Three-dimensional Transient Seepage Numerical Simulation Of Deep Foundation Pit Dewatering In Complicated Quaternary Loose Sediments With Great Thickness

Thick and incompact quaternary sediments in Yangtzi River delta create a complex aquifer system. Because the horizontal flow barrier around the deep foundation pit is difficult to penetrate into the bottom of the aquifer, deep foundation pit dewatering usually uses partially penetrating wells and this results in very complex changes in groundwater seepage with obvious characteristic of three-dimensional flow. Analysis method and two-dimensional model have not contented simulation and prediction demond of deep foundation pit any more.Aiming at deep foundation pit dewatering in complicated quaternary loose sediments with great thickness, three-dimensional numerical simulation theories of deep foundation pit dewatering has been discussed and a deep research has been done by using finite element method. 'Remove row and column' and 'put a big number' have been used for the first boundary;a continuous Regional Identification Function is used on the basis of non-variant nodal virtual flux method for the free surface boundary;Gauss formula is used to make the curved surface integral of the free surface boundary transformed as the difference between volumn integral and other curved surface integral to calculate the integral item on boundaries, which avoiding finding the position of free surface, while the continuous Regional Identification Function is applied to calculate volumn integral and surface integral;Crout and PCG method is used for solusion. Then three-dimensional transient confined and unconfined seepage analysis procedure is developed.The procedure is applied in two projects: (1) Shanghai Huanqiu finance center tower foundation pit dewatering, the complicated flow state of groundwater is simulated under these conditions which the aquifers deposite layer by layer, the bottom of the aquifers goes deep to 149m, the horizontal flow barrier goes deep to 34m, the extraction wells are penetrated to 55m, the filters of the extraction wells are located between 34m to 55m, and the water level in the deep foundation pit is decreased to 26m under land surface, the result of 8 pumping wells optimization project is designed. (2) The forth subway of Dong Jia-du tunnel repair foundation pit dewatering in Shanghai, the complicated flow state of groundwater is simulated under these conditions which the aquifers deposite layer by layer, the bottom of the aquifers goes deep to 144.45m, the horizontal flow barrier goes deep to 65m, the extraction wells are penetrated to 60m, the filters of the extraction wells are located between 45m to 60m, and the water level in the deep foundation pit is decreased to42.45m under land surface, then an optimization project is designed. Both of them are proved to be correct and reliable.Research results indicate that continuous Regional Identification Function is used in nodal virtual flux method, volumn integral and surface integral enhances the convergence and stabilization;using 'put a big number' to deal with the first boundary is faster than 'Remove row and column', the more the first boundary nodes are the more obvious advantage will display;PCG method is quite faster than Crout method to solve equations;it is reliable to use three-dimensional finite element simulation theories to simulate and predict deep foundation pit dewatering in such areas.