A performance summary of reinforced soil structures in the greater Los Angeles area after the Northridge earthquake

This paper contains a summary of observations on segmental retaining walls and geogrid reinforced soil slopes which were subjected to the shaking of the Northridge earthquake. Eleven structures were visually examined for evidence of distress resulting from the earthquake. The structures highlighted provide a wide range of slope and wall systems using commercially available and widely marketed products. Specific details of two structures located within 35 km (22 miles) of the epicenter and subjected to strong ground motions are discussed. General comments on their construction and performance are provided. It is concluded from the survey that reinforced soil structures perform excellently even when subjected to seismic loads far in excess of those.for which they were designed.     

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.     

软土地基加筋土挡墙数值模拟及稳定性探讨

 对一软土地基加筋土挡墙建立二维数值模型,模拟其在分级堆载情况下挡墙和地基内的沉降、水平位移、土压力,以及土工格栅轴向应变的变化规律,模拟结果与现场实测结果基本吻合。采用有限元强度折减法计算的挡墙稳定性和滑裂面位置与实测情况一致,表现为深层滑动失稳。模拟和实测的各层筋材最大应变出现在距墙面4～6 m的位置,与目前土工合成材料加筋挡墙设计理论的朗肯破坏面位置不同,其原因是目前的挡墙设计理论基于刚性地基假定,未考虑地基变形对筋材应变分布及稳定性的影响。采用该数值模型探讨加长挡墙底部筋材对其稳定性的影响,得出挡墙稳定性与底部筋材加长长度和层数关系密切。得到的挡墙稳定性与筋材加长长度和层数的关系曲线,对于软土地基加筋土挡墙设计有指导意义。     

Hydro-mechanical behavior of geogrid reinforced soil barriers of landfill cover systems

This paper examines the hydro-mechanical behavior of soil barriers with and without the inclusion of geogrid reinforcement within the soil barrier of landfill cover systems. The effect of geogrid type on the deformation behavior of the soil barrier subjected to various ranges of distortion levels was examined through centrifuge tests carried out at 40 g. An overburden pressure equivalent to that of landfill cover systems was applied to all the soil barriers tested in this study. The performance of the soil barrier with and without geogrid layer was assessed by measuring water breakthrough at the onset of differential settlements during centrifuge tests. Un-reinforced soil barriers of 0.6 m and 1.2 m thickness were observed to experience single narrow cracks penetrating up to full -depth of soil barriers at distortion levels of 0.056 and 0.069 respectively. In comparison, soil barriers reinforced with geogrids restrained cracking better than unreinforced soil barriers. However, degree of restraining of cracks in the soil barriers was found to be strongly depending on the geogrid type and the thickness of the soil barrier. Limiting distortion levels for 0.6 m and 1.2 m thick soil barriers reinforced with a low strength geogrid was found to be 0.095 and 0.108 respectively. When the soil barrier of both thicknesses was reinforced with a geogrid having relatively high tensile load-strain characteristics, the integrity of the geogrid reinforced soil barrier was observed to be retained even after inducing a distortion level of 0.125. The results from the present study suggest that the hydro-mechanical behavior of the soil barriers can be improved with a suitable geogrid layer having adequate tensile load-strain characteristics.     

土体地震大变形分析中Seed有效循环次数方法的局限性

采用地震波输入动三轴实验并结合实际震害现象,研究常用的Seed有效循环次数方法在分析地震下土体大变形和地基差降中的合理性和局限性。结果表明有效循环次数方法的主要局限有:不适于大脉冲强地震动下土体残余变形的分析,不能反映地震动本身不规则性和不对称性、土的各向异性特性以及土层横向分布不均匀带来的土体永久变形和地基震陷的差异,不适应土体变形时程分析的要求,用于软弱地基上建筑物不均匀震陷的计算容易导致不合理的结果。     

加筋土坡动态稳定性拟静力分析

 加筋土工结构被广泛采用的原因不仅是其具有良好的静力性能,且也在于出色的动力稳定性能,现有研究较少考虑竖向地震效应对加筋土坡动态稳定性的影响。基于塑性极限分析上限理论,假定不同的破坏面,同时考虑水平和竖向地震影响并结合不同加筋模式,采用拟静力分析方法推导一定加筋强度条件下的边坡临界高度和一定边坡高度条件下的临界加筋强度计算公式,并对所导公式采用序列二次规划法进行了优化计算,数值计算与分析表明：简单静态和动态条件下,该结果与现有研究成果有较好的一致性,可以证明该方法的正确性;水平和竖向地震、岩土材料强度特性、边坡倾斜度均对加筋土坡的动态稳定性有重要影响,特别当边坡较陡,岩土填筑材料质量较差和地震影响强度较大时,忽视竖向地震影响将会导致设计偏于不安全;最后针对工程实际,提出相应的工程建议。     

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.     

Field behaviors of a geogrid reinforced MSW slope in a high-food-waste-content MSW landfill: A case study

A one-year field monitoring of a geogrid reinforced municipal solid waste (MSW) slope was conducted in the Xingfeng Landfill. Settlement tubes, strain gauges and earth pressure cells were used to measure the vertical settlement, the reinforcement strains and the vertical earth pressures in the reinforced MSW slope, respectively. During the monitoring period, the waste sliding occurred and the fresh MSW was dumped at the top of the reinforced slope. The vertical settlement along the reinforcement was nonlinear and the peak settlement occurred at the central part of the reinforcement. The reinforcement strains and the vertical earth pressures at various positions were affected by the sliding and the waste dumping to differing extents. Along the lengths of the geogrid reinforcements, the reinforcement strains showed single-peak distributions. The peak strains were attained in the central part of the reinforcements and the minimum strains were attained at the tail ends. The vertical earth pressures mainly depend on the overlying loads; however, the distributions of them along the reinforcement were nonlinear. Based on the monitoring results, the slope stability evaluation was conducted. It shows that the internal stability of the reinforced MSW slope might be sufficient, while the external stability was insufficient, meaning that this reinforced project was unsuccessful. Finally, various lessons and design suggestions learned from this unsuccessful project were discussed, which could provide valuable references for the future practice of geosynthetic reinforced MSW.     

Liquefaction-induced damage evaluation of earth embankment and corresponding countermeasure

Liquefaction of sandy soils is a big threat to the stability and the safety of an earth embankment laid on saturated soils. A large number of liquefaction-induced damages on embankment due to different types of earthquakes have been reported worldwide. In this research, the dynamic behaviors of earth embankment and the reinforcement effects of grouting as remediation method, subjected to moderate earthquake EQ₁ and strong earthquake EQ₂, were numerically investigated. The seismic behaviors of ground composed of cohesionless sandy soil and cohesive clayey soil were uniformly described by the cyclic mobility (CM) model, which is capable of describing accurately the mechanical property of the soil due to monotonic and cyclic loadings by accounting for stress-induced anisotropy, over-consolidation, and soil structure. It is known from the numerical investigation that the embankment would experience destructive deformation, and that the collapse mode was closely related to the properties of input seismic motion because high intensities and long durations of an earthquake motion could lead to significant plastic deformation and prolonged soil liquefaction. Under the strong seismic loading of EQ₂, a circular collapse surface, combined with huge settlement and lateral spread, occurred inside the liquefication zone and extended towards the embankment crest. In contrast, in moderate earthquake EQ₁, upheaval was observed at each toe of the embankment, and instability occurred only in the liquefied ground. An anti-liquefaction remediation via grouting was determined to significantly reduce liquefaction-induced deformation (settlement, lateral spreading, and local uplift) and restrain the deep-seated circular sliding failure, even though the top sandy soil liquefied in both earthquakes. When the structure was subjected to EQ₂ motion, local failure occurred on the embankment slope reinforced with grouting, and thus, an additional appropriate countermeasure should be implemented to further strengthen the slope. For both input motions, the surface deformation of the considered embankment decreased gradually as the thickness of reinforcement was increased, although the reinforcement effect was no longer significant once the thickness exceeded 6 m.     

格宾加筋土挡墙抗震性能及数值分析

基于有限差分程序FLAC3D动力分析模块,建立水平地震作用下格宾加筋土挡墙足尺数值模型,通过抗震模型试验结果验证数值模型的可靠性,分析不同强度地震波作用、不同竖向加筋间距时,格宾加筋土挡墙的水平位移响应、震陷、加速度响应及破坏模式,在此基础上,提出格宾加筋土挡墙抗震设计相关措施与建议。结果表明:在不同峰值加速度作用下,格宾加筋土挡墙没有出现倒塌破坏,在较大的水平位移及沉降发生后仍能继续承载,表现出良好的抗震性能;在地震波频率特性基本不变的情况下增长加速度峰值,墙面加速度放大系数有减小的趋势;格宾加筋土挡墙建造于7度及以下、8度、9度及以上抗震设防区时,格宾网竖向间距分别不宜大于1.0m、0.75m、0.5m;水平地震作用下挡墙潜在破裂面为双线段组合形式;提出格宾加筋土挡墙抗震设计位移控制标准。     

下伏基岩堆积体边坡抗滑桩加固前后地震响应特征离心模型试验

 为研究抗滑桩加固上覆堆积体--下伏基岩二元结构边坡的抗震机制,开展2组1∶50比尺的离心振动台模型试验,以对比分析下伏基岩堆积体边坡在抗滑排桩加固前后的地震响应特征与抗滑桩的桩身弯矩分布规律。试验时,输入4级加速度峰值连续增大的El Centro波,监测边坡模型坡面与坡体内的加速度响应、坡顶沉降变形以及抗滑桩上静、动弯矩的分布。试验结果显示由于抗滑桩抑制了上覆堆积体的下滑,坡顶的加速度峰值(PGA)放大系数、加速度反应谱以及竖向沉降变形均有不同程度的降低。抗滑桩一方面加固了上覆堆积滑体另一方面在坡体内产生了地震波的反射叠加效应,使得边坡水平响应加速度放大系数出现了桩前增大桩后减小的现象。下伏基岩堆积体边坡坡顶沉降与Arias烈度在抗滑排桩加固前后均具有良好的正相关线性关系。地震荷载作用过程中抗滑桩动力响应弯矩变化幅值明显大于地震作用后的静弯矩增量,且静弯矩与动弯矩变化幅值的分布均在基岩面附近达到峰值,易在基岩面附近造成抗滑桩的破坏,类似工况下抗滑桩的抗震配筋设计应充分考虑这一特点。     

Influence of Geogrid Layer on the Integrity of Compacted Clay Liners of Landfills

The objective of this paper is to examine the influence of geogrid layer on the integrity of clay liners of landfills. A series of centrifuge model tests were performed on model clay liners subjected to non-uniform settlements with and without a geogrid layer embedded within the top one-third portion of the clay liner moist-compacted on the wet side of its optimum moisture content at 40 g. The model clay liner material has been selected in such a way that it envelopes the material characteristics of the clay liners, which are used for constructing an impermeable barrier in a lining system. By maintaining type and location of the geogrid within the clay liner as constant, the thickness of clay liner is varied to check the possibility of reducing the thickness of a geogrid reinforced clay liner. Digital image analysis technique was employed to ascertain the initiation of cracking and to compute strains both on the surface and along the cross-section of the clay liner with and without any geogrid layer. It was observed that clay liners compacted at moulding water content towards wet side of their OMC found to experience multiple cracking at the onset of non-uniform settlements. Contrary to this, geogrid reinforced clay liner was observed to sustain large distortions and experience only tiny cracks limited up to a location of a geogrid. With an increase in thickness of the clay liner reinforced with a geogrid, geogrid reinforced compacted clay liner was observed to retain its integrity and restrains cracking completely.     

Seismic behavior of geosynthetic-reinforced retaining walls backfilled with cohesive soil

This study investigates the seismic performance of geosynthetic-reinforced modular block retaining walls backfilled with cohesive, fine grained clay-sand soil mixture. Shaking table tests were performed for three ½ scaled (wall height 190 cm) and ¼ scaled model walls to investigate the effects of backfill type, the influence of reinforcement length and reinforcement stiffness effects. The El Centro and Kobe earthquake records of varying amplitudes were used as base acceleration. Displacement of the front wall, accelerations at different locations, strains on the reinforcements, and the visual observations of the facing and the backfill surface were used to evaluate the seismic performance of model walls. The model walls were subjected to rigorous shaking and the walls did not exhibit any stability problems or signs of impending failure. The maximum deformations observed on the models with cohesive backfill was less than half of the deformation of the sand model. The load transfers between the geogrid and cohesive soil was comparable to that of sand and hence the needed reinforcement length was similar as well. As a result; the model walls with cohesive backfills performed within acceptable limits under seismic loading conditions when compared with granular backfilled counterparts.     

格栅条带式加筋胎面挡土墙变形性能数值计算

采用土工格栅加筋的方法提高废旧轮胎挡墙的承载性能,促进废旧轮胎挡墙的推广应用,通过数值计算方法分析了不同墙顶荷载下有无土工格栅加筋的废旧轮胎挡墙的水平变形与竖向沉降反应特征,得出铺设土工格栅加筋的方法可显著减小墙体的水平变形和竖向沉降,提高废旧轮胎挡墙结构的承载能力,随着外荷载的增加,墙体变形模式依次呈凹凸微小变化型、“弯弓”型、“似弯弓”型和“鼓腮”型和直线型。考虑土工格栅的加筋长度、竖向加筋间距以及格栅加筋刚度3种因素对废旧轮胎+土工格栅加筋土挡墙的水平变形的影响,得出在废旧轮胎加筋土挡墙设计中,建议土工格栅的加筋长度选取范围为0.5H～0.7H,土工格栅竖向间距的选取范围为0.4 m~0.7 m,格栅刚度不宜大于5 000 kN/m。     

两种加筋土挡墙的动力特性比较及抗震设计建议

为分析比较条带式和包裹式加筋土挡墙的地震动力响应特征,开展了两种加筋土挡墙模型的大型振动台试验.结合震害调查的结果,发现砌块式加筋土挡墙在地震作用下的破坏模式主要表现为局部砌块的松动变形,很少会出现整体垮塌的情况.相比条带式加筋土挡墙,包裹式加筋土挡墙在地震作用下产生的变形量要小.在相同地震量级作用下,包裹式加筋土挡墙相应部位的水平加速度放大系数要小于条带式加筋土挡墙,但峰值动土压力却要比条带式加筋土挡墙大,这是因为包裹式加筋土挡墙面板在地震作用下的变形量小,对土体的约束能力强所致.因此,在抗震设防区,特别是是高地震烈度区进行加筋土挡墙的选型时,包裹式加筋土挡墙应作为一种优选结构.分析认为加筋土挡墙的抗震设计除了要进行整体稳定性的验算外,还应注重墙体变形量的控制,加筋土挡墙在地震作用下的最大变形量应小于允许的变形量.为维持线路的正常使用,加筋土挡墙的变形指数应控制在4%以内.若验算得到的变形量超出允许值,可采取增大墙后填土的压实度和增加拉筋长度,以及加厚墙体和降低墙体坡率等措施.     

Independent Reinforced Soil Structure with Pile Foundation—Piled Geo-Wall: An Experimental Study on the Application to Seismic Measure for Embankment—

Our aim in this study was to achieve an independent reinforced soil structure with pile foundation that can be applied to such structures as earth retaining walls and countermeasures against the collapse of embankments or rockfall impact built on narrow construction sites, such as on slopes. In order to confirm the effectiveness of the application of pile foundation to reinforced soil structures by geogrid for improvement of the lateral resistance of the structure and to investigate the interaction between pile and reinforced soil structure, a dynamic centrifuge model test (25 G) was carried out. Two geogrid reinforced soil, one with piles and one without, were used in a countermeasure to reduce the deformation of a road embankment built on a slope in the event of an earthquake, and the effectiveness of the pile foundation to the reinforced soil structure was considered with regard to it affected the road surface. The details and the results of the dynamic centrifuge model test, as well as the interaction between pile and reinforced soil structure are described, and the effectiveness of the application of pile foundation to reinforced soil structure is discussed in this paper.     

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.     

土工格栅纵横肋的加筋土力学特性仿真研究

为了研究土工格栅纵横肋布筋形式对加筋土力学特性的影响,采用非线性有限元软件ABAQUS建立模型,对不同工况下加筋土体的应力、位移以及土工格栅的拉应力进行仿真分析。研究结果表明:纵肋对加筋土力学性能的影响起主导作用,横肋起辅助作用。在逐级上覆荷载的作用下,完整格栅的加筋土结构稳定性较好;减少横肋时,加筋土结构力学特性变化不明显;减少纵肋时,加筋土体应力、位移增大,土工格栅拉应力增大,加筋土结构朝着整体破坏的方向发展,布筋时应避免这种情况。     

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.     

Estimation of seismic active earth pressure on reinforced retaining wall using lower bound limit analysis and modified pseudo-dynamic method

Present study estimates seismic active earth pressure on the reinforced retaining wall by combining the lower bound finite element limit analysis and the modified Pseudo-dynamic method. A series of parametric analyses are performed by varying seismic acceleration coefficient, time period of seismic loading, soil friction and dilation angles, reinforcement spacing, length of reinforcement, soil-reinforcement interface, damping ratio of soil, soil-wall interface, wall inclination, and ground inclination. Maximum active earth pressure is exerted when natural time period of reinforced soil matches with the time period of an earthquake. Reinforcement is found to be effective in terms of reducing active earth pressure significantly on the wall subjected to seismic loading. Effectiveness of reinforcement depends upon two factors, namely vertical spacing and soil-reinforcement interface friction angle. For relatively smaller reinforcement spacing, soil-reinforcement behaves like a composite block, which helps to constraint stresses within a small area behind the wall. Maximum tensile resistance is developed when fully rough interface condition is assumed between soil and reinforcement layer. Failure patterns are provided to understand the behaviour of reinforced retaining wall under different time of seismic loading.