Behaviour of geogrid reinforced soil retaining wall with concrete-rigid facing

Monitoring was carried out during construction of a cast-in-situ concrete-rigid facing geogrid reinforced soil retaining wall in the Gan (Zhou)-Long (Yan) railway main line of China. The monitoring included the vertical foundation pressure and lateral earth pressure of the reinforced soil wall facing, the tensile strain in the reinforcement and the horizontal deformation of the facing. The vertical foundation pressure of reinforced soil retaining wall is non-linear along the reinforcement length, and the maximum value is at the middle of the reinforcement length, moreover the value reduces gradually at top and bottom. The measured lateral earth pressure within the reinforced soil wall is non-linear along the height and the value is less than the active lateral earth pressure. The distribution of tensile strain in the geogrid reinforcements within the upper portion of the wall is single-peak value, but the distribution of tensile strain in the reinforcements within the lower portion of the wall has double-peak values. The potential failure plane within the upper portion of the wall is similar to “0.3H method”, whereas the potential failure plane within portion of the lower wall is closer to the active Rankine earth pressure theory. The position of the maximum lateral displacement of the wall face during construction is within portion of the lower wall, moreover the position of the maximum lateral displacement of the wall face post-construction is within the portion of the top wall. These monitoring results of the behaviour of the wall can be used as a reference for future study and design of geogrid reinforced soil retaining wall systems.     

DIA of centrifuge model tests on geogrid reinforced soil walls with low-permeable backfills subjected to rainfall

Geogrid reinforced soil walls (GRSWs) constructed using low-permeable backfills often experience failures when subjected to rainfall. The objective of this paper is to employ centrifuge modelling to investigate the effect of geogrid types on the performance of GRSW models constructed with low-permeable backfill, when subjected to rainfall intensity of 10 mm/h. A 4.5 m radius large beam centrifuge facility was used, and rainfall was simulated using a custom-designed rainfall simulator at 40 gravities. Digital Image Analysis (DIA) was employed to understand the deformation behaviour of GRSWs with low stiffness geogrid layers with and without drainage provision subjected to rainfall. Additionally, the effect of varying stiffness of geogrid reinforcement layers across the height of GRSW was also investigated. The interpretation of DIA helped to quantify displacement vector fields, face movements, surface settlement profiles and geogrid strain distribution with depth. Irrespective of drainage provision, GRSWs reinforced with low stiffness geogrid layers experienced a catastrophic failure at the onset of rainfall. However, GRSW reinforced with geogrid layers of varying stiffness was observed to perform well. This study demonstrates the effective use of DIA of GRSWs subjected to rainfall along with centrifuge-based physical model testing.     

静动载下筋材变化对加筋土挡墙性能影响分析

为了研究动静荷载下,加筋长度及筋材类型变化对加筋土挡墙工作性能的影响,进行了7种工况下的加筋土挡墙模型试验,对比分析了加筋土挡墙的水平土压力、水平土压力系数、墙面水平位移和加载板竖向沉降及筋材应变等参数的发展规律。试验结果表明:动载下加筋土挡墙筋材应变随着加载时间的增长、加筋长度的减小、位置高度的增加而增大,且顶层筋材应变远远大于其他层;加筋长度及筋材横肋的减少明显降低挡墙的承载性能,格栅横肋减少导致挡墙极限承载力降低18% ,加筋长度减少使面板水平位移最大增大了2.2倍;与静载作用下相比,动载下土工格栅的侧向约束作用及网兜效应能够得到更好地发挥。     

Experimental investigation of a reinforced soil retaining wall with a flexible geogrid-wrapped ecological bag facing

This paper presented a field study of the reinforced soil wall (RSW) with a geogrid wrap-around facing. In addition to the conventional monitoring content, the strain of the face-wrapping geogrid, which was neglected in most previous studies, was monitored during the construction process. The positional relationship between the maximum vertical earth pressure and horizontally laid geogrid strain was revealed by using the proposed oblique dragging effect. It was found that the strain on the face-wrapping geogrid occurred mainly in the early stage of construction. The oblique dragging effect existing in the flexible RSWs increased the vertical earth pressure and changed its distribution law, resulting in the position of the maximum vertical soil pressure appearing behind the position of the maximum horizontally laid geogrid strain. The horizontal earth pressure at different positions behind the wall was obviously lower than the theoretical result by using Rankine's theory. Also, a modified 0.3H method (where H refers to the wall height) was presented to account for the slope of the wall face, which could be used to determine the potential sliding surfaces for single-stage RSWs and two-tier RSWs with small offset.     

超高无面板式土工格栅加筋路堤现场试验研究

结合在建宜巴高速公路50 m高的加筋填土断面进行现场试验,对超高无面板式土工格栅加筋路堤的格栅变形、垂直土压力、水平土压力、分层沉降以及深层水平位移等内容进行了近2 a的测试,研究超高无面板式土工格栅加筋土路堤的受力、变形规律,分析了其作用机理。结果表明：不同层位土工格栅的最大拉应变出现在离返包面约4~6 m处,格栅应变沿筋长呈双峰值分布,施工期土工格栅应变具有明显的滞后性,且工后1.5 a格栅出现明显的收缩回弹;土工格栅的存在对土压力分布具有明显的调整作用,格栅末端附近实测垂直土压力值略超过理论值,中间和近坡面部位实测土压力值小于理论值;水平土压力沿路堤高度呈非线性形式分布,路堤中部的水平土压力值略大于顶部,其值小于主动土压力;分层沉降量在施工期存在较大波动,在垂直高度上,上部和底部偏小,中下部偏大;深层水平位移随着深度的增加逐渐减小,填土结束后深层水平位移仍有一定程度增大。     

Centrifuge model studies on the performance of soil walls reinforced with sand-cushioned geogrid layers

This paper is to investigate the effectiveness of encapsulating geogrid layers within thin sand layers, for enhancing the deformation behavior of vertical reinforced soil walls constructed with marginal backfills. Centrifuge model tests were performed on vertical soil walls, reinforced with geogrid layers, using a 4.5 m radius large beam centrifuge available at IIT Bombay at 40 gravities. The backfill conditions, height of soil wall, reinforcement length, and reinforcement spacing, were kept constant in all the tests. A wrap-around technique was used to represent flexible facing. Three different geogrid types with varying stiffness were used in the present study. The walls were instrumented with vertical linear variable differential transformers to monitor surface settlements during the tests. Marker-based digital image analysis technique was used to determine face movements and distribution of geogrid strain along the wall height. The deformation behavior of soil walls, reinforced with geogrid layers encapsulated in thin layers of sand, were compared against a base model having no sand-cushioned geogrid layers. Provision of sand-cushioned geogrid layers and increase in geogrid stiffness were found to limit normalized face movements (S_f/H), normalized crest settlements (S_c/H), and change in maximum peak reinforcement strain (dε_pmax). Sand-cushioned geogrid layers were also found to limit the development of tension cracks behind and within the reinforced zone. Significant reduction in rate of maximum face movement (dS_fmax/dt) and rate of maximum peak reinforcement strain (dε_pmax/dt) was observed, with an increase in value of normalized reinforcement stiffness (J_g/γH²) of geogrid layers. The analysis and interpretation of centrifuge model tests on soil walls, constructed with marginal backfills and reinforced with sand-cushioned geogrid layers, indicate that their performance is superior to the walls without sand-cushioned geogrid layers.     

土工合成材料大型直剪界面作用宏细观研究

利用大型直剪模型试验设备，在不同竖向压力下进行一系列的土工合成材料直剪试验，应用数码可视化跟踪技术，结合土体变形无标点量测技术来研究双向土工格栅与砂土直剪界面作用的宏细观特性，同时分析界面附近土压力分布规律，并研究界面颗粒运动变化规律和细观组构演化特征与宏观特性的关联。分析结果表明，直剪筋土界面附近竖向压力分布从前端依次向后端减少；直剪界面位移达25 mm时，形成了稳定的剪应变集中带；在筋土界面（6～8）D₅₀粒径厚度范围内，界面颗粒以旋转和平动方式同时位移，该范围外颗粒以平动方式沿剪切方向位移，且位移较小；在剪切过程中，界面颗粒发生旋转，土体发生剪胀，孔隙率增大，平均接触数减小，颗粒重新被压密，孔隙率减小，平均接触数增多，颗粒长轴排列趋于水平方向，各细观组构处于相对稳定状态。     

双级土工格栅加筋土挡墙的测试分析

通过对梅坎铁路双级土工格栅加筋土挡墙的墙面板水平土压力、垂直土压力及土工格栅变形的原位观测 ,分析了墙面板水平土压力的变化及分布规律、垂直土压力变化及土工格栅的变形随填高及时间的变化规律 ,并对挡土墙的破裂面进行了探讨     

Centrifuge tests to investigate global performance of geosynthetic-reinforced pile-supported embankments with side slopes

Three centrifuge model tests were conducted to investigate the influence of the number of geosynthetic layers and the pile clear spacing on the global performance of Geosynthetic-Reinforced Pile-Supported (GRPS) embankments with side slopes constructed on soft soil foundations. This study found that the change of the geogrid number from one to two did not significantly affect the foundation settlement, the geogrid deflection, and the vertical stress at the embankment base. For the GRPS embankment with a single geogrid layer, the geogrid strain distribution at the embankment base showed an “M” shape along the transverse direction with the maximum strain near the embankment shoulder. When two geogrid layers with sand in between were used, the upper and lower layers showed different strain distributions with the maximum strains happening near the embankment shoulder and at the center of the embankment for the upper and lower layers respectively. The strains of the upper geogrid were smaller than those of the lower geogrid. Smaller pile clear spacing reduced the geogrid deflection and the foundation settlement. Despite the change of the pile clear spacing, the progressive development of soil arching with the normalized displacement at the embankment base followed a similar trend without an obvious stress recovery stage.     

土工格栅加筋石灰土挡墙工程特性试验研究

以河北省保（定）沧（州）高速公路模块式土工格栅加筋石灰土挡墙为工程依托,以现场原型试验为手段,系统研究了该结构工作状态下的基底竖向土压力、墙面板背部侧向土压力和土工格栅拉筋应变分布规律。试验结果表明：基底竖向土压力沿筋长近似呈梯形分布,其大小一般小于理论值,最大值发生在墙背附近,且随竣工后时间的延续有下降的趋势;实测墙背侧向土压力沿墙高呈非线性增长分布,数值小于主动土压力;实测拉筋应变沿筋长呈单峰值分布,且数值均小于0.6%。试验结果可以为类似工程的设计、研究提供参考。     

Enhanced performance of reinforced soil walls by the inclusion of short fiber

This paper presents the effects of the inclusion of short fiber in sandy silt (SM) soil on the performance of reinforced walls. The inclusion of short fiber in soil is expected to increase soil strength and improve stability when it is used as the backfill material. Short fiber of 60 mm length was used and the mixing ratio of the fiber was 0.2% by weight of the soil. The finite element method was used to examine the influence of the reinforced short fiber on reinforced walls. The vertical and horizontal earth pressure, displacement and settlement of the wall face were analyzed. These results were compared to the measured results from two full-scale tests. It is shown that use of short fiber reinforced soil increases the stability of the wall and decreases the earth pressures and displacements of the wall. This effect is more significant when short fiber soil is used in combination with geogrid.     

某河道护岸工程加筋土挡墙的原型观测

通过对某河道护岸工程加筋土挡墙的筋带拉力、面板土压力、基底压力、填土分层沉降、面板水平位移以及潮位变化等进行施工期间的原型观测 ,分析并研究了筋带拉力、面板土压力和基底压力随填土厚度变化的规律 ,以及填土分层沉降、面板水平位移以及潮位变化的分布规律及其影响因素     

Novel soil-pegged geogrid (PG) interactions in pull-out loading conditions

In current study, large-scale pull-out tests were conducted to examine the behavior of a novel reinforcement system named “Pegged Geogrid” (PG) under pull-out loading condition. Metal pegs were combined with geogrid to enhance resistance to pull-out. Incorporating pegs with geogrids alleviates bolting, welding, clamping, increasing geogrid length or making alterations to the geogrid as recommended by previous researchers. Peg roots are simply inserted/driven through apertures into the soil, nailing the geogrid to the lower and subsequently the upper backfill layers. Effects of soil particle size, normal pressure, peg length, width, and numbers has been evaluated using two sandy and a gravely soil. Results show that inclusion of pegs significantly enhances soil passive resistance contribution to pull-out. Increasing the width and number of pegs, resulted in enhancing passive soil resistance activation in front of the bearing surfaces and thus greater pull-out resistance augmented by soil particle size and normal pressure. Displacements corresponding to maximum pull-out forces gradually improved by peg width and strain distribution along geogrid in the PG system progressively became linear in contrast to the non-linear distribution in the conventional soil-geogrid (NG) system. Normal pressure was more influential on enhancing pull-out resistance in coarser soil.     

Performance of a 33m high geogrid reinforced soil embankment without concrete panel

Geosynthetic reinforced soil embankment are extensively applied in the construction of high-speed railway and highway in mountainous regions but limited field monitoring is conducted on high and steep cases. Aiming to acquire better understanding, a 33-m-high single-tiered wrapped-facing geogrid reinforced soil embankment with the slope of 1 V:0.5H in China was monitored for 2 years during and after construction. Vertical earth pressure, strain of geogrids, horizontal displacement and settlement per layer were recorded and analysed. The results show that the geogrid tensile strains gradually increased during construction. And they were still developing after completion due to creep and subsequent vehicle surcharge load. The predictions of reinforcement loads by the FHWA methods were much higher than the estimated ones from measured strains. The vertical earth pressures linearly grew during construction and then stabilized fast. The horizontal displacement increases with height and the largest value achieved around the top of the slope two years after the construction is 0.14% the total height approximately. The settlement per layer is larger in the lower and middle portion of the embankment and no obvious change is observed over time. This study hopes to serve as a case reference for design and construction of similar reinforcement projects in the future.     

土工格栅加筋土地基载荷试验的数值模拟

采用FALC^3D对土工格栅加筋土地基载荷试验进行了进一步的数值模拟分析。根据计算结果,针对原型试验中难以量测的试坑变形及筋土界面摩阻力分布特征进行了讨论。利用数值模拟技术的优势,求解加筋地基的应变场,研究了加筋地基的破坏模式。结果表明:在竖向荷载作用下,试坑会发生侧向位移,通过加筋能有效减小试坑的侧向位移;筋土界面摩阻力的分布与筋土之间的相对位移直接相关;加筋地基的破坏机构因筋材的存在而发生改变,“深基础”效应以及“扩散层”效应都是加筋地基的增强机理,但地基的破坏模式随筋材的布置形式改变而有所不同。     

Three-dimensional physical modeling of load transfer in basal reinforced embankments under differential settlement

This study developed a large-scale laboratory apparatus to evaluate the load transfer behavior of basal reinforced embankment fill because of soil arching and geogrid tensile force. A 3D trapdoor-like test system performed five scaled model tests using analogical soil. The instrumentation system involved multiple earth pressure cells, dial gauges, multipoint settlement gauges, and geogrid strainmeters. Comprehensive measurements were performed to investigate the evolution of soil stress and displacement at specific fill elevations with variations in the area replacement ratio and geogrid stiffness. The critical height of the soil arching was determined to be ～1.1–1.94 times the clear pile spacing based on the soil stress and displacement profiles. The distribution of the geogrid tensile strain between and above the adjacent caps demonstrated that the maximum geogrid strains occur on top of the caps, and the tensioned geogrid effect on the load transfer efficiency was evaluated. The cap size and center-to-center pile spacing affect the pile efficacy more significantly than the stiffness of the geogrid. The measured critical heights of arching, stress concentration ratios, and geogrid strain matched those calculated by several well-recognized analytical methods. This experimental program facilitates the development of arching models that account for differential settlement impact.     

台阶格栅加筋土墙土压力的模型试验研究

 为研究多级台阶式格栅加筋墙的工作机制和力学特性，在室内修建一个长8.0 m，宽3.0 m，高4.5 m的模型槽。在模型槽中对3.0 m×1.5 m的3级台阶格栅加筋土挡墙和2种格栅网格尺寸进行系列模型试验。测试格栅加筋土挡墙面板后的土压力、加筋体后土压力、加筋土各分层土压力及地基应力等。试验发现加筋体内各点的土压力与传统的土压力理论计算值不一致，土压力分布与基础条件及加筋体高度密切相关；加筋体基础的位移对加筋体的力学特性影响较大，它会引起加筋土体内应力的重分配，即当位移较大时挡墙地基应力呈现V型分布，较小时按斜线分布；面板后侧竖向和侧向土压力沿墙高均呈现顶部和底部小中间大的外凸型分布，加筋体后土压力亦有同样的趋势，只是外凸程度较小；加筋体内力学特性变化与格栅网格尺寸也有一定关联。     

软土地基加筋土挡墙现场试验研究

 结合一软土地基反包式土工格栅加筋黏性土挡墙现场测试,对挡墙填筑过程中原地表沉降、墙趾水平和垂直位移、墙面水平位移、挡墙内部垂直土压力和墙背水平土压力,以及筋材应变分布等进行分析,探讨其工作性状及其稳定性。分析结果表明：软土地基加筋土挡墙的破坏形式表现为外部失稳;挡墙墙面出现“鼓肚”现象,其最大水平位移位于挡墙中部墙高位置;格栅应变在距墙面0.8 H(H为挡墙的高度)处最大,设计上的0.3H法不能适用于深厚软土地基加筋土挡墙。研究成果可为今后类似工程的研究、设计与施工提供参考。     

根据桩身侧向位移反演非极限状态下土压力分布

根据支护桩侧向位移,反演了非极限状态下桩身所受土压力从而进行了支护桩设计,并结合工程实例进行了验算,结果表明：利用非极限状态下土压力与桩身位移的关系式,反演出桩身的受力状态处于静止土压力与主动土压力之间,符合非极限状态下土压力处于静止土压力与极限土压力之间的基本假设;并根据对反演土压力进行的验算,得出了其在基坑不同深度情况下计算的准确性。     

应用应力圆的准静止土压力计算

垂直挡土墙,墙后土体表面水平,不产生侧向变形为最简单情况下的静止土压力;当墙背倾斜、填土表面倾斜,或两者兼而有之的复杂计算条件为实际状态下静止土压力;本文分析了墙后土体不产生线应变而仅有剪切应变的土压力,称之为准静止土压力。具体分析方法如下:根据单元体在侧压力作用下的应力圆、极点及应变计算,得出在线应变为零的条件下,表面倾斜的墙后填土体作用在斜墙上的准静止土压力计算式。所导出满足线应变为零的准静止土压力计算式是静止土压力计算的延伸,有理论意义及使用价值。