Foundation Pit Supporting Structure. Under Non-steady Seepage Stability Analysis

The excavation in the zone of high groundwater table will induce saturate-unsaturated unsteady seepage due to the difference and variation of water head between the both sides of retaining walls of foundation pit. It has been shown by a great amount of engineering practical that the influence of the seepage on the stability of retaining structure is significant. However, there is little relevant study made so far. In this dissertation, the actual unsteady seepage fields in the process of lowering the ground water level is simulated by using the finite element method (FEM), the influence of the unsteady seepage on the stability of retaining structures of foundation pit, for example, the cement-soil retaining wall, is systematically and thoroughly researched. The main original work and results are as follows:1. Based on the fundamental theories of unsteady seepage and finite element, the condition of water pressure acting on the retaining wall is discussed by simulating the unsteady seepage field using the software SEEP/W. The distinct comparisons between the present results with the unsteady seepage and those with the steady seepage or without the seepage indicate that the unsteady seepage condition is very important for the water pressure.2. On the basis of Coulomb's concept, the calculating formulae for the soil pressure acting on the retaining structure with the unsteady seepage are obtained by using the method of level-layer analysis, and the relevant Fortran programs are developed to find the theoretical answers to the nonlinear distribution of the earth pressure and the position of the application point of the resultant earth pressure. The influence of some parameters such as the project of lowering the ground water level, the coefficient of permeability, the internal friction angle of soils and the angle of wall friction on the distribution of the earth pressure and the application point of its resultant is mainly discussed. Then, the influence of the aforementioned parameters on the safety factors of resisting the wall's overturn and horizontal slipping in the process of lowering the ground water level is further investigated. Numerical results show that the seepage due to lowering of the ground water level is advantageous to the security of the retaining wall and it is conservative if the seepage is taken no account in the design practice.3. By using the software SLOPE/W and SEEP/W, the condition of the global stability of foundation pit and the influence of the project of lowering the ground water level and the coefficient of permeability on its safety factor are researched. The computed results show that, opposite to its favorable influence on the stability of the wall's overturn and horizontal slipping, the unsteady seepage is adverse to the global stability of foundation pit, and therefore it is dangerous if the seepage is taken no account in the design practice. In a word, the importance of the seepage should be rightly understood in the deep foundation engineering.