦合。     

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.     

Failure behavior and mechanism of slopes reinforced using soil nail wall under various loading conditions

Soil–nailing technology is widely applied in practice for reinforcing slopes. A series of centrifuge model tests was conducted on slopes reinforced with a soil nail wall under three types of loading conditions. The behavior and mechanism of failure process of the reinforced slopes were studied using image-based observation and displacement measurements for the slope, nails, and cement layer. The nailing significantly increased the stability level and restricted the tension cracks of the slopes. Increasing the nail length improved the stability of the reinforced slopes with deeper slip surfaces. The reinforced slope exhibited a significant failure process, in which slope slippage failure and cement layer fracture occurred in conjunction with a coupling effect. The deformation localization was induced by the loading within the slope and ultimately developed into a slip surface. The nailing reinforced the slope by significantly delaying the occurrence of the deformation localization within the slope. The failure of nails was recognized as a combination of pull-out failure and bend deformation. The loading conditions were shown to have a significant effect on slope deformation and nail deflection, and they consequently influenced the failure behavior and its formation sequence.     

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。     

库水位升降条件下边坡失稳离心模型试验研究

 三峡库区频发的滑坡地质灾害已愈来愈引起学者的高度重视,其受水位反复升降的影响是库区边坡不同于其他陆地自然边坡的一个显著特点。以离心模型试验为手段,基于三峡库区典型滑坡的工程地质特征,建立相应的土质边坡离心模型。在试验过程中实现对库水位循环升降的控制,模拟库区边坡在水位升降作用下的失稳过程。通过数码摄像、数字图像处理和传感元件测试,获得该试验条件下的土坡在水位升降过程中典型位置的孔隙水压力变化、全断面位移矢量演化(水平位移和沉降)、滑面形态及裂缝形成发展过程,并详细分析边坡在这种外部水环境影响下的变形演化、失稳和破坏模式。试验结果表明,若仅考虑水位升降作用的影响,该试验条件下的库区土质边坡的变形呈现典型的渐进牵引破坏模式,并具备较强的水土软化影响特征;裂缝在变形演化过程中出现交替张开和闭合现象,该失稳模式下的滑面呈折线形态,并在变形破坏过程中次生多级滑面。研究结果为库区滑坡地质灾害机制的深入认识、以及滑坡预防和控制提供了重要依据。     

加筋软岩粗粒土路堤填料大型三轴试验研究

 为研究加筋粗粒土填料的强度变形特性及加筋效果,进行加筋强风化软岩粗粒土固结不排水和固结排水大三轴试验。试验表明：加筋填料的应力–应变关系表现为应变硬化型;轴向应变较小(ea＜1%)时,加筋填料效果不明显,随着轴向应变的逐渐增大加筋效果逐渐发挥。加筋填料的孔隙水压力均高于素填料,随着加筋层数的增加均有不同程度的提高。加筋效果系数均＞1.0,一层加筋填料加筋效果系数为1.09～1.21,二层加筋填料加筋效果系数为1.30～1.71,三层加筋填料加筋效果系数为1.31～1.72。加筋前后填料的内摩擦角j基本不变,填料的黏聚力增大。加筋填料的本构关系可以用Duncan-Chang模型来描述,依据试验结果求得模型参数。     

Evaluation of geosynthetic reinforcement in unpaved road using moving wheel load test

The common cause of failure of the unpaved road is associated with undesirable ruts and deformations. Use of geosynthetic reinforcement is a solution to this pavement distress problem as experienced in limited research works, especially in the laboratory studies. This study presents the performance of geosynthetic-reinforced unpaved roads subjected to moving wheel load tests to investigate the effect of geosynthetic reinforcement on the pavement surface deformation of the unpaved roads. Unreinforced and geosynthetic-reinforced unpaved road test sections consisting of varied reinforcements were constructed in a test pit, 9 m long and 2.7 m wide. Geogrid and geotextile were used for reinforcing the unpaved road test sections. The rut depth was measured in the transverse direction of the wheel path after certain number of wheel passes. Traffic Benefit Ratio (TBR) and Performance Index (PI) were employed in the study for the evaluation of the effectiveness of geosynthetic reinforcement in unpaved roads. After 350 vehicle passes, the geotextile-reinforced and geogrid-reinforced test sections get rutting reduced by 44.89% and 28.57%, respectively. The test results indicate that inclusion of geosynthetic reinforcement significantly improves the rutting resistance and stability of reinforced test sections compared to the unreinforced test sections.     

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.     

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.     

A comprehensive method for analyzing the effect of geotextile layers on embankment stability

Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of these programs consider the tensile strength of geotextiles and soil–geotextile interface friction. However, currently available commercial software generally does not consider the drainage function of nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile strength and soil–geotextile interface friction. The second method also considers the drainage function. In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of the most important conditions with regard to stability of channel embankments. It is shown that for this type of application, modeling a nonwoven geotextile reinforced embankment using commercial software which neglects the drainage function of the geotextile may be unrealistic.     

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.     

抗滑桩加固边坡变形破坏特性离心模型试验研究

抗滑桩加固边坡的增强作用与抗滑桩设置位置、桩头条件等密切相关,通过离心模型试验对不同桩位与桩头条件下采用抗滑桩加固边坡的变形破坏特性进行研究。试验表明,设在边坡中部的抗滑桩在边坡破坏前桩头水平位移最大,而在相同的破坏载荷作用下,设在中部的抗滑桩比设在上、下部以及桩头固定比桩头自由时边坡破坏后的位移矢量要小,这表明抗滑桩设边坡中部以及桩头固定对边坡土体的遮挡效果及边坡安全系数的提高最好。     

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。