开挖前降水引发基坑变形机制模型试验研究

Mechanism of foundation pit deformation caused by dewatering before soil excavation:an experimental study

现有基坑相关研究主要关注土方开挖过程引起的变形,认为围护结构变形起点是土方第1次开挖。然而,一些工程实测表明,基坑开挖前降水阶段即可引起围护结构及周边地层发生厘米级的变形。显然,未考虑开挖前变形的基坑监测数据将低估基坑施工的环境效应。为了研究基坑开挖前降水引发基坑变形的机制,开展了室内模型试验,对基坑开挖前降水过程进行了缩尺精细化模拟。通过微型降水井的设置与调控,模型试验真实再现了实际基坑降水过程中井流效应对围护结构受力变形的影响。试验过程中发现,随着降水的进行,坑外降水漏斗不断扩展,围护结构悬臂式侧移及坑外拱肩式地面沉降也随之产生。另外,降水导致墙前水压力明显减小,并诱发墙前侧向总压力重分布(以减小为主),围护结构为此发生指向坑内的悬臂式运动以寻求新的受力平衡,并通过墙后土体损失诱发坑外地层变形。

The existing research on foundation pit mainly focuses on the deformation caused by soil excavation.It is often believed that the retaining wall deformation would commence after soil excavation.However,some field measurements show that the dewatering conducted before soil excavation might have already induced retaining wall deflection and surrounding ground deformation,which could be at centimeter level.Apparently,the monitoring data excluding deformations caused before excavation will underestimate the environmental effect of the construction work.In order to investigate the mechanism of foundation pit deformation caused by dewatering before excavation,a similarity model test is conducted to simulate the dewatering process.In the laboratory experiment,by means of mini dewatering wells,the seepage process around wells is reproduced and the seepage effect on retaining wall deformation is studied.The test results indicate that the depression cone outside the foundation pit enlarges continuously during the dewatering.Meanwhile,cantilever-type wall deflection and spandrel-type surface settlement are observed.In addition,the dewatering leads to an apparent decrease of the pore water pressure in front of the wall,which further induces decrease of the lateral total pressure.In response to the stress change,the retaining wall moves towards the pit in a cantilever type to achieve a new equilibrium state,and further induces the surrounding ground deformation as a result of the ground loss behind the wall.