基坑开挖前潜水降水引起的地下连续墙侧移研究

Lateral displacement of diaphragm wall by dewatering of phreatic water before excavation

在天津地铁3号线某车站基坑工程开展了现场潜水预降水试验与测试,发现基坑开挖前的潜水预降水即可引起可观的地下连续墙向坑内的位移,进而引起坑外地面及建(构)筑物沉降;并对比了预降水前在地下连续墙墙顶设置水平支撑与否及分段降水时对应的地连墙侧移模式及大小,发现降水前提前在墙顶设置水平支撑能够大幅度减小预降水过程中地连墙墙顶侧移,并使得墙体侧移模式由悬臂型转化为内凸型。对试验依托的实际工程,结合有限元软件ABAQUS建立考虑降水井瞬态降水的三维流固耦合模型,研究了基坑开挖前潜水降水过程地连墙侧移机理。研究表明,基坑开挖前潜水降水导致坑内土体在土体自重和墙体侧压力作用下发生三向固结,从而导致墙体发生侧向位移,同时坑内外墙、土压力发生重分布。基坑提前设置第一道水平支撑再降水、结合信息化施工分层分段降水等均可有效减小基坑开挖前潜水降水引起的变形。

Based on the in-situ dewatering tests on phreatic water in an excavation of Tianjin Metro Line 3, it is found that considerable lateral inward displacement of diaphragm wall and the consequential ground settlement around the excavation occur during the dewatering of phreatic water before its construction. Further field measurements indicate that the magnitude of the lateral displacement of diaphragm wall can be significantly reduced by applying struts at the top of wall before its construction and by staged dewatering and in the meanwhile, the pattern of displacement is transformed from cantilevered to bulged one in the middle of wall. Then, three-dimensional fluid-solid coupling numerical analysis is performed to simulate the field dewatering tests, and the mechanism of the lateral displacement of wall induced by dewatering of phreatic water before excavation is studied. Numerical results show that the soils inside the excavation are consolidated in three-dimension during and after dewatering, leading to the redistribution of earth pressure between the wall and the soils. The displacement of diaphragm wall can be effectively reduced by the installation of the first level of struts and staged dewatering.