中低压缩性土地区桩承式加筋路堤现场试验研究

将桩承式加筋路堤技术应用于中低压缩性土地区高速铁路桥台和涵洞之间填方路基的处理,通过逐渐改变CFG桩桩长形成刚度均匀变化的地基加固区,严格控制线路纵向差异沉降。通过现场试验对桥台、涵顶和路基中心地基沉降进行了长期观测,同时对桩承式加筋路堤桩间土沉降、孔隙水压力、格栅上下表面土压力和格栅变形进行了长期监测分析。研究结果表明:桩承式加筋路堤可有效减小中低压缩性土地基沉降,总沉降小且很快趋于稳定;桩承式加筋路堤通过土拱效应和张拉膜效应将路堤荷载向桩帽传递,格栅下桩土应力比明显高于格栅上,张拉膜效应明显,格栅上桩土应力比接近1.0,土拱效应较弱;格栅在路肩处发挥的作用强于线路中心处。     

深厚软土层端承摩擦桩桩承式加筋低路堤现场试验

为揭示桩承式加筋低路堤的荷载传递机理及土工格栅的加固效应,选取武汉智能网联赛车道项目主赛道连接段某低路堤路段开展了一个承载单元的现场试验,分别测试了施工期桩帽顶部与格栅上层的土压力分布、路堤分层沉降、格栅应变与挠曲。数据分析表明：随着填土高度的增加,土拱效应与张拉膜效应逐渐发挥,桩帽顶部的土压力开始超过桩间土压力并在填筑结束后逐渐趋于稳定。在端承摩擦桩桩承式加筋低路堤的荷载传递机制中,桩间土承担主要荷载,占比62.23%,张拉膜效应仅占很小一部分,占总荷载的3.77%。土工格栅的最大应变出现在桩帽边缘处,但最大值仅为0.195%,这表明土工格栅性能难以得到充分发挥。在该桩承式加筋低路堤中,两桩中心处的桩间土沉降要明显大于四桩对角线中心处。     

加筋形式对桩承式路堤工作性状影响的试验研究

对无加筋和采用不同加筋材料、加筋层数下桩承式路堤的工作性状进行了三维模型试验研究,侧重分析了桩土应力比、应力折减系数、填土中竖向应力分布、地基沉降等内容。结果表明加筋材料的设置有利于荷载向桩顶的转移,可有效减小沉降,但不同加筋形式下桩承式路堤的工作性状有所不同。使用单层或双层土工布时,路堤的荷载传递机理主要是填土的土拱效应和加筋材料的拉膜效应,但拉膜效应发挥相对较晚。使用双层格栅时,加筋材料与周围砂土形成半刚性平台。单层格栅的作用介于两者之间。试验结果与常规拉膜效应设计方法的对比表明,若假设荷载只由相邻桩间的加筋材料条带承担,计算的拉力将偏大,过于保守。     

移动荷载作用下桩承式加筋路堤的动力特性

为了研究桩承式加筋路堤在移动荷载作用下的特性,采用FLAC 3D软件建立了移动荷载作用下道路的三维动力流固耦合分析模型,对桩承式加筋路堤和天然路堤在移动荷载作用下的竖向变形、桩土应力比、超孔隙水压力、加速度等进行了对比分析,并研究了不同轴载对路堤竖向变形的影响。分析结果表明：移动荷载作用下,桩承式加筋路堤通过桩体土拱效应和格栅张拉膜效应的联合作用,其路面竖向变形、桩土应力比、超孔隙水压力、加速度均比天然路堤的结果明显减小;随着轴载的增加,桩承式加筋路堤路面竖向变形不断增大。     

基于有限元方法的桩承式加筋拓宽路基数值模拟分析

针对桩承式加筋路基这一复杂的工作系统,结合连霍高速某段桩承式路堤拓宽工程,通过有限元软件ABAQUS建立了桩承式加筋路堤整体三维有限元分析模型;从土拱效应、土工格栅的拉膜效应,以及桩土间刚度差异引起的不均匀沉降等方面分析了系统的工作机理。结果表明土工格栅最大变形和最大拉力发生在桩帽边缘处;拓宽路堤的施工阶段对地表沉降影响较小;软土的长期固结是造成不均匀沉降的主要原因等结论。     

桩承式加筋路堤土拱及格栅受力模型试验研究

介绍了桩承式加筋路堤足尺模型实验装置,该实验装置利用PVC材料水袋模拟桩间软土,从而在一定程度上能够控制桩土差异沉降。路堤填筑过程中测试了路堤内部土压力以及格栅拉力,并且重点分析了桩帽和桩间不同位置处土压力以及格栅拉力随填筑高度的变化规律。实验结果表明,路堤在填筑过程中发生了明显的土拱效应,路堤填筑完成后桩土应力比约为8.46,土拱高度约为1.125倍桩间净距;单向土工格栅能够进一步将桩间上方土压力传递到桩顶上方;随着路堤填筑高度的增加,格栅拉力增长并不大,路堤横向滑移引起的格栅拉力可以忽略不计。     

桩承式加筋路堤中土拱形态及荷载传递机理的数值分析

通过建立平面应变条件下桩承式加筋路堤土拱形态分析的弹塑性有限元模型,对桩承式加筋路堤中的土拱形态进行有限元数值模拟,并分析加筋对桩帽-土荷载分担特性的影响。计算结果表明:平面应变条件下桩承式加筋路堤中的土拱形态为半椭圆,土拱高度小于无筋桩承式路堤。土拱高度随路堤高度的增大先增高后稳定不变。加筋刚度对土拱高度的影响较小。加筋将桩间土承担的路堤荷载向桩帽上转移。由加筋传递到桩帽上的竖向应力随桩间距和加筋刚度的增大而增大,随桩帽宽度的增大而减小。     

基于混合试验的桩承式加筋路堤参数影响分析

桩承式加筋路堤的路堤土拱效应、加筋体张拉膜效应以及桩土地基承载效应之间存在着较为复杂的耦合效应,由于土拱效应随路堤加载与桩间土下沉的演化,目前的计算理论与方法难以真实地评价桩承式路堤荷载调节与沉降稳定的全过程。引入混合试验的技术思路,开发一套阵列式多活动门试验装置及桩土地基混合试验方法,实现加筋垫层路堤物理模型与桩土地基数值模型的适时数据交换,开展9组不同路堤高度与加筋体拉伸模量的参数影响试验。试验结果表明：所建立的混合试验方法能够较好地实现桩承式加筋路堤的全组件参与、全效应耦合工作性能模拟,大大地节省了试验成本和时间,为桩承式加筋路堤的长期承载性能演化以及沉降预测提供了一种新的试验手段。同心圆力学模型能够较为准确地评价路堤的最大土拱效应。当路堤高度较低时,采用拉伸模量较高的加筋体能够提高桩的荷载分担比。采用拉伸模量较高的加筋体,在不同的加筋高度条件下均能够显著地降低桩间土沉降与桩土差异沉降。随着路堤高度的增加,不同加筋体模量试验的桩、土荷载分担比趋于一致,采用地基反力系数法不能可靠地反映桩间土的真实承载机制。     

桩承式加筋路堤桩体荷载分担比计算

为反映多层加筋材料分布层位和应变差异对桩承式加筋路堤的影响,基于土拱形态和筋材应变分布的假设,推导桩土荷载分担比的计算公式。首先根据极限平衡理论和同心圆二维土拱模型假设,推导筋上桩体荷载分担比的计算公式;再假设帽间格栅变形为圆弧曲线,考虑帽上格栅变形对帽间格栅应变的贡献,推导更符合常用大桩帽疏桩结构特点的格栅应变计算公式;进一步以筋下填土产生的土压力和桩间土应力为附加应力计算底层格栅挠度,提出以底层格栅挠度为自变量的任意层格栅挠度表达式;最终依据格栅应变计算格栅拉力及其竖向分量,得到考虑格栅拉膜传荷作用的筋下桩体荷载分担比计算公式。该方法可适用于单层以及非单层加筋的桩承式加筋路堤,经工程实例验证,计算值与实测值吻合较好。     

桩承式加筋路堤土拱效应的缩尺模型试验研究

通过缩尺模型试验,主要研究桩承式加筋路堤中正方形布桩的桩帽净间距、路堤填土性质对土拱效应的影响,分析填土黏聚力和桩帽间距对土拱效应的影响及其随附加荷载变化的规律。研究结果表明:(1)路堤填土的黏聚力使填土中土拱效应增强,桩帽上的土压力增大,而桩间土土压力减小;与黏聚力为0的填土相比,填土内黏聚力不等于0时,桩体荷载分担比增大,在相同荷载条件下桩间土和路堤表面的沉降均减小。(2)填土的成拱条件应与黏聚力无关,土拱高度应该大于1.0倍且小于1.5倍的桩帽净间距。(3)桩帽间距越大,土拱效应越弱,桩体荷载分担比越小。(4)不管路堤填土是否具有黏聚力以及桩帽净间距如何变化,桩承式加筋路堤中桩帽边缘处的土压力均大于桩帽中心处的土压力。     

Full-scale model tests of load transfer in geogrid-reinforced and floating pile-supported embankments

Well-designed field full-scale model tests were carried out to enhance the understanding of geogrid-reinforced and floating pile-supported (GRFPS) embankments constructed on medium compressibility soil (MCS). Two comparative test sections of GRFPS embankments with and without pile caps were built over silty clay with medium compressibility for field monitoring, over 25 months. The heavily instrumented embankments produced comprehensive high-quality data. Settlement, earth pressure, and geogrid strain measurements during embankment filling stages and the postconstruction placement stage were conducted. The influence of pile cap installation on the differential deformation and load transfer behaviour of the GRFPS embankment was evaluated. The results demonstrate the installation of pile caps can significantly improve the evolution characteristics of the stress increment ratio on the pile, facilitating a change in load sharing of the pile top from a “softening” feature to a “hardening” feature. The state of the “arching structure” heavily depends on the relative displacement. After the maximum arching is formed, although the subgrade load continuously increases, the arching enters the damage and recovery state, and the transfer of the overburden load increment is largely conducted by the tensioned membrane effect.     

Effect of geogrid reinforcement on the load transfer in pile-supported embankment under cyclic loading

This paper investigated effects of geogrid reinforcement on the load transfer in pile-supported embankment under cyclic loading using self-moving trapdoor tests. In the self-moving trapdoor test setup, the trapdoor between two stationary portions (which were equivalent to the piles) was supported by compression springs to simulate the subsoil. Quartz sand and a biaxial geogrid were used as the test fill and reinforcement material, respectively. Tests results show that soil arching above the geogrid reinforcement and load transfer to the stationary portions (caused by the soil arching and tensioned membrane effect) experienced a process of “relatively enhancing - relatively degrading” with an increase in the number of cycles and maintained similar degrees within each complete cycle of cyclic loading. Moreover, the inclusion of geogrid reinforcement reduced the mobilization of soil arching, but increased the degree of load transfer to the stationary portions. In addition, cyclic loading with a higher frequency tended to mobilize more soil arching and induce a higher degree of load transfer to stationary portions. Also observed was that a higher frequency cyclic loading tended to decelerate the degradation of load transfer to stationary portions and caused less surface settlement, which indicating increased load-carrying capacity of pile-supported embankment.     

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.     

Two-dimensional soil arching evolution in geosynthetic-reinforced pile-supported embankments over voids

Soil arching and tensioned membrane effects are two main load transfer mechanisms for geosynthetic-reinforced pile-supported (GRPS) embankments over soft soils or voids. Evidences show that the tensioned membrane effect interacts with the soil arching effect. To investigate the soil arching evolution under different geosynthetic reinforcement stiffness and embankment height, a series of discrete element method (DEM) simulations of GRPS embankments were carried out based on physical model tests. The results indicate that the deformation pattern in the GRPS embankments changed from a concentric ellipse arch pattern to an equal settlement pattern with the increase of the embankment height. High stiffness geosynthetic hindered the development of soil arching and required more subsoil settlement to enable the development of maximum soil arching. However, soil arching in the GRPS embankments with low stiffness reinforcement degraded after reaching maximum soil arching. Appropriate stiffness reinforcement ensured the development and stability of maximum soil arching. According to the stress states on the pile top, a concentric ellipse soil arch model is proposed in this paper to describe the soil arching behavior in the GRPS embankments over voids. The predicted heights of soil arches and load efficacies on the piles agreed well with the DEM simulations and the test results from the literature.     

基于Hewlett土拱模型桩承式加筋路堤桩土应力比计算

桩土应力比是桩承式加筋路堤荷载传递以及地基沉降计算的重要参数。基于Hewlett土拱模型,单独分析拱顶或拱脚土单元,假设拱顶土单元处于极限状态(拱脚土单元处于弹塑性状态),以均匀超载模拟交通荷载,推导桩土应力比计算公式;基于抛物线模型,考虑筋-土界面摩擦以及地基反力,改进张拉膜效应分析方法,推导加筋条件下桩土应力比计算公式。最后与相关文献实测数据进行对比验证,结果表明该方法与相关文献实测结果除桩间净距为100mm存在误差外,变化规律基本一致,当桩间净距大于100mm时,误差不超过8%。     

A simple design approach to analyse the piled embankment including tensile reinforcement and subsoil contributions

There is not one generally accepted approach for the design of geogrid-reinforced pile-supported (GRPS) embankments. Relevant mechanisms include arching of the embankment material, but also the effect of geogrid reinforcement and potentially a contribution from the underlying subsoil. This paper presents a simple design approach to identify the contribution of all three mechanisms, in which the contribution of multi-layered geogrid reinforcement is also presented. To validate the theoretical predictions for the effect of geogrid reinforcement and the potential contribution of underlying subsoil, a series of three-dimensional finite element analyses are conducted. It is found that a point of ‘maximum arching’ is increased with the height of embankment. This study also presents that the reinforcement could reduce the ultimate stress on the subsoil. However, this requires significant sag of the reinforcement. It is found that the sag of reinforcement is very sensitive to the span of the reinforcement between piles, but relatively insensitive to the stiffness of the reinforcement. For a case with three layers of geogrid, the upper two grids carry relatively little tension compared to the bottom layer. This in turn leads to an approximate but simple equation of vertical equilibrium which may be of use in design.     

土工网在桥头引道路堤中的应用研究

以室内试验研究了路基土与土工网之间的综合摩阻力、张力膜效应及不同土工网铺设间距对复合土反应模量的影响，并依托上海市城市外环线1期工程，研究了桥头引道加筋路堤和不加筋路堤的地基顶面垂直土压力、水平土压力、地基顶面沉降、路堤坡脚侧向位移和路面工后沉降等．结果表明，将土工网加筋路堤用于解决桥头跳车效果明显。     

桩承路堤土拱效应现场试验与分析

桩承路堤在路堤自重作用下形成竖向土拱,并通过土拱传递荷载,其浅层荷载传递机制直接影响到桩土的协调工作和地基的加固效果。为研究桩承路堤中竖向土拱效应的特性,通过设置试验段进行了现场试验,对采集到的应力与变形数据进行了分析处理。研究结果表明,刚性桩支承路堤中存在明显的竖向土拱效应,且在路堤浅层采用粗粒土填料填筑有利于路堤中连续、稳定的土拱的形成,能够起到更好的加固效果。最后提出了一些合理化建议,可供设计与施工人员参考。