下覆倾斜地层软土桩-网复合地基破坏机理试验

Model test study on the failure mechanisms of the geogrid-reinforced and pile-supported foundation on soft soil with overlying sloping base

摘要:

为研究嵌岩桩和摩擦桩同时存在下桩-网复合地基的破坏机理,以浙江沿海某高速铁路工段为背景,基于典型断面开展相似比尺为1∶100的土工离心机模型试验,分析下覆倾斜地层软土桩-网复合地基的变形特征及破坏模式.结果表明:路堤顶面的竖向、水平位移主要发生在施工阶段及工后静置阶段,且进入运营阶段后亦有明显发展,路堤顶面竖向沉降差随运营时间的推移逐渐增长；复合地基的沉降呈非对称分布,形成一个偏向下覆倾斜地层斜坡向下一侧的不对称沉降盆；在上部荷载作用下,复合地基的沉降主要由下卧层的压缩、桩体的向上向下刺入及桩体倾倒或横移3部分组成；桩顶水平位移沿下覆地层斜坡向下方向逐渐增大并伴随不同程度的淤泥质黏土绕流破坏,桩顶竖向位移呈现偏向下覆地层斜坡向下一侧的不对称“V”字型分布；嵌岩桩的潜在破坏模式为弯拉或弯剪破坏,斜坡桩的潜在破坏模式为弯剪或倾倒破坏,摩擦桩的潜在破坏模式为倾倒及横移破坏.通过试验可知,采用等长桩的桩网复合结构对下覆倾斜地层软土地基进行处理是不合理的,将威胁高铁的安全运营.此外,对此类既有铁路地基的维护及有效加固措施亟待进一步研究.

Abstract:

To investigate the failure mechanisms of the geogrid-reinforced and pile-supported foundation in which the embedded piles and floating piles exist at the same time, the centrifuge model test with the similar constant of 1∶100 was performed based on a typical project of a high speed railway located in the coastal areas of Zhejiang Province. The deformation characteristics and failure modes of such foundation on soft soil with overlying sloping base were analyzed in detail. According to the centrifuge model test, the conclusions are drawn as follows: most vertical and horizontal displacement occurred during the construction phase and the standing time, and increased continuously in operating phase. Meanwhile, the differential settlement increased gradually with the operation time. The distribution of the composite foundation settlement was asymmetric, and a settlement basin appeared after the test. Under the upper load, the settlement of the composite foundation was mainly caused by the underlayer compaction, the upward and downward pricking destruction of the pile, and tilting or transversal displacement of the pile. The horizontal displacement of the pile head increased gradually along the downward direction of the overlying sloping base, and the soil flowed around the piles in varying degrees. The pile head vertical displacement distributed unevenly and presented an asymmetric shape of "V". The embedded piles suffered bending tensile failure or bending-shear failure; piles on the overlying sloping base suffered bending-shear failure or tilting failure; and floating piles suffered tilting failure and transversal displacement failure. Test results indicate that it is unreasonable that this kind of foundation is reinforced by equilong piles, and the effective reinforcement measures for this kind of high speed railway foundations still need further research.