Deformation Characteristics and Stress Path Analysis of Divided Foundation Pit in Soft Soils

Abstract:

Based on a case of a super large underground complex construction adjacent to a historical building in Shanghai， a three-dimensional finite element model considering small strain characteristics of soil was established to examine the deformation characteristics and stress path of soils during excavation. The field measurement data were utilized for the validation of model parameters， and six different construction conditions were calculated and analyzed. The results showed that the effect of only optimizing the excavation sequence on controlling the deformation of the foundation pit was limited and not comprehensive. It was necessary to consider the influence of integral stiffness of underground structure and boundary condition change during the whole construction process. The comparison that the optimal construction scheme was to excavate smaller pits first， followed by the completion of the underground structure and the construction of larger pits， which could reduce the lateral displacement of the retaining structure close to the sensitive building by 30 % compared with the adverse condition. In addition， different excavation sequences， boundary conditions， and drainage conditions made the stress state of soils inside and outside the pit more complex and varied. The soils inside the pit presented diverse stress paths in different unloading conditions， while the soil outside the pit showed a multiple unloading stress path. The application of the servo prestressed steel support could control the deflection of the retaining wall close to the protection object within 0.1% H_e （excavation depth）， and the retaining wall might move towards the outside of the pit under the combined action of steel supports at each level. Moreover， the soil outside the pit would exhibit the stress path of horizontal unloading first and then loading.