Effects of seismic amplification on the stability design of geosynthetic-reinforced soil walls

Many researches of geosynthetic-reinforced soil (GRS) walls under earthquakes demonstrate seismic acceleration amplification along the wall height. Current design methods of GRS walls often neglect the amplification effect on seismic stability and could yield an unconservative result. A pseudo-static method based on limit equilibrium (LE) analyses is carried out to calculate the distribution of required tension of seismic GRS walls following a top-down procedure. The connection load between the reinforcement and facing is correspondingly determined by the front-end pullout capacity. The approach assumes that the horizontal seismic acceleration coefficient varies linearly from the bottom to the top of GRS walls. The obtained results of the required tension involving the seismic amplification are in good agreement with other LE results in previous studies. Parametric studies are conducted to investigate the effects of horizontal seismic coefficient, primary and secondary reinforcement lengths and wall batter on the seismic stability of GRS walls. The seismic amplification yields more required reinforcement tension, significantly for the lower layers of the GRS wall subjected to strong earthquakes. In this situation, lengthening the bottom 1/2 of reinforcement layers could reduce the required tension to avoid tensile breakage of the reinforcements.     

Influence of toe restraint conditions on performance of geosynthetic-reinforced soil retaining walls using centrifuge model tests

Current design regulations most often require use of limit equilibrium methods for the internal stability analyses of geosynthetic-reinforced soil (GRS) walls. However, the limit-equilibrium based approaches generally over-predict reinforcement loads for GRS walls when comparing with measured data from full-scale instrumented walls under working stress conditions. Wall toe resistance has an important influence on the performance of GRS walls but is ignored in limit equilibrium-based methods of design. This paper reports centrifuge modelling of GRS walls which have different toe restraint conditions but are otherwise identical. The GRS wall models prepared in this study isolate the influence of wall toe resistance on the performance of walls. Based on measured data from four centrifuge wall model tests, a reduction in wall toe resistance (by reducing the interface shear resistance at the base of the wall facing or removing the soil passive resistance in front of the wall toe or both) induces larger maximum facing deformation and reinforcement strain and load. The results also demonstrate that the wall models with typical toe restraint conditions are most likely operated under working stress conditions while those with poor toe restraint conditions may experience (or be close to reach) a state of limit equilibrium.     

拉压变轴力下小剪跨比预应力混凝土墙往复受剪试验研究

为了研究预应力混凝土（PC）剪力墙的抗震性能,提出剪力墙在拉压变轴力作用下的水平往复加载试验加载制度,完成3片剪跨比为1.0的预应力混凝土墙在恒定轴拉力、恒定轴压力和拉压变轴力作用下的水平往复加载试验,研究其破坏模式、滞回性能、承载力、变形能力、刚度和残余裂缝宽度,并与型钢混凝土（SRC）墙和普通RC墙的抗震性能进行了对比。试验结果表明：恒定轴拉力试验中,预应力混凝土墙发生了腹板剪切破坏;恒定轴力试验中墙体发生了斜压破坏;拉压变轴力试验中,墙体在压剪方向加载时发生剪压破坏。拉压变轴力加载导致预应力混凝土墙拉剪和压剪承载力分别降低了18.7%和10.5%。预应力混凝土墙在恒定轴拉力和拉压变轴力作用下的极限位移角为1.2%~1.6%,变形能力大于JGJ 3—2010《高层建筑混凝土结构技术规程》规定的弹塑性位移角限值(1/100);恒定轴压力试验中水平峰值荷载超过了墙体截面受剪承载力限值,出现斜压破坏,极限位移角仅为0.6%。预应力混凝土墙试件与SRC墙试件的刚度、承载力和变形能力接近,前者的残余裂缝宽度小于后者的,表现出更好的震后可修复性。由于预应力有效抑制了墙体水平贯通裂缝的形成、防止出现沿水平裂面的滑移破坏,因此在较大轴拉力水平时预应力混凝土墙比普通RC墙的抗侧刚度和承载能力均显著提高。总体来看,预应力混凝土墙抗震性能优良,是一种改善高层建筑中受拉剪力墙抗震性能的有效手段。     

Evaluation of oblique pullout resistance of reinforcements in soil wall subjected to seismic loads

Pullout resistance is one of the most important factors governing seismic stability of reinforced soil walls. The previous studies on the seismic stability of reinforced soil walls have focused on the axial resistance of the reinforcement against the pullout. However, the kinematics of failure causes the reinforcement to be subjected to the oblique pullout force and bending deformation. Considering the kinematics of failure and bending deformation of the reinforcement, this paper presents a pseudo-static seismic analysis for evaluating the pullout resistance of reinforcements in soil wall subjected to oblique pullout forces. A modified horizontal slice method (HSM) and Pasternak model are used to calculate the required force to maintain the stability of the reinforced soil wall and shear resistance mobilized in the reinforcements, respectively. In addition, this paper studies the effect of various parameters on the pullout resistance of the reinforcements in soil wall subjected to seismic loads. Results of this study are compared with the published data and their differences are analyzed in detail.     

纤维增强混凝土剪力墙抗震性能试验研究与理论分析

为根本改善混凝土基体的脆性,提高混凝土剪力墙的抗震性能和损伤容限,设计制作6个局部纤维增强混凝土（FRC）剪力墙试件,在试件变形关键部位采用FRC替代普通混凝土,并考虑高轴压比下剪力墙受压钢筋屈曲和受拉纵筋应力集中的问题,在塑性铰区纵向钢筋上设置钢套管,以改善受力钢筋的稳定性和变形性能。通过对悬臂剪力墙试件的拟静力试验,研究此类剪力墙的破坏现象、受力机理和滞回特性,探讨轴压比、FRC区高度、纵筋强度和钢套管长度等因素对墙体变形能力及耗能能力的影响。研究表明,与普通混凝土剪力墙试件相比,塑性铰区采用FRC的剪力墙试件具有较高的损伤容限和变形能力;提高钢筋强度和延性以及在纵筋上设置钢套管,对其抗震性能和耗能能力均具有明显的改善作用。     

Three-dimensional finite element analysis of geosynthetic-reinforced soil walls with turning corners

The paper presents in-depth three-dimensional finite element analyses investigating geosynthetic-reinforced soil walls with turning corners. Validation of the 3D numerical procedure was first performed via comparisons between the simulated and reported results of a benchmark physical modeling built at the Royal Military College of Canada. GRS walls with corners of 90°, 105°, 120°, 135°, 150°, and 180° were simulated adopting the National Concrete Masonry Association guidelines. The behaviors of the GRS walls with corners, including the lateral facing displacement, maximum reinforcement load, factor of safety, potential failure surface, vertical separation of facing blocks, and types of corners were carefully evaluated. Our comprehensive results show (i) minimum lateral displacement occurs at the corner; (ii) lower strength of reinforcements are required at the corner; (iii) higher corner angles lead to lower stability; (iv) potential failure surface forms earlier at the end walls; (v) deeper potential failure surfaces are found at the corners; (vi) larger numbers of vertical separations are found at walls with smaller corner angles. The paper highlighted the salient influence of the corners on the behaviors of GRS walls and indicated that a 3D analysis could reflect the required reinforcement length and the irregular formation of the potential failure surfaces.     

Numerical study on maximum reinforcement tensile forces in geosynthetic reinforced soil bridge abutments

This paper presents a numerical study of maximum reinforcement tensile forces for geosynthetic reinforced soil (GRS) bridge abutments. The backfill soil was characterized using a nonlinear elasto-plastic constitutive model that incorporates a hyperbolic stress-strain relationship with strain softening behavior and the Mohr-Coulomb failure criterion. The geogrid reinforcement was characterized using a hyperbolic load-strain-time constitutive model. The GRS bridge abutments were numerically constructed in stages, including soil compaction effects, and then loaded in stages to the service load condition (i.e., applied vertical stress?=?200?kPa) and finally to the failure condition (i.e., vertical strain?=?5%). A parametric study was conducted to investigate the effects of geogrid reinforcement, backfill soil, and abutment geometry on reinforcement tensile forces at the service load condition and failure condition. Results indicate that reinforcement vertical spacing and backfill soil friction angle have the most significant effects on magnitudes of maximum tensile forces at the service load condition. The locus of maximum tensile forces at the failure condition was found to be Y-shaped. Geogrid reinforcement parameters have little effect on the Y-shaped locus of the maximum tensile forces when no secondary reinforcement layers are included, backfill soil shear strength parameters have moderate effects, and abutment geometry parameters have significant effects.     

Model tests of geosynthetic-reinforced soil walls with marginal backfill subjected to rainfall

A series of model tests were conducted to investigate the performance of geosynthetic-reinforced soil (GRS) walls with marginal backfill subjected to rainfall infiltration. The effectiveness of improvement measures—such as decreasing reinforcement spacing and increasing sand cushion thickness—to prevent the GRS wall failure due to heavy rainfall was evaluated. The distribution and variation of the volumetric water content, porewater pressure, wall deformation, and reinforcement tensile strain were monitored during the test. The advancement of the wetting front and the drainage function of sand cushions were visually observed using the fluorescent dyeing technique. For the baseline case, the wall began to deform as rainfall proceeded, causing the potential failure surface to gradually move backward. When the potential failure surface moved beyond the reinforced zone, the pullout of the topmost reinforcement layers occurred, resulting in the collapse of the GRS wall in a compound failure mode. Decreases in reinforcement spacing and increases in sand cushion thickness effectively reduced wall deformation and enhanced wall stability. The placing of sand cushions between the reinforcement layers can also delay water infiltration and reduce the accumulation of porewater pressure inside the wall. Suggestions for designing rain-resistant GRS walls are also proposed based on the findings.     

Reinforcement load and deformation mode of geosynthetic-reinforced soil walls subject to seismic loading during service life

A Finite Element procedure was used to investigate the reinforcement load and the deformation mode for geosynthetic-reinforced soil (GRS) walls subject to seismic loading during their service life, focusing on those with marginal backfill soils. Marginal backfill soils are hereby defined as filled materials containing cohesive fines with plasticity index (PI) >6, which may exhibit substantial creep under constant static loading before subjected to earthquake. It was found that under strong seismic loading reinforced soil walls with marginal backfills exhibited a distinctive “two-wedge” deformation mode. The surface of maximum reinforcement load was the combined effect of the internal potential failure surface and the outer surface that extended into the retained earth. In the range investigated, which is believed to cover general backfill soils and geosynthetic reinforcements, the creep rates of soils and reinforcements had small influence on the reinforcement load and the “two-wedge” deformation mode, but reinforcement stiffness played a critical role on these two responses of GRS walls. It was also found that the “two-wedge” deformation mode could be restricted if sufficiently long reinforcement was used. The study shows that it is rational to investigate the reinforcement load of reinforced soil walls subject to seismic loading without considering the previous long-term creep.     

Simplified Procedure to Evaluate Earthquake-Induced Residual Displacement of Geosynthetic Reinforced Soil Retaining Walls

Based on a series of shaking table model tests, it was found that the effects of 1) subsoil and backfill deformation, 2) failure plane formation in backfill, and 3) pullout resistance mobilized by the reinforcements on the seismic behaviors of the geosynthetic reinforced soil retaining walls (GRS walls) were significant. These effects cannot be taken into account in the conventional pseudo-static based limit equilibrium analyses or Newmark's rigid sliding block analogy, which are usually adopted as the seismic design procedure. Therefore, this study attempts to develop a simplified procedure to evaluate earthquake-induced residual displacement of GRS walls by reflecting the knowledge on the seismic behaviors of GRS walls obtained from the shaking table model tests. In the proposed method, 1) the deformation characteristics of subsoil and backfill are modeled based on the model test results and 2) the effect of failure plane formation is considered by using residual soil strength after the failure plane formation while the peak soil strength is used before the failure plane formation, and 3) the effect of the pullout resistance mobilized by the reinforcement is also introduced by evaluating the pullout resistance based on the results from the pullout tests of the reinforcements. By using the proposed method, simulations were performed on the shaking table model test results conducted under a wide variety of testing conditions and good agreements between the calculated and measured displacements were observed.     

火灾下钢筋混凝土墙计算模型及影响因素分析

采用有限元软件ABAQUS对火灾下钢筋混凝土墙的变形全过程进行了计算,计算结果与以往实验结果吻合较好.在此基础上,系统分析了轴压比、侧向荷载比、高厚比、墙厚度、混凝土抗压强度、钢筋屈服强度、配筋率和混凝土保护层厚度对钢筋混凝土墙变形和耐火极限的影响规律.研究结果表明,受火过程中,钢筋混凝土墙在无侧向荷载且轴压比或墙厚度...     

双面加筋路堤的地震动力响应分析

双面加筋路堤作为加筋土挡土墙的一种衍生结构,沿袭了加筋土挡土墙优良的抗震性能,被广泛应用于道路建设工程,然而国内外关于双面加筋路堤的抗震设计还不够完善,采用的基于极限平衡法的抗震设计仍存在诸多问题。采用基于PLAXIS软件的有限元分析方法,对双面加筋路堤进行了较为全面的动力特性分析,结果表明,地震作用下双面加筋路堤的各层筋材最大内力分布、单侧面板侧移形式及路面沉降形式同单一的加筋土挡土墙表现形式相似;通过对不同宽高比结构筋材内力的分析得出,在地震作用下,加筋区及非加筋区之间存在第二潜在破裂面发育的可能。基于单自由度强迫振动理论及数值分析结果,建立了整体最大筋材内力与地震动及结构参数的关系。     

地震作用下模块式面板土工合成材料加筋土挡墙的内部稳定分析

由于造价低廉,性能优良且外表美观,模块式面板土工合成材料加筋土挡墙在我国交通及城建等领域有着广泛的应用前景。大量的工程实践证明,土工合成材料加筋土挡墙的抗震性能良好,但仍有必要进行合理的抗震设计,而内部稳定校核是加筋土挡墙抗震设计的一个重要环节,它一定程度上决定了高烈度地震区加筋土挡墙的配筋方式及配筋密度。应用非线性动力有限元法分析不同加筋长度、加筋间距及不同地震作用下模块式土工合成材料加筋土挡墙在地震作用下的内部稳定,研究了筋材蠕变对加筋土挡墙动力内部稳定的影响,并将有限元分析的结果与国外规范建议的内部稳定校核结果进行比较。研究结果表明,在正常配筋密度条件下,各层筋材最大内力的位置与规范建议的位置有一定的区别,墙体下部更加远离面板;且筋材的最大内力沿高度的分布与该规范计算结果差别较大;而筋材蠕变使筋材的内力出现重分布。     

Analysis of back-to-back mechanically stabilized earth walls

Back-to-back Mechanically Stabilized Earth (MSE) walls are commonly used for embankments approaching bridges. However, available design guidelines for this wall system are limited. The distance between two opposing walls is a key parameter used for determining the analysis methods in FHWA Guidelines. Two extreme cases are identified: (1) reinforcements from both sides meet in the middle or overlap, and (2) the walls are far apart, independent of each other. However, existing design methodologies do not provide a clear and justified answer how the required tensile strength of reinforcement and the external stability change with respect to the distance of the back-to-back walls. The focus of this paper is to investigate the effect of the wall width to height ratio on internal and external stability of MSE walls under static conditions. Finite difference method incorporated in the FLAC software and limit equilibrium method (i.e., the Bishop simplified method) in the ReSSA software were used for this analysis. Parametric studies were carried out by varying two important parameters, i.e., the wall width to height ratio and the quality of backfill material, to investigate their effects on the critical failure surface, the required tensile strength of reinforcement, and the lateral earth pressure behind the reinforced zone. The effect of the connection of reinforcements in the middle, when back-to-back walls are close, was also investigated.     

Experimental study of the performance of geosynthetics-reinforced soil walls under differential settlements

The deformation performance and settlement failure mechanism of geosynthetics-reinforced soil (GRS) walls are the two key points of engineering design under the differential settlement. This paper presents model tests of deformation performance and failure mechanism of the GRS wall with and without lateral restriction under differential settlement conditions. The observation and measurement results, including force and vertical displacement of geosynthetics and lateral deformation of facing panels, indicate good settlement control performance of GRS wall during construction and under differential settlement. Results indicate that the influence of the stress state of facing panels on the settlement control performance of GRS wall cannot be ignored. And the differential settlement failure of GRS wall is likely to occur in the joint of facing panels and geosynthetics. For good illustrations, two analytical approaches about deformation and stress of geosynthetics were proposed based on elastic cable theory, in GRS wall with and without lateral restriction. The expressions exclude the necessity to carry out sophisticated numerical analyses to stress and deformation and may help to develop the design guidelines for such GRS wall.     

Case history on failure of geosynthetics-reinforced soil bridge approach retaining walls

Six geosynthetic-reinforced soil (GRS) retaining walls supporting bridge approach roads of an overpass bridge in China exhibited a series of structural problems after 18 years of service. Field investigations demonstrated that the major structural problems consist of excessive lateral facing displacement, settlement and damage of facing panels, and pavement cracks above the GRS retaining walls. The structural problems were mainly caused by inadequate backfill compaction behind the facing, rain water infiltration, the settlement of foundation soil, and reinforcement ageing. Among the six GRS walls, a 22-m-long section collapsed after mild rain in July 2016, and the failure surface in the collapsed zone was mainly located 0.5–0.9 m away from the back of facing panels along the wall height. The field investigation found that external water filtration into the backfill behind the facing panels, and the breakage of connection between reinforcement and facing panels were the main causes of the failure. The connection breakage resulted from the ageing of PP reinforcement strips, and the critical issue of PP reinforcement ageing in complex backfill environment was pinpointed. Remedial measures of the failed section and reinforcing techniques of the remaining GRS walls were briefly presented in the end.     

中等剪跨比RC剪力墙拉-弯-剪受力性能试验研究

为研究中等剪跨比钢筋混凝土(RC)剪力墙的拉-弯-剪受力性能,对4个RC剪力墙开展了在恒定轴拉力和往复水平力作用下的拟静力试验。RC墙剪跨比为1.5,尺寸和配筋均相同,仅轴拉力变化。结果表明：RC墙分别发生了剪切破坏、弯曲-剪切破坏和弯曲破坏;轴拉力致使RC墙的水平承载力降低,竖向钢筋平均拉应力比ns从0.20增大到0.80时,RC墙峰值荷载降低了约55%;中等剪跨比RC墙弯曲-剪切耦合效应明显,墙底部截面弯曲屈服后,塑性铰区的剪切变形也表现出显著的非线性;轴拉力和往复水平力作用下墙体发生显著的轴向伸长,引起墙体受剪承载力退化,竖向钢筋平均拉应力比ns=0.40的RC墙,其受力由弯曲机制向剪切机制转变,出现了弯曲-剪切破坏,基于转动角软化桁架模型和轴向伸长的实测数据,定量计算了该类墙体的受剪承载力退化,揭示了弯曲-剪切破坏机理。最后,验证了美国ACI 318—14和中国JGJ 3—2010中RC墙正截面拉弯承载力计算方法和公式的适用性。     

中等剪跨比RC剪力墙拉-弯-剪受力性能试验研究

为研究中等剪跨比钢筋混凝土(RC)剪力墙的拉-弯-剪受力性能，对4个RC剪力墙开展了在恒定轴拉力和往复水平力作用下的拟静力试验。RC墙剪跨比为1.5，尺寸和配筋均相同，仅轴拉力变化。结果表明：RC墙分别发生了剪切破坏、弯曲-剪切破坏和弯曲破坏；轴拉力致使RC墙的水平承载力降低，竖向钢筋平均拉应力比ns从0.20增大到0.80时，RC墙峰值荷载降低了约55%；中等剪跨比RC墙弯曲-剪切耦合效应明显，墙底部截面弯曲屈服后，塑性铰区的剪切变形也表现出显著的非线性；轴拉力和往复水平力作用下墙体发生显著的轴向伸长，引起墙体受剪承载力退化，竖向钢筋平均拉应力比ns=0.40的RC墙，其受力由弯曲机制向剪切机制转变，出现了弯曲-剪切破坏，基于转动角软化桁架模型和轴向伸长的实测数据，定量计算了该类墙体的受剪承载力退化，揭示了弯曲-剪切破坏机理。最后，验证了美国ACI 318—14和中国JGJ 3—2010中RC墙正截面拉弯承载力计算方法和公式的适用性。     

Deterministic and probabilistic assessment of margins of safety for internal stability of as-built PET strap reinforced soil walls

The paper demonstrates deterministic and reliability-based assessment of strength limit states (tensile resistance and pullout) and the service limit state for soil failure for mechanically stabilized earth (MSE) walls constructed with polyester (PET) strap reinforcement. The general approach considers the accuracy of the load and resistance models that appear in each limit state equation plus uncertainty in the estimate of nominal load and resistance values at time of design. Reliability index is computed using a closed-form solution that is easily implemented in a spreadsheet. Three PET strap MSE wall case studies are used to demonstrate the reliability-based assessment approach and to compare margins of safety using different load and resistance model combinations. In some walls using the Coherent Gravity Method to compute loads, the recommended nominal factors of safety for tensile strength and pullout limit states were not satisfied. However, reliability analyses showed that the walls satisfy recommended minimum target reliability index values for the limit states investigated, usually by large amounts. The most critical limit state is the soil failure limit state which is used in the Simplified Stiffness Method to keep the reinforced soil zone at working stress conditions assumed for geosynthetic MSE walls under operational conditions.     

On the required connection strength of geosynthetically reinforced walls

The use of geosynthetics for the reinforcement of retaining walls, bridge abutments, and wing walls has progressed rapidly over the past 10 years. Design of the reinforcement, along with the requisite testing, has also advanced in a parallel manner. There is, however, an issue that has been raised over the required connection strength of the reinforcement to the wall facing. Theoretically it is felt, and shown numerically herein, that the required force is very small. Essentially all known wall systems can mobilize such forces.

What is of concern is a number of short- and long-term issues during and after the wall is constructed which can possibly add to the theoretically required connection strength. They are individually described herein. Furthermore, they are grouped into three types of stress mobilizing situations: uniform settlement, localized asymmetric deformation and localized symmetric deformation. A parametric evaluation of each situation is offered illustrating the possible magnitude of increase of connection stresses beyond the theoretical value under a hypothetical set of conditions. While the values can become high, all scenarios can (and should) be avoided.

Clearly, there should be no pressing concern over connection strength with proper design followed by proper construction practice.