加筋风积砂地基承载力试验研究及计算分析

针对沙漠地区风积砂土特殊的物理力学性质，以土工格栅为加筋材料对风积砂土进行加固。通过室内模型试验，对未加筋的风积砂土和15种布筋方式下的加筋风积砂地基承载力进行了试验研究。测定了各种布筋方式下加筋风积砂土的极限破坏荷载，分析了加筋土的变形以及应力扩散情况。根据试验结果，总结了不同布筋方式及不同埋深条件下，加筋风积砂地基承载力的变化规律，并推荐片式双层格栅为施工中有效的布筋方式，此布筋方式下的加筋风积砂地基承载力较风积砂地基承载力增加1．2倍。提出了加筋风积砂土的强度机理和破坏模式，建立了无埋深条件下片式单层格栅加筋风积砂地基承载力的计算公式。经试验验证，所得结果具有实用价值。     

土工格室加筋砂土地基沉降试验研究

土工合成材料可以有效提高地基的承载力与减小地基的表面沉降差异。在静荷载作用下,采用室内模型试验方法对纯砂地基和土工格室加筋地基的地基承载力和沉降情况进行了对比分析,研究了格室埋深、格室高度及筋材层数对距离基础不同远近处地基沉降的影响。研究结果表明,在荷载较小时,土工格室加筋地基作用效果相近;在荷载较大时,土工格室加筋效果提高显著;土工格室加筋地基不仅有效控制了基础沉降,而且减小了基础附近地基的沉降差异;筋材调节地基不均匀沉降的加筋效果随筋材埋深减小、筋材层数增加、格室高度增加而有不同程度的提高。     

基于Terzaghi极限承载力理论的加筋地基承载力研究

为研究筋带对地基承载力的影响,基于Terzaghi极限承载力理论,采用极限平衡法和条分法计算了无黏性土加筋地基的极限承载力。首先,将地基滑裂土体分为主动区、过渡区和被动区。其中,主动区侧面与水平面的夹角δ_1不再为固定值,而应满足地基破坏时承载力最小的原则。其次,过渡区滑面为对数螺旋线,计算时采用折线代替从而将过渡区划分为若干三角条块。最后,将筋带等效为作用在三角条块上的拉力,利用极限平衡理论计算被动区、过渡区、主动区各条块的受力,求出不同δ_1对应的地基极限承载力,进而可得到最小地基承载力。同时,分析了筋带层数、埋深和间距、长度及内摩擦角对地基承载力的影响。通过与试验数据对比,验证了本文方法的可行性。     

软土地基土工带加筋碎石垫层试验研究

加筋垫层能够提高地基承载力,增加地基稳定性。对某土质条件的加筋垫层进行现场试验,研究加筋垫层的作用和影响加筋效果的主要因素。分析了加筋层数、筋带间距、加筋线密度等参数对加筋地基强度和变形的影响。试验结果表明,垫层加筋后地基承载力显著增加,而且双层加筋效果更好。对于双层加筋,随着荷载的增加,靠近基底的筋带才先发挥作用。影响加筋效果的主要因素有加筋层数和加筋带的间距等。     

静载作用下土工格栅加筋防护埋地管道力学性能实验研究

针对土工格栅加筋防护埋地管道开展静力载荷实验,研究管周填土相对密实度(Dr)、管道埋深(H)、筋材长度(L)、加筋层数(n)以及首层筋材埋深(u)等对埋地管道防护性能的影响。实验结果表明:首层筋材最佳埋深为0.4B(B为加载板宽),筋材最佳铺设长度为4D(D为管道外径),筋材层数以3~4层为宜;同等条件下随着Dr持续增加,管道极限承载力增加,加载板沉降相应减少,且二者变化率明显降低,表明管周土相对松散时加筋效果愈加明显;同等条件下管道水平和竖向径向变形均随地表载荷增加而增加,且竖直径向比水平径向变形略大,通过增加筋材层数能显著提高土体刚度,能有效地分散管道上方载荷,为管道提供减载保护;管道外壁监测点环向应变值为-1.5%~1.0%,顶部以压缩变形为主,其两侧45°处为压缩和拉伸变形过渡区,而水平径向以拉伸变形为主;随着Dr增加,管周环向应变减小,且应变的对称性愈加显著,表明因Dr增加引起土体自身刚度增加,能有效地限制管道移动及变形。     

循环荷载作用下格栅防护柔性埋地管道的性能分析

交通循环荷载下埋地管道性能与防护是当前研究的重点问题,首先针对格栅加筋柔性管道开展试验研究,分析管道埋深H为3D(D为管道外径)时循环荷载水平和频率、首层格栅埋深、长度、层间距和筋材层数对管道力学与变形性能的影响,试验结果表明:首层格栅最佳埋深u为0.4B(B为加载板宽度),最佳层间距ug为0.5B,最佳铺设长度L为5D;增加格栅层数能显著增强土体,从而有效减少管道变形和加载板沉降;提高荷载水平或降低荷载频率使管道变形、加载板沉降和格栅应变整体显著增加;格栅应变随其与加载板中心的距离增加而减小,格栅中心点应变随循环次数增加呈现先增加后减少的趋势。进而,基于有限元数值模拟分析管道埋深H、加载板宽度B和管径D对管道力学性能的影响,数值结果表明增加管道埋深或减小加载板宽度,管道径向变形减小;同等荷载作用下,减小管径时管道径向变形增大,筋材加筋效果减弱,适当增加管道直径,有利于筋材加筋作用的充分发挥,从而减小管道径向变形。     

静载作用下土工格栅加筋防护埋地管道力学性能的实验研究

 针对土工格栅加筋防护埋地管道开展了静力载荷实验,研究管周填土相对密实度(Dr)、管道埋深(H)、筋材长度(L)和层数(n),以及首层筋材埋深(u)等对埋地管道防护性能的影响。实验结果表明：首层筋材最佳埋深为0.4B(加载板宽),筋材最佳铺设长度为4D(管道外径),筋材层数以3～4层为宜;同等条件下随着Dr持续增加,管道极限承载力增加,加载板沉降相应减少,且二者变化率明显降低,表明管周土相对松散时加筋效果愈加明显;同等条件下管道水平和竖向径向变形均随地表载荷增加而增加,且竖直径向比水平径向变形略大,通过增加筋材层数能显著提高土体刚度,能有效地分散管道上方载荷,为管道提供减载保护;管道外壁监测点环向应变值位于－1.5%～1.0%之间,顶部以压缩变形为主,其两侧45°处为压缩和拉伸变形过渡区,而水平径向以拉伸变形为主;随着Dr增加,管周环向应变减小,且应变的对称性愈加显著,表明因Dr增加引起土体自身刚度增加,能有效地限制管道移动及变形。     

不同含水量土工格栅加筋粘土三轴试验研究

通过土体三轴受压条件下的无侧向静止土压力系数和摩尔-库仑定理,将加筋粘土体在三轴受压条件下的总体抗剪承载力分为加筋土体发生侧向变形前的承载力和与土工格栅密切相关的土体发生侧向变形后的承载力两部分进行研究.通过不同含水量下加筋粘土的三轴试验及其所得相关数据,提出了利用加筋土体发生侧向变形后的抗剪承载力与极限抗剪承载力的比值p2随加筋层数的变化情况作为评价加筋效果的重要指标.试验结果表明,当粘性土在最优含水量状态下时,其加筋效果最为显著.     

加筋地基极限承载力的变分解法

针对条形基础下均质软土加筋地基的极限承载力问题,根据塑性极限平衡原理,考虑各层筋材的拉力关系及拉力方向,在Mohr-Coulomb破坏准则的基础上,将加筋地基极限承载力问题等价为一个泛函极值问题。利用变分原理得到与平衡方程相等价的积分约束条件以及相应的欧拉方程与横截条件,在引入边界条件后,求得了加筋地基破坏时的滑裂面、滑裂面上法向应力及加筋地基极限承载力。与此同时,研究了土体内摩擦角、土工材料受拉方向、土工材料加筋层数及铺设层间距等因素对地基极限承载力的影响,为软土地基加筋工程设计提供理论参考。     

斜坡地基极限承载力下限解计算

为了研究斜坡地基的破坏机理，基于极限平衡法理论，建立了一个新的能考虑坡后土体坡度影响的斜坡地基承载力的计算模式，通过FORTRAN语言编制了斜坡地基坡后土体的发挥系数的计算程序，分析了斜坡地基内摩擦角、基础下侧土体坡度、基础上侧土体坡度、相对坡顶距及基础相对埋深等因素对斜坡地基极限承载力性能的影响，得出了不同地基情况下斜坡地基坡后土体的发挥系数。得到的斜坡地基的承载力系数和发挥系数可用于斜坡地基的理论分析和设计中。     

A quasi-2D exploration of optimum design settings for geotextile-reinforced sand in assistance with PIV analysis of failure mechanism

Many earlier studies were focused on testing different types of geosynthetics to investigate effect of reinforcement on bearing capacity, but the effect of tensile strength on the failure mechanism has not been examined sufficiently. Within this scope, a test setup was prepared to apply strip loads on densely compacted reinforced sand under the plane strain condition. The tank containing the reinforced sand was a rectangular prism with perfect transparency, and its interior dimensions were 960 × 200 × 650 mm³. Firstly, optimum values of design variables (depth of first sheet, length and number of sheets, space between sheets, tensile strength of sheets) for the woven geotextile reinforced sand were determined experimentally. Then, the failure mechanisms of the soil, which were reinforced with geotextiles of different tensile strengths, were observed and analyzed with particle image velocimetry (PIV) technique. Consequently, the failure mechanism of the sand with a single geotextile reinforcement was similar to general shear failure of unreinforced soil. Contrarily, the failure surfaces were deeper and longer. Additionally, the deep-footing mechanism reached out large depth in the case of four reinforcement layers. The failure mechanism converted into a punching type due to a hypothetic increase in the bearing depth of reinforcement.     

Load-settlement response of shallow square footings on geogrid-reinforced sand under cyclic loading

To study the settlement and dynamic response characteristics of shallow square footings on geogrid-reinforced sand under cyclic loading, 7 sets of large scale laboratory tests are performed on a 0.5?m wide square footing resting on unreinforced and geogrid reinforced sand contained in a 3?m?×?1.6?m?×?2?m (length?×?width?×?height) steel tank. Different reinforcing schemes are considered in the tests: one layer of reinforcement at the depth of 0.3B, 0.6B and 0.9B, where B is the width of the footing; two and three layers of reinforcement at the depth and spacing both at 0.3B. In one of the two double layered reinforcing systems, the reinforcements are wrapped around at the ends. The footings are loaded to 160?kPa under static loading before applying cyclic loading. The cyclic loadings are applied at 40?kPa amplitude increments. Each loading stage lasts for 10?min at the frequency of 2?Hz, or until failure, whichever occurs first. The settlement of the footing, strain in the reinforcement and acceleration rate in the soil have been monitored during the tests. The results showed that the ultimate bearing capacity of the footings was affected by the number and layout of the reinforcements, and the increment of bearing capacity does not always increase with the number of reinforcement layers. The layout of the reinforcement layers affected the failure mechanisms of the footings. Including more layers of reinforcement could greatly reduce the dynamic response of the foundations under cyclic loading. In terms of bearing capacity improvement, including one layer of reinforcement at the depth of 0.6B was the optimum based on the test results. It is found that fracture of geogrid could occur under cyclic loading if the reinforcement is too shallow, i.e. for the cases with the first layer of reinforcement at 0.3B depth.     

Two dimensional experimental study for the behaviour of surface footings on unreinforced and reinforced sand beds overlying soft pockets

This paper presents results of a comprehensive investigation undertaken to quantify the efficiency of using reinforcement layers in order to enhance the bearing capacity of soils that are characterised by the existence of localised soft pockets. Small-scale model experiments using two dimensional tank were conducted with beds created from well graded sand with mean particle size of 300 μm but prepared with different dry densities. A relatively softer material was embedded at predetermined locations within the sand beds so as to represent localised soft pockets. Various arrangements of soil reinforcement were tested and compared against comparable tests but without reinforcement. In total 42 tests were carried out in order to study the effect of the width and depth of the soft pocket, the depth of one reinforcing layer and the length and number of reinforcing layers on the soil bearing capacity. The results show clearly that the ultimate bearing capacity reduces by up to 70% due to the presence of a soft pocket. It was also noted that the proximity of the soft pocket also influenced the bearing capacity. Reinforcing the soil with a single layer or increasing the length of reinforcement is not as effective as was anticipated based on previous studies. However, bearing capacity increased significantly (up to 4 times) to that of unreinforced sand when four layers of reinforcement were embedded. The results suggest that rupture of the bottom reinforcement layer is imminent in heavily reinforced sand beds overlying soft pockets and therefore its tensile strength is critical for successful reinforcement.     

Laboratory investigation of bearing capacity behaviour of strip footing on reinforced flyash slope

The paper presents the results of laboratory model tests on bearing capacity behaviour of a strip footing resting on the top of a geogrid reinforced flyash slope. A series of model footing tests covering a wide range of boundary conditions, including unreinforced cases were conducted by varying parameters such as location and depth of embedment of single geogrid layer, number of geogrid layers, location of footing relative to the slope crest, slope angles and width of footing. The results of the investigation indicate that both the pressure–settlement behaviour and the ultimate bearing capacity of footing resting on the top of a flyash slope can be enhanced by the presence of reinforcing layers. However the efficiency of flyash geogrid system increases with the increasing number of geogrid layers and edge distance of footing from the slope. Based on experimental results critical values of geogrid parameters for maximum reinforcing effects are established. Experimental results obtained from a series of model tests have been presented and discussed in the paper.     

土工袋加固砂性土质边坡模型试验与上限解

 通过模型试验比较有土工袋加固和无土工袋加固时边坡的破坏形态及承载力,验证土工袋对边坡的加固效果,并基于试验结果建立边坡的许可破坏模式及其速度场,利用极限分析上限法求解了边坡的极限高度上限解,利用模型试验结果对边坡极限高度进行验证。试验及计算结果均表明：边坡越陡,有土工袋加固较无土工袋加固时边坡的极限高度和承载力提高越大,表明土工袋加固效果越好。试验结果与计算结果基本吻合,表明该计算方法的可信性,可为边坡的稳定性设计问题提供理论依据。     

Bearing capacity of strip footings on sand slopes reinforced with geogrid and grid-anchor

This paper presents the effect of a new type of geogrid inclusion on the bearing capacity of a rigid strip footing constructed on a sand slope. A broad series of conditions, including unreinforced cases, was tested by varying parameters such as geogrid type, number of geogrid layers, vertical spacing and depth to topmost layer of geogrid. The results were then analyzed to find both qualitative and quantitative relationships between the bearing capacity and the geogrid parameters. A series of finite element analyses was additionally carried out on a prototype slope and the results were compared with the findings from the laboratory model tests and to complete the results of the model tests. The results show that the bearing capacity of rigid strip footings on sloping ground can be intensively increased by the inclusion of grid-anchor layers in the ground, and that the magnitude of bearing capacity increase depends greatly on the geogrid distribution. It is also shown that the load-settlement behavior and bearing capacity of the rigid footing can be considerably improved by the inclusion of a reinforcing layer at the appropriate location in the fill slope. The agreement between observed and computed results is found to be reasonably good in terms of load-settlement behavior and optimum parameters.     

Bearing capacity of square footings on geosynthetic reinforced sand

The results from laboratory model tests and numerical simulations on square footings resting on sand are presented. Bearing capacity of footings on geosynthetic reinforced sand is evaluated and the effect of various reinforcement parameters like the type and tensile strength of geosynthetic material, amount of reinforcement, layout and configuration of geosynthetic layers below the footing on the bearing capacity improvement of the footings is studied through systematic model studies. A steel tank of size 900 × 900 × 600 mm is used for conducting model tests. Four types of grids, namely strong biaxial geogrid, weak biaxial geogrid, uniaxial geogrid and a geonet, each with different tensile strength, are used in the tests. Geosynthetic reinforcement is provided in the form of planar layers, varying the depth of reinforced zone below the footing, number of geosynthetic layers within the reinforced zone and the width of geosynthetic layers in different tests. Influence of all these parameters on the bearing capacity improvement of square footing and its settlement is studied by comparing with the test on unreinforced sand. Results show that the effective depth of reinforcement is twice the width of the footing and optimum spacing of geosynthetic layers is half the width of the footing. It is observed that the layout and configuration of reinforcement play a vital role in bearing capacity improvement rather than the tensile strength of the geosynthetic material. Experimental observations are supported by the findings from numerical analyses.     

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.     

Bearing capacity of strip foundations in horizontal-vertical reinforced soils

The formula for calculating the ultimate bearing capacity of horizontal-vertical reinforced soil is investigated based on the failure mode and the mechanism of sand beds reinforced with horizontal-vertical reinforcement. Two components of soils and reinforcement are calculated separately. The ultimate bearing capacity of a shallow, concentrically loaded strip footing on homogeneous soil is commonly determined using the Terzaghi superposition method. The contribution of horizontal-vertical reinforcement is calculated based on the bearing resistance of the soil against the transverse members. A vertical inclusion is treated as a retaining wall, the confinement being calculated using Rankine's earth pressure theory. An analytical solution is presented including the traditional factors of soil, unit soil weight, footing width, number of horizontal-vertical reinforcement layers, and reinforcement geometry. The results were validated against experimental results and the mean error of the theoretical model was about 10%, with a maximum error of about 20%.     

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。