软土区基坑预降水引起支护墙侧移的典型参数影响研究

A parametric study of lateral displacement of support wall induced by foundation pre-dewatering in soft ground

基于天津地区透水层和弱透水层交替出现的场地水文地质条件,通过大量数值计算与工程实测对比,结合非线性回归分析,研究了预降水时间t、预降水深度H_d、基坑长宽L,基坑宽度b等参数对基坑预降水过程中支护墙侧移发展的影响规律。随t延长,最大支护墙侧移δ_(hm)呈增长速率不断减小的非线性增长,在预降水刚开始的几天内,支护墙侧移即可占据其预降水过程中稳定侧移的很大比重。其次,H_d对δ_(hm)及支护墙悬臂侧移深度H_c的发展有重要影响,总的来说,δ_(hm)及H_c均随H_d增大而增大,具体地,当H_d达到场区第2透水层前(后),δ_(hm)随H_d的线性增长比例较小(大),当H_d达到场区深厚弱透水层厚度一半之前(后),H_c的增长接近于(明显快于)H_d的增长。另外,对于同一宽度b基坑,当基坑长度L达到一定程度后,预降水引起的基坑长度方向边角效应总是发生在基坑两侧距离坑角某一确定范围之内,在预降水过程中,若支护墙变形需要严格控制时,可通过布置横隔墙,使得相邻横隔墙间距S_(cw)与b之比在4以内,或S_(cw)/H_d在3.7以内,利用基坑边角效应减小预降水引起的最大支护墙侧移。

Strata in Tianjin area are featured as an alternated multi-aquifer aquitard system. Based on this strata condition, lots of numerical calculations investigate the effect of deep foundation pre-dewatering on lateral displacement of support wall in soft ground under different construction conditions, including dewatering time（t）, dewatering depth（Hd）, pit length（L） and pit width（b）. The calculation results were compared with relevant field data, and nonlinear regression analysis was then conducted based on the calculation and observation results. The results indicate that the maximal lateral displacement of support wall（δhm） increases nonlinearly with t, and the growth rate decreases continuously. In the first few days of the dewatering, lateral displacement of support wall development can occupy large proportion of its stable value during pre-dewatering. Moreover, Hd has a great influence on the dhm development and the depth of wall with cantilever-type deflection（Hc）. In general, δhm and Hc will be larger with the increase of Hd. Specifically, before Hd reaches the second permeable stratum of the field, δhm increases linearly with Hd by a small scale. Otherwise, the growth proportion of δhm with Hd increases greatly. Before Hd reaches the center location of the thick aquitard, Hc increases synchronously with Hd increased, and otherwise, the Hc increment is apparently greater than the Hd increment. Furthermore, as to the same pit width（b）, when the pit length（L） reaches a specific value, the corner effect induced by dewatering always appears at a specific distance from the pit corner along the pit length direction. In practical pre-dewatering, cross wall can be used to partition the pit into some parts, and let the ratio of cross wall spacing（Scw） to b be less than 4, or let Scw/Hd be less than 3.7, using the corner effect to effectively reduce the maximal lateral displacement of support wall induced by pre-dewatering.