考虑开挖卸荷影响的桩侧摩阻力等效计算方法

基坑开挖后的卸荷回弹会导致桩侧摩阻力减小，但这一影响仅局限于基坑底部一定深度范围内。在计算开挖后的桩侧摩阻力时，可将卸荷视为作用在坑底的均布上拔荷载。当荷载作用在地表以下，土体附加应力应采用Mindlin应力解。定义基于Mindlin解计算开挖后桩侧摩阻力与开挖前变化为5%时的深度为开挖影响计算深度Hc，开挖深度和宽度对Hc的影响明显，土体内摩擦角的影响较小，而泊松比的影响可以忽略。据此推导出Hc的计算公式。开挖卸荷导致的竖向附加应力，Boussinesq解求得的结果大于Mindlin解，但随着深度增加，二者差异逐渐减小。分别针对开挖深度、宽度和内摩擦角不同取值的216种组合进行计算，结果表明，Hc范围内基于Mindlin解求得的桩侧摩阻力与基于Boussinesq解结果的比值? 最大为1.124，最小为1.001。工程实践中为偏于安全，可直接采用Boussinesq解进行桩侧摩阻力计算。

An equivalent method for calculating side friction of pile considering excavation-induced unloading effcet

The side friction will decrease due to unloading rebound deformation after excavation, though it occurs only within a certain depth range near the bottom of pits. Unloading action can be considered as a uniformly distributed upward pressure on the bottom of pit when the side friction of piles after excavation is calculated. The additional stress of soils can be expressed by Mindlin solution when the load is under the ground. Excavation calculation depth Hc is defined as the depth at which change ratio of side friction is 5% between after and before excavation, and all the influential factors on Hc such as excavation depth, excavation wide, internal friction angle and Poisson’s ratio are considered. In addition, the calculation formula of Hc is derived. Although the result of vertical additional stress from Boussinesq solution is bigger than that from Mindlin solution, but the difference becomes smaller with the depth. The side frictions after excavation for 216 cases are calculated. It is shown that the maximum ratio ? of results from Boussinesq solution to that from Mindlin solution is 1.124, and the minimum value is 1.001. Therefore the side friction of pile after excavation can be calculated by Boussinesq solution for safety.