OPTIMIZATION DESIGN FOR DEWATERING OF LARGE DEEP FOUNDATION PIT UNDER COMPLEX GEOLOGICAL CONDITIONS

A large deep foundation pit, located on the bank of lower reaches of Yangtze River, is of a typical dual structural stratum. An impermeable layer between two confined aquifers involved in the pit excavation is partial loss so that the two confined aquifers have a very close hydraulic connection. Because the condition of the formation under the pit site is very complex, and foundation pit dewatering has a direct influence on the safety of foundation pit engineering. The pit is divided into three zones for separate dewatering design. Here is the detailed plan. The hydraulic connection to the outside is cut off by the retaining structure in the two confined aquifers at Region Ⅰ and the first confined aquifer at zone Ⅱ. Unwatering wells are used in these layers. The second confined aquifer in zone Ⅱ is not cut off, and the two confined aquifers in zone Ⅲ where the impermeable layer is lost, have a close hydraulic connection. Two dewatering schemes are designed for these two regions. In scheme 1, wells are set in zone Ⅱ to pump the groundwater of the second confined aquifer, and in zone Ⅲ to pump the groundwater of the first aquifer. However, in scheme 2, wells are only set in zone Ⅲ and pump the groundwater of the first aquifer. For a better comparison between the two schemes and verifying their feasibility, a numerical simulation is conducted. The results show that both of the schemes are feasible and can lower the groundwater level to 1m below the bottom of the foundation pit. So the two schemes can both meet the requirement of dewatering. But scheme 2 needs less wells and results in smaller water table decline of aquifers outside the foundation pit, so that it has less impact on the environment surroundings, and it's more suitable than scheme 1.