Research on Safety of High Earth-rock Cofferdam under Complicated Working Conditions

Based on the nonstable-nonsaturated seepage theory and the ultimate balance theory, the slope stabilization of high earth-rock cofferdam under complicated operating conditions was studied by using finite element method. The Wudongde upstream earth-rock cofferdam was chosen as an instance in this paper. Numerical simulations were carried out corresponding to several working conditions, such as foundation pit drainage, foundation pit excavation and flood fluctuations, etc. The main conclusions are as follows. The Wudongde upstream earth-rock cofferdam has the seepage stability and the stability against sliding. The pumping speed has obvious influence on the seepage field so that it should be less than 3 meters per day. The maximum gradient of spill point can reach to 0.42 when the foundation level decreases at 0.5 meters per day. Therefore the water level of overburden is suggested to reduce before excavating foundation pit. Water break with large speed in the upstream of cofferdam will make inverted flow occur. The maximum inverted seepage gradient is 0.45 as the water break speed is 6 meters per day. The unsteady seepage has more disadvantages for skid resistance than that of steady seepage. The water level varies more quickly, the value of safety factor become littler. The minimum factor of safety occurs during the variation of water level instead of at the minimum water level.