Studies On The Behavior Of Foundation Pit With Excavation Considering Seepage

The excavation in the zone of high groundwater table will induce unsteady seepage due to the difference and variation of water head between the inside and the outside of a foundation pit. It has been shown by a great amount of practical excavations that the influence of the seepage on the behavior of foundation pit is significant. However, there is little relevant study made so far. In this dissertation, the seepage influence of groundwater on the behavior of foundation pit involved with excavation was systematically and thoroughly studied both by analytical method and by FEM. The main original work and results are as follows:1. Based on analytical method and soil mechanics principle, analytical formulas for calculating negative excess pore water pressure induced by single-stage excavation and multi-stage one were obtained respectively, and the influence of negative pore pressure dissipation on the lateral pressure acting on retaining structure and the heave of pit base was analyzed in detail. It was indicated that the dissipation of negative pore pressure is harmful to the stability of a foundation pit during excavation.2. Analytical formulas for calculating the water head in active and passive zones of a foundation pit were derived under the assumption that groundwater seepage was one-dimensional and steady. The influence of seepage on the pore water pressure and earth pressure acting on retaining structure and the heave of pit base was investigated. The results showed that whether seepage is favorable to the stability of retaining structure is concerned with the thickness, the permeability coefficient, the effective stress strength index of soil stratum. It was also demonstrated that seepage causes the decrease of vertical effective, stress and thus the increase of heave in soil mass beneath pit base.3. Based on Biot's consolidation theory, computer program 3DBCPE3.0 was developed by 3-D FEM that can be used to perform the coupled analysis of soil mass deformation and pore pressure dissipation. In this program, more complicate conditions such as variation of water head difference between the inside and the outside of a foundation pit and variation of seepage free surface outside of pit can be taken into consideration.4. The time and spatial effects of excavation considering the coupling of pore pressure dissipation and deformation were investigated. The influence of soil permeability and construction period was then discussed. The results of the three-dimensional analysis were compared with the ones of two-dimensional analysis, which showed that the result obtained from two-dimensional FEM analysis is in overestimated side.5. The time and spatial effects of excavation considering the variation of water head difference between the inside and the outside of a foundation pit were then investigated. By comparing the analysis results considering the variation of water head difference with the ones without considering it, the conclusions were drawn that the pore pressure distribution is distinctly different from the latter and the horizontal displacement of retaining structure, the ground surface settlement as well as the pit base heave are all larger than the latter. So theresult without considering the variation of water head difference is in unsafe side.6. The time and spatial effects of excavation considering variation of seepage free surface outside of foundation pit were further investigated. The variation of seepage free surface with time was analyzed, and the influence of soil permeability on seepage free surface and deformation was then discussed. The comparisons between the results considering variation of seepage free surface and the ones without considering the variation showed that when soil permeability is low, the variation of seepage free surface is so small that its influence on the behavior of excavation is negligible. Finally, an exact excavation case with field measurements was analyzed using the program developed and the horizontal displacements of retaining structure at...