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

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.