既有地下室增层开挖引起的桩基侧摩阻力损失分析

既有高层建筑地下增层开挖,不可避免地引起地基应力场改变,导致桩土接触面和桩端土层应力变化,从而降低原基础底板下桩基承载力。由于实际工程情况限制,增层开挖后桩基的承载力无法通过现场试验获得,因而采用理论分析和有限元模拟增层开挖后桩基承载力就显得尤为重要。采用经典桩侧摩阻力计算公式,简单地考虑上覆土压变化以及应用Mindlin应力解考虑开挖引起的附加应力,分别计算桩侧极限摩阻力,然后利用有限元法计算桩侧摩阻力并和前述计算结果相对比,最后分析了增层开挖条件下桩侧极限摩阻力损失比随不同增层开挖宽度和深度的变化规律,结果表明：损失比随增层开挖宽度的增大而增大,开始呈线性增大然后趋于稳定；损失比随增层开挖深度的增大而增大,大致呈线性趋势；损失比沿桩身由上往下逐渐减小。

Shaft resistance loss of piles due to excavation beneath existing basements

The excavation beneath the existing high-rise buildings, which changes the ground stress field, will result in changes of the stresses at the pile-soil surface and the pile end so as to reduce the bearing capacity of piles. Considering the field conditions, the bearing capacity of piles can not be obtained by field tests after excavation, so it is particularly necessary to evaluate the resistance loss using the theoretical analysis and the finite element simulation. The classical theory is used to calculate the ultimate shaft resistance of the piles. The Mindlin’s stress solution and a simplified method are adopted to consider the additional stress caused by excavation. Subsequently, the results of theoretical analysis are compared with those of the finite element analysis. The relationship among loss ratio of the ultimate shaft resistance, excavation width and depth is also analyzed. The results show that the loss ratio increases with the increase of excavation width and exhibits an initial linear increase followed by a more nonlinear increase until a constant value. Moreover, the loss ratio increases with the increasing excavation depth in a linear manner and decreases along the shaft from top to bottom.