坑外群桩阻隔效应对基坑内抽水引发变形影响机制

Barrier effects of surrounding group piles on deformation of foundation pits induced by dewatering

城市已有地下结构会对地下水流动和地层运移带来阻隔作用,该条件下的基坑设计与施工应考虑该阻隔效应的影响,但传统研究尚未充分揭露邻近结构阻隔作用对地下水渗流及基坑变形的影响机制。依托天津某地铁车站基坑工程开展了室内模型试验,通过在基坑-土-邻近结构(以桩基结构为例)物理模型中布设应力与变形传感器以全过程实时监测基坑抽水过程中坑内外水位变化、围护结构侧移、坑外地表沉降和围护结构两侧水土压力,探讨了有/无桩基结构时基坑降水引起的围护结构及周边土体变形规律,并揭示桩基结构阻隔效应对基坑抽水引发变形的影响机制。研究发现:①当基坑外存在桩基结构时,由于其对地下水运动的阻隔效应,较小的基坑涌水量可引起较大的坑内水位下降,此时,可适当减少坑内降水井的数量来实现同样的降水效果;②邻近桩基结构对地层运动的阻隔效应将导致基坑抽水过程中围护墙墙前侧向总压力更大、围护墙墙后侧向总压力更小,为此,抽水引起的围护结构侧移及坑外地面沉降更小。基坑设计时应考虑邻近结构对基坑施工环境效应的影响,从而得到更合理的基坑支护与施工方案。

The existing underground structures in urban areas can block the groundwater flow and soil movement(i.e.,barrier effects).On this occasion,the design and construction of foundation pits should consider the influences of the barrier effects.However,the mechanism of groundwater seepage and deformation of foundation pits under the barrier effects of the adjacent structures has not been fully revealed.In this study,two laboratory-scale tests are conducted based on a metro foundation pit in Tianjin.A series of stress and deformation sensors are placed in the physical model for pit-soil-structures(taking pile foundation as an example)to monitor the real-time variation of water level,deformations of retaining wall,settlements of ground surface,and pore-water and earth pressures at both sides of the retaining wall during dewatering of the foundation pit.Based on the tests,the deformation laws of the retaining wall and surrounding soil caused by dewatering considering and without considering the adjacent pile foundations are discussed.The mechanism of deformation of the foundation pit induced by dewatering under the barrier effects of pile foundations is also revealed.Two main conclusions are drawn:(1)Larger groundwater drawdown in the pit can be caused by smaller water yield due to the barrier effects of pile foundations on groundwater flow.In this case,the number of pumping wells in the pit can be appropriately reduced to achieve the same pumping effects.(2)The barrier effects of the adjacent pile foundations on the ground movement may lead to a larger lateral pressure in front of the retaining wall and a smaller lateral pressure behind the enclosure wall.On this occasion,the deformation of dewatering-induced retaining wall and settlement of ground surface outside the pit may be much smaller.The effects of the adjacent underground structures on the foundation pits should be considered in the design so that a more reasonable supporting scheme and construction plan can be obtained.