地铁车站深基坑桩锚支护结构内力试验研究

为研究深大复杂基坑桩锚支护结构内力演化规律和受荷特性，以总面积约为16×104 m2、最大开挖深度为26 m的基坑工程为依托，在支护桩和锚杆钢筋上预埋钢筋计，分别对基坑开挖过程中和桩头侧向加载、不同工况锚杆拉拔过程中桩的内力和锚杆内力进行监测。结果表明：（1）随着基坑的开挖，悬臂阶段3根支护桩外侧桩身应力呈拉-压-拉变化，内侧桩身应力呈压-拉变化；同一测点钢筋应力逐渐增大，最大值位置略微下移，应力零点出现的位置随桩长的不同而不同。单支点阶段随着基坑暴露时间的增加，外露桩身应力增大，桩身钢筋应力峰值出现在开挖面附近区域，嵌固段桩身应力变化复杂且应力零点比悬臂阶段出现的早。两支点阶段桩身钢筋应力变化更复杂，主要受基坑开挖时间和预应力锚杆的张拉锁定等因素的影响。（2）支护桩、锚杆支护结构设计需考虑其最大允许变形量；满足锚固长度临界值要求后，自由段越长，锚固效果越好，锚固段越短越经济。（3）在未施加拉力和不同拉力作用过程中，锚杆受力发生重分布，与以往土质或岩质基坑认识不同。（4）锚杆侧摩阻力中性点和潜在滑移面的出现与移动是一致的，可用于确定基坑潜在滑移面位置和锚杆临界长度。

Internal force test research on pile-anchor retaining structure of metro station deep foundation

In order to study the evolution of internal force and the load characteristics on pile-anchor retaining structure of complex, large and deep foundation, based on the deep foundation of which the total area is about 160 000 m2 and the maximum excavation depth reaches 26 m, we have monitored the internal forces of the pile and anchor respectively in the process of foundation pit excavation, pile head with lateral loaded and different working conditions of the bolt drawing by embedded bar gauge on the retaining pile and anchor reinforcement. The results show that: (1) during the cantilever pile stage, as the foundation pit excavation the lateral pile body stress of three retaining piles presents “tension-compression-tension” and the inside pile body stress exhibits “compression-tension”. The same point stress increases. Max stress position moves down slightly. The stress-zero appearance varies depending on the length of pile. During single-fulcrum stage, along with increasing exposure time of foundation excavations, the exposed pile stress increases. The peak stress of pile appears in the near excavation area. Excavation of pile body stress tends to complex and the zero-stress point arise earlier than the cantilever pile stage. During two-fulcrum stage, pile’s reinforcing stress tends to be more complicated. Main reasons are that the excavation time of foundation excavations and the impact of factors such as tension locking of prestressed anchor. (2) structural bolting design needs to consider the maximum deformation; After meeting the threshold length of anchorage, the longer the free section, the better anchoring effect, the shorter anchoring section, the more economical. (3) without pulling force or during different tensile force, redistribution occurs in bolt stress, which is different from the those in soil or rock excavation. (4) the appearance and movement of the neutral point of lateral friction force in anchor arm are in consistence with those of the potential slip surface. Therefore, the location of slip surface and anchor arm threshold length in a deep excavation can be determined.