土体侧移条件下被动桩桩-土相互作用机理的透明土模型试验研究

Lateral spreads of top soils induced by deep excavation, surface loading and tunnel construction are common in the process of urbanization, which may cause additional stresses or deformations to the pile foundation systems of adjacent buildings. It is within the regime of typical passive piles. Various empirical, analytical (pressure or displacement-based), and numerical methods have been adopted to design and evaluate the efficiency of passive piles. However, more attention should have been focused on the deformation of soils around the pile and the soil-pile interaction mechanism. This research program relies on the physical model test combining the uses of transparent soil and the PIV (particle image velocimetry) technique. The equipment includes a lateral loading system and an optical measurement instrument with an optical platform, a CCD (charge-coupled device) camera and a linear laser device. The deformation characteristics of pile surrounding soil under lateral load are non-intrusively measured. Based on multi-slice soil displacement fields, a three-dimensional displacement field is obtained and model test results are comparatively analyzed with findings of numerical simulations. For group passive piles, the soil arching effect and the shadowing effect induced by pile-soil-pile interaction will be investigated from aspects of influencing parameters such as pile spacing, layout and soil conditions. The conclusions arrived at in this program can give preliminary design guidelines for structural optimization and stress/deformation analysis of passive piles.

城市建设中经常会遇到由于基坑开挖、地面堆载等原因而引起的土体侧移问题。土体侧移会使邻近建筑物的桩基产生附加内力或变形，属于典型的被动桩问题。被动桩分析方法主要包括经验公式法、土压力法、位移法以及整体法。然而这些方法往往基于假定的桩土相对位移曲线以及桩侧土压力-桩土相对位移曲线而对桩土相互作用机理揭示不足。本项目基于透明土材料和粒子图像测速(PIV)技术，对被动单桩在加载过程中桩周土体位移场发展变化进行非插入式量测；同时通过多切面土体位移场测量，得到桩周土变形的真实三维位移场，并研究单桩设计变量对桩土相互作用的影响；考虑被动桩群桩的桩-土-桩相互影响导致的土拱效应和遮拦效应，分析群桩设计变量对土拱效应和遮拦效应的影响。本项目研究对揭示被动桩与周围土体的作用规律以及深入了解桩土相互作用机理有参考价值，并为优化被动桩结构设计及内力计算提供科学依据。

本项目基于透明土材料和粒子图像测速(PIV)技术，利用高分辨率数字相机进行透明土激光散斑场图像采集，并通过先进图像分析方法对试验过程中获得的图像进行处理，得到了被动桩受荷过程中桩周土体各切面的位移以及位移矢量图，揭示了水平受荷作用下桩周土体颗粒的运动趋势和位移大小，建立了土体内部三维连续变形的持续量测；通过不同深度的位移层析图片，得到了受荷桩桩周土体的真实三维位移场,并研究单桩设计变量对桩土相互作用的影响；通过模型试验，确定水平加载条件下的土体自由场位移模式，验证文献中对土体侧移曲线所做假设的合理性与有效性；测得了被动桩受荷过程中桩体和桩周土的侧移曲线，分析桩土相对位移关系曲线的形态随受荷变化情况，确定桩侧土压力-桩土相对位移 p-关系曲线。结合土体三维位移场规律，揭示了水平受荷作用下桩土相互作用机理; 依托模型试验结合数值模拟，确定桩侧向极限抗力沿深度分布以及达到极限土压力所需的桩土相对位移量，揭示了被动单桩设计变量如桩截面形状、桩径、桩长、桩土相对刚度、桩周粗糙度、堆载的大小和堆载距离等因素对桩土相互作用机理影响；结合模型试验和数值模拟，得到被动群桩的基桩不同位置、不同深度处的水平位移等值线分布规律，研究群桩设计变量如桩间距、布置、长细比以及桩位等因素对土拱效应形态和遮拦效应的影响。此外，基于有限元法和有限差分法研究了被动桩受临近开挖引起的土体变形的影响，包括各向异性黏土中邻近隧洞开挖施工引起的桩基响应的数值分析; 软黏土中深基坑支护开挖对临近被动桩群桩影响的三维数值分析;以及考虑岩土体的空间变异性的隧道开挖对邻近桩基影响的随机有限差分法分析。