Use of Compressible Expanded Polystyrene Blocks and Geogrids for Retaining Wall Structures

The use of a compressible layer such as expanded polystyrene blocks behind a rigid retaining wall and geogrid layers embedded in a dense granular backfill is examined as a reinforcement technique for retaining wall structures. The mobile model retaining walls adjacent to reinforced model specimens are subjected to different surcharge pressures, and are caused to move laterally to measure the lateral earth pressure during the wall movement. The coefficients of earth pressure at rest and active earth pressure are carefully inferred from test results. Three series of tests are conducted; one test series with expanded polystyrene blocks installed behind the wall, another with geogrid layers embedded within model specimens, and the last series with expanded polystyrene blocks installed behind the wall and geogrid layers fixed between two adjacent expanded polystyrene blocks and embedded within model specimens. The reductions in the earth pressure at rest and the active earth pressure due to various patterns of reinforcement are interpreted in relation to the concept of controlled yielding of compressible expanded polystyrene blocks, tensile strains induced along geogrid layers, fixity between expanded polystyrene blocks and geogrid layers, and a facing unit consisting of expanded polystyrene blocks.     

短加筋土挡墙墙后连接作用的离心模型试验研究

短加筋土挡墙是一种在既有稳定墙体/陡坡前修建的短加筋土挡墙,但目前缺乏对短加筋土挡墙与稳定墙体/陡坡间连接形式及其作用的统一认识,对其工作机理的研究也有待深入。利用离心模型试验,结合系统的监测数据,对不同连接形式的模型墙顶沉降、墙面水平位移、土压力分布等规律进行了分析,探讨了短加筋土挡墙的行为特征以及墙后机械连接的作用。研究发现:短加筋土挡墙的竖向沉降和水平位移均较常规加筋土挡墙大且分布不均匀,墙后连接形式对短加筋土挡墙的变形存在明显的影响;短加筋土挡墙和其后稳定墙体/陡坡之间的压力远小于理论水平土压力值,稳定墙体/陡坡与短加筋土挡墙之间仅存在接触压力;短加筋土挡墙墙后设置机械连接可以显著提高整体稳定性,减小差异沉降,控制水平变形,而不设连接时短加筋土挡墙变形较大且易于垮塌破坏。     

Numerical analysis of a laterally loaded shaft constructed within an MSE wall

Drilled shafts have been widely used to support lateral loads from superstructures. For typical applications, design methods are available to generate lateral load versus displacement curves and to estimate ultimate lateral capacity and displacement of the drilled shaft under a certain lateral load. However, occasionally drilled shafts have to be constructed within the reinforced zones of MSE walls, for instance drilled shafts supporting sound walls, traffic signs, billboards, and other superstructures. Under these circumstances, existing design methods are not applicable because of: (1) the limited horizontal extent of the soil mass; (2) the resistance from reinforcement; and (3) the influence of MSE wall facing. In this regard, a full-scale field study was conducted to investigate the behavior of shafts within the MSE wall, subjected to lateral loads. The test wall was 43 m long and 6 m high and constructed with layers of uniaxial geogrid and selected backfill. Three-dimensional numerical analyses were performed prior to the construction of this test wall (i.e., Class-A prediction) to guide its design and after the field test using the actual material properties (i.e., Class-C prediction). The selected test shaft for the analyses in this study was located at 1.8 m behind the wall facing. The numerical results from the Class-A and Class-C predictions are compared with the field data. The study showed that the Class-A prediction provided useful information for the design of the test wall and development of field test details. The Class-C prediction improved the overall accuracy of the calculations and could serve as a reference for future study.     

微型桩–加筋土挡墙的模型试验和数值模拟分析

微型桩–加筋土挡墙是一种适用于山区陡坡地带公路工程的新型路基支挡结构,为了初步检验其工作机制和效果,通过模型试验,并结合有限元模拟,对微型桩加固前后的斜坡加筋土挡墙在多级附加荷载下的静力响应进行对比研究。结果表明:数值计算手段能够较好地再现模型试验结果;在2~10 kPa附加荷载下,实测微型桩–加筋土挡墙的沉降比加固前减小11%~40%,面板侧向位移减小50%~66%,且附加荷载越大,减小的比例越大;同时,基底压力和侧向土压力也比加固前明显降低;倾斜桩对挡土墙变形的限制发挥着更为重要的作用,实测2~10 kPa附加荷载导致的倾斜桩身弯矩比竖直桩高出25.9%~40.3%,因而有必要采用较高强度的微型桩作为倾斜方向的加固构件。研究结论初步验证了微型桩–加筋土挡墙的有效性,可为其在实际工程中的应用提供指导。     

主动侧向受荷桩模型试验与颗粒流数值模拟研究

分别采用室内模型试验与颗粒流数值模拟的方法,对砂土中受水平荷载的桩顶自由短桩桩周土体在加载过程中的应力和位移的发展变化规律以及桩土相互作用性状进行了较系统和深入的研究。在室内模型试验中,研究了短桩在松砂、中密砂、密砂中的位移荷载规律,桩前、桩后土压力的分布和发展规律;并重点通过数字图像无标点量测技术密切关注桩周土体的位移产生和发展变化过程。通过开发颗粒流程序模拟室内模型试验在加载过程中的桩土相互作用过程中应力和位移的发展变化规律,与模型试验的结果进行了对比。给出了砂土中受水平荷载的桩顶自由短桩较完整的变形和破坏模式及极限土压力分布形式。     

Experimental investigation on the performance of multi-tiered geogrid mechanically stabilized earth (MSE) walls with wrap-around facing subjected to earthquake loading

Construction of mechanically stabilized earth (MSE) walls in multi-tiered configurations is a promising solution for increasing the height of such walls. The good performance of this type of walls after recent major earthquakes was reported in a number of technical studies. In the present study, an experimental approach was adopted to compare the seismic performance of single-tiered and multi-tiered MSE walls using physical modeling and through conducting a series of uniaxial shaking table tests. To do so, several geogrid-reinforced soil walls with wrap-around facing (i.e., three-, two-, and single-tiered) with a total height of 10 m were designed in the form of prototypes of 1-m-height wall models. The step-wise intensified sinusoidal waves were applied to the models in 14 typical forms. Comparing the shaking table test results confirmed the post-earthquake advantages of multi-tiered MSE walls. The results revealed that tiered walls exhibited better behaviors under earthquake loading in terms of the seismic stability of the wall, displacement of the wall crest, horizontal displacement of the wall facing, deformation mode and failure mechanism of the wall, settlement of backfill surface, and seismic acceleration responses.     

Study of the behavior of mechanically stabilized earth (MSE) walls subjected to differential settlements

The limit equilibrium (LE) analysis has been used to design MSE walls. Presumably, the deflection of MSE walls can be limited to an acceptable range by ensuring sufficient factors of safety (FOSs) for both external and internal stabilities. However, unexpected ground movements, such as movements induced by excavations, volume changes of expansive soils, collapse of sinkholes, and consolidations of underlying soils, can induce excessive differential settlements that may influence both the stability and the serviceability of MSE walls. In this study, a numerical model, which was calibrated by triaxial tests and further by a specially-designed MSE wall tests, investigated the behavior of an MSE wall as well as the influence of various factors on the performance of the MSE wall when the wall facing settled relatively to the reinforced zone. The numerical results showed that the differential settlement would cause substantial vertical and horizontal movements for the MSE wall, as well as an increase in lateral earth pressure and geosynthetic reinforcement strain. The maximum horizontal movement and increase of the lateral earth pressure occurred at about 1.0 m above the toe. The differential settlement resulted in a critical plane that coincided with the plane of 45°+?/2. The maximum increase of the strain for each geogrid layer occurred in that plane, and the bottom layer had the greatest strain increase among all layers of reinforcement. The study further indicated that the surcharge, backfill friction angle, tensile stiffness of geogrid, reinforcement length and MSE wall height had noticeable influences on horizontal and vertical movements, and strain in geosynthetics. According to the results, the MSE wall that had a higher factor of safety would have less movements and geosynthetic strain increase. In contrast, only the friction angle, tensile stiffness and MSE wall height showed some degree of influence on the lateral earth pressure due to differential settlements.     

砂土中大直径单桩水平受荷离心模型试验

大直径桩基在海洋工程中已越来越广泛应用。针对目前API规范p-y曲线对水平受荷大直径单桩的不适用性,通过离心模型试验研究了砂性土中大直径单桩分别在水平静力和循环荷载作用下的受力和变形特性。验证了通过实测桩身弯矩推算桩身变形和桩周土反力的有效性,分别获得了干砂和饱和砂的大直径单桩水平静力p-y曲线。在修正p-y曲线初始刚度的基础上,采用双曲线型p-y曲线分析了水平受荷大直径单桩的内力和变形。揭示了水平单向循环荷载下大直径单桩的桩身变形及内力变化特性,试验结果显示桩身变形和最大弯矩近似与循环次数的对数线性相关。最后,由各循环次数下的桩身弯矩获得了大直径单桩水平循环p-y曲线,提出了循环应力比相关的p-y曲线循环弱化因子,以及相应的桩基变形累积和内力变化分析方法。     

Large-scale tests of pile-supported earth platform with and without geogrid

In recent years, concrete piles, such as cast-in-place piles and precast concrete piles, have been increasingly used to support superstructures and embankments when they are constructed on soft soils. On the top of pile head elevation, a certain thick granular cushion including geosynthetic reinforcement is usually installed to transfer more external load onto the piles through soil arching effect and membrane effect. This technique involving the use of rigid piles, gravel cushion and geosynthetics is usually referred to as geosynthetic-reinforced and pile-supported earth platform. This paper presents two well-instrumented large-scale tests of pile-supported earth platform with and without geogrid reinforcement. The performance of the pile-supported platform with geogrid and its load transfer behavior were investigated and compared with those for the test without geogrid. The validation of the EBGEO (2010) calculation was performed based on the test results. The test results indicate that under lower applied load, the loads carried by the piles in the test with geogrid were close to those in the test without goegrid, while with an increase in external load the loads carried by piles in the test with geogrid increased faster than those in the test without geogrid. The negative skin friction for the test with geogrid was smaller than that for the test without geogrid. Based on the contours of earth pressures on foundation base the maximum earth pressures were distributed along the edge of central cap in the test with geogrid. The minimum earth pressures were on midway subsoil between two caps in both tests. Based on the test results, the efficacy for the test with geogrid was 2.5% greater than that for the test without geogrid at the end of loading. The efficacies predicted by the EBGEO (2010) calculation agreed well with the measured efficacies.     

海洋高桩基础水平单调及循环加载现场试验

开展了海洋软黏土中 2 根大直径高桩基础的现场水平单调和循环加载试验,实测获得了桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力和孔隙水压力,揭示了水平单调和循环荷载作用下桩土相互作用规律及桩基水平位移和桩身弯矩发展规律。利用实测桩身水平位移推算了桩周土反力,在此基础上提出了相应的双曲线型 p – y 曲线,通过引进 Poulos 循环弱化模型建立了水平循环荷载作用下的桩基双曲线型 p – y 曲线分析模型,水平单调及循环荷载作用下桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力等计算结果与实测值均吻合良好。通过现场试验发现规范 p – y 曲线法计算结果偏保守的主要原因是所采用的 p – y 曲线的刚度偏小;不同时段的循环荷载对桩基循环累积变形有叠加效应。 建议设计中应考虑桩基全寿命服役期内所承受的所有循环荷载的影响, 对于重要工程应开展相应的现场水平加载试验,实测桩身水平位移或桩身弯矩,进而利用所推算的桩周土反力来分析桩基受力变形及承载力。     

上拔与水平力组合作用下单桩静载试验

通过现场单桩水平静载荷试验及上拔与水平力组合作用下单桩静载试验,分析了水平荷载作用下不同桩身抗弯刚度灌注桩的变形特性和承载特性,探讨了有、无上拔荷载作用两种情况下单桩水平承载力及变形之间的差异。结果表明:桩身抗弯刚度的大小是影响单桩水平承载力及水平位移、桩顶转角的一个重要因素;当水平荷载较小时,与水平荷载同时作用的上拔荷载对桩的水平位移及桩顶转角无较大影响;而当水平荷载较大时,上拔荷载的主要影响在于降低桩周土体的强度,使得土体的水平位移增大。     

Behaviour of Pile Group behind a Sheet Pile Quay Wall Subjected to Liquefaction-Induced Large Ground Deformation Observed in Shaking Test in E-Defense Project

This paper aims to illustrate a large-scale test on a pile group and a sheet pile quay wall which were subjected to liquefaction-induced large ground deformation. The sheet pile quay wall was displaced laterally and the 2×3 pile group was forced by the flow of liquefied soil. This experiment was conducted in March 2006 at the National Research Institute for Earth Science and Disaster Prevention (NIED), Hyogo Earthquake Engineering Research Center, Japan. Liquefaction-induced lateral spreading was achieved, and soil moved laterally about 1.1 m behind the sheet pile quay wall. Lateral soil displacement was measured by the inclinometers, and results were in close agreement with the directly observed values. Soil lateral displacement and velocity of soil flow decreased as the distance from the quay wall increased toward the landside. Bending strain records were able to explain the damages to the piles, yielding at the top and buckling at the middle height. Lateral force of the liquefied soil exerted on the piles was obtained using earth pressure (EP) sensors and it is shown that rear row piles (close to the quay wall) received larger lateral forces than front row piles (far from the quay wall). This behaviour is explained by the distribution of displacement and velocity of the liquefied soil throughout the shaking. In addition, the lateral soil force was back calculated from strain gauge data and the results are compared with the ones directly measured by the EP sensors. Then, the limitations and advantages of the back-calculation approach are elaborated in this study. Moreover, the time history of lateral soil force showed no correlation with either soil or pile displacements, while it demonstrated a fairly close correlation with the relative velocity until a specific time. This interesting finding would confirm the rate-dependent behaviour of the liquefied soil, though more data from large scale experiments, field testing and centrifuge model tests are needed in this regard.     

长期竖向循环荷载作用下黄土中单桩沉降特性模型试验研究

长期竖向循环荷载作用下单桩的沉降特性与静载情况下明显不同,通过黄土及饱和黄土中单桩的竖向循环荷载试验,分析了循环荷载作用下单桩的累积位移发展模式,研究了桩顶位移s及循环位移幅值c?随循环次数N的变化规律。试验发现:黄土饱和后单桩的承载力明显下降;饱和前后单桩循环荷载试验的s–N曲线都可采用幂函数进行描述;相同荷载大小时,饱和前单桩的循环荷载位移小于静载位移,循环位移比(循环荷载位移与静载位移的比值)介于0.6与0.8之间,而饱和后单桩的循环荷载位移明显大于静载位移,且循环位移比随循环荷载比线性增长;饱和前单桩的循环位移幅值明显大于饱和后的位移幅值,主要受循环荷载幅值及桩周土性质的影响。     

Full-scale mechanically stabilized earth (MSE) walls under strip footing load

This study analyses two full-scale model tests on mechanically stabilized earth (MSE) walls. One test was conducted with a rigid and one with a flexible wall face. Other parameters were the same in these two tests, like the number and type of geogrid layers, the vertical distance between the layers and the soil type. The loads and strains on the reinforcement are measured as function of the horizontal and vertical earth pressure and compared with analytical models. Specifics regarding the behavior of the geogrids under the compaction load during the construction of the model and under strip footing load are included in the study. Results are compared with AASHTO and the empirical K-stiffness method. In this study, an analytical method is developed for the MSE walls taking into account the facing panel rigidity both after backfill construction and after strip footing load. There is good agreement between the proposed analytical method and the experimental results considering the facing panel rigidity. The results indicate that the tensile force on reinforcement layers for rigid facing is less than the flexible facing. The maximum strains in the reinforcement layers occurred in the upper layers right below the strip footing load. The maximum wall deflection for the flexible facing is more than for the rigid facing. The maximum deflection was at the top of the wall for the rigid facing and occurred at z/H?=?0.81 from top of the wall for the flexible facing.     

集约式微型桩群水平承载性能试验研究

 对矩形布置的集约式微型桩群支护体系采用分级加载的方法,进行水平荷载原型试验及数值计算,研究滑坡推力作用下集约式微型桩群抗滑作用机制,并分析桩间土体参数、桩身强度、桩排间距等设计参数变化对桩群水平承载性能影响的敏感程度。结果表明,在滑体介质为粉质黏土的地层中,当滑坡推力超过某一定值时前排桩才会发挥支挡作用,桩群前方土压力峰值的位置深于桩群内部,微型桩群与土体水平荷载分担比约为7：3;控制最佳排间距,相比优化其他设计参数对提高集约式微型桩群水平承载力、减小桩顶位移效果明显。试验成果为深入分析集约式微型桩群的抗滑机制和优化工程设计提供了参考。     

单桩静载荷试验的理论模拟及影响因素分析

应用基于 Mindlin 位移解的弹性理论法建立了堆载法和锚桩法单桩静载测试过程的理论模拟方法,对两种测试方法进行了对比分析。重点研究了基准桩布置对两种测试方法结果的影响,结果表明：堆载法和锚桩法测试均夸大了单桩刚度,且锚桩法比堆载法更明显;另外,按照现有桩基检测规范进行的桩基测试在一定的条件下对单桩刚度的影响较大。要充分利用压重平台卸荷或锚桩上拔引起基准桩的上抬和测试桩桩身应力引起基准桩的下沉叠加来降低对测试结果的影响。     

Performance of two-tiered reinforced-soil retaining walls under strip footing load

In the current study, an attempt was made to investigate the performance of two-tiered mechanically stabilized earth walls (T-TMSEWs) under static footing loading using reduced-scale model tests. For this purpose, twenty-four T-TMSEW models were constructed with three different types of reinforcement (metal strips, geogrid and geostraps) and were loaded using the rotatable and non-rotatable strip footings in different distances to the wall crest. Findings indicated that, although decreasing the reinforcement stiffness and the soil-reinforcement interaction reduces the ultimate bearing capacity of footings, the use of extensible reinforcements with low pull-out capacity and allowing the footing to tilt can be two effective solutions in T-TMSEWs to minimize deformations of backfill surface and connection loads as well as lateral pressures. It was observed that the use of a two-tiered configuration in MSE walls and also reducing tensile stiffness and soil-reinforcement interaction simultaneously, not only lead to change in the slip surface geometry but also prevent the development of deep slip surfaces in the lower tier. On the other hand, increasing the footing distance to the wall crest in the range of reinforced zone was found to be another influential solution to improve the bearing capacity, reduce wall deformations and also minimize lateral pressures.     

Effect of earth reinforcement,soil properties and wall properties on bridge MSE walls

Mechanically stabilized earth (MSE) retaining walls are popular for highway bridge structures. They have precast concrete panels attached to earth reinforcement. The panels are designed to have some lateral movement. However, in some cases, excessive movement and even complete dislocation of the panels have been observed. In this study, 3-D numerical modeling involving an existing MSE wall was undertaken to investigate various wall parameters. The effects of pore pressure, soil cohesion, earth reinforcement type and length, breakage/slippage of reinforcement and concrete strength, were examined. Results showed that the wall movement is affected by soil pore pressure and reinforcement integrity and length, and unaffected by concrete strength. Soil cohesion has a minor effect, while the movement increased by 13–20 mm for flexible geogrid reinforced walls compared with the steel grid walls. The steel grid stresses were below yielding, while the geogrid experienced significant stresses without rupture. Geogrid reinforcement may be used taking account of slippage resistance and wall movement. If steel grid is used, non-cohesive soil is recommended to minimize corrosion. Proper soil drainage is important for control of pore pressure.     

钢筋混凝土钻孔灌注桩桩端桩侧后压浆技术

结合工程实例，介绍了钢筋混凝土钻孔灌注桩桩端桩侧后压浆技术的原理及应用，该技术是采用通过桩身预埋铁管，用高压力强行将水泥浆压入桩底桩侧土层中，在桩身质量达到规范要求的前提下大幅度提桩端及桩侧土层承载力，从而提高单桩群桩承载力和刚度。并对试桩进行了静载试验，得出了一系列试验数据，从而证明钻孔灌注桩桩端桩侧后压浆技术的可行性。     

高速铁路桥桩在轴向循环荷载长期作用下的承载和变形特性试验研究

针对高速列车通过小跨度桥梁时列车活载对桥桩的影响分析来获得动力加载参数,进而对位于软粘土地层中的钻孔灌注桩进行了轴向循环荷载长期作用下的动力试验,测试和研究了循环荷载长期作用下桩的动位移幅值、桩顶沉降、桩身轴力、桩侧动摩阻力和单桩极限承载力等参数的发挥和变化情况。试验结果表明:列车循环荷载长期作用下,灌注桩的桩身轴力发生了局部调整,砂性土层的桩侧摩阻力具有增强效应,淤泥质粘性土的桩侧摩阻力具有退化效应;列车循环荷载对软土地区单桩的承载能力和桩基的工后沉降影响甚微,但会使单桩竖向刚度降低。