WATER-HEAD RISE CAUSING SOIL BURST AT DOWN-SIDE PLACE OF FOUNDATION PIT WITH UNCLOSED WATERPROOF CURTAIN DURING RELIEF DEWATERING PROCESS

When unclosed waterproof curtain is used for pit dewatering, some defects may exist in the curtain. Bursting issues caused by water-head rise in the pit have been recorded multiple times. In the subway airshaft discussed in this article, at the end of the excavation process, large inflow occurred at west side of the pit. A three-dimensional numerical model with anisotropic permeability into consideration of unsteady flow of underground water is built to simulate and analyze seepage field under seven working conditions. They are no defect on the waterproof curtain, defect at 28~33m deep with/without increased water drawing volume, defect at 33~38m deep with/without increased water drawing volume, defect at 38~43m deep with/without increased water drawing volume. The research shows that when the waterproof curtain has no defect, the waterproof curtain inside and outside water-head difference is largest at top of aquifer and the difference decreases with depth. Contours are dense near the seepage path near the defect. The water inflow raises water-head in the pit and lower water-head outside the pit. Seepage vectors of the water coming into the pit through the defect are mainly horizontal. When the defect is at the aquifer of high permeability, the corresponding water inflow is large. Increasing water drawing volume decreases impact area of the water-head rise, but excessive drawing can incur soil-piping flow. The larger the vertical distance between the defect and bottom of the curtain, the larger the rising potential is, and vice versa. The results provide guidance to incident mechanism analysis, predication and dynamic risk assessment of soil burst at down-side place of foundation pit.