基坑开挖对坑内工程桩影响的实测及有限元分析

Field Observation and Finite Element Analysis of Effect of Overlying Excavation on Piles

基坑开挖对坑内工程桩的影响越来越受到工程界的重视.对天津站地下换乘中心盖挖逆作法基坑工程开挖过程中支承于桩基上的钢管柱的竖向位移进行了实测,在此基础上进行了有限元分析.实测结果及有限元分析表明:基坑开挖可导致工程桩产生较大隆起,截至底板浇筑完毕本工程实测钢管柱顶最大值达到33,mm,钢管柱之间及钢管柱与地下连续墙之间产生了可观的差异竖向变形.随着盖挖逆作法基坑分层降水、分层开挖及分层施做地下结构,钢管柱的竖向位移呈波动式发展.坑底以下土体的隆起除在引起桩发生较大的竖向位移外,在桩身还可引起一定的拉力.不同位置处工程桩对应桩身拉应力和侧阻分布相差较大,基坑边部桩受地连墙变形影响较大.在桩顶作用有较大荷载的情况下,基坑开挖后,桩身中下部仍会产生较大的拉力,在设计时须加以考虑.

The effect of overlying excavation on piles has recently attracted more and more attention. The construction of underground Passenger Transfer Center of Tianjin Transport Hub was carried out by top-down method. The vertical displacement of steel pipe columns supported by the piles beneath the bottom of excavation was measured. The corre-sponding numerical analysis was also conducted. The measured data and numerical analysis show that excavation can induce a large heave of piles, which is 3 3 mm in this project until the bottom slab is finished. Considerable differen-tial heave occurred among steel pipe columns and between steel pipe columns and diaphragm walls. The heave of steel pipe columns developed with fluctuation during staged dewatering, excavation and slab casting. The basal heave of soil also induced considerable tensile force in the piles. At different locations of excavation, the tension stress in the piles and the shaft friction of the piles are obviously different. This difference is partly due to the fact that the piles near the diaphragm wall were affected by the deformation of diaphragm walls. Although there was a large load on the top of the piles, large tensile force still occurred in the middle and lower part of the piles after excavation, which should be considered in the design.