Abstract
The excavation unloading of deep foundation pits in soft soil areas often produces negative excess pore water pressure. The rebound deformation of soil on the excavation surface of the foundation pit can be predicted reliably through the accurate expression of relevant variation laws. In combination with the principle of effective stress and the general equation of unidirectional seepage consolidation, an equation for calculating the rebound deformation from the bottom in the process of foundation pit excavation unloading was obtained. Additionally, a triaxial unloading test was adopted to simulate the excavation unloading processes for actual foundation pit engineering. After studying the variation law of the excess pore water pressure generated by excavation unloading, it was found that the negative excess pore water pressure increased with increasing unloading rate, while the corresponding peak value decreased with increasing confining pressure. The equation for rebound calculation was verified through a comparison with relevant measured data from actual engineering. Therefore, it is considered that the equation can reliably describe the rebound deformation law of the base. This paper aims to guide the design and construction of deep foundation pits in soft soil areas.
Highlights
According to the principle of effective stress, the total stress generated by the external load, σ, is borne jointly by the effective stress (σ′) and the pore water pressure (Pw), while the pore water pressure includes the hydrostatic pressure (cw (H − z)) and the excess pore water pressure (u). e hydrostatic pressure does not change with the consolidation process, and often, only the excess pore water pressure, which changes with consolidation degree, is considered for convenience. erefore, it is considered that, during this process, the excess pore water pressure dissipates gradually with time, the effective stress increases, and the soil is continuously compressed until it becomes stable
Based on the basic hypotheses of Terzaghi’s one-dimensional consolidation theory, it is considered that, during the loading process, the change in thickness of the saturated clay layer (H) is negligible, and the external forces cause only upward or downward seepage and compression of the soil, while the permeability coefficient does not change with depth. us, the differential equation for seepage consolidation in terms of the excess pore water pressure (u) under the external load is as follows: z2u zu zσ
During the foundation pit excavation, the decreasing process for the overburden pressure of the soil at the bottom of the pit can be taken as a decreasing process for σz,t, the applied load in the one-dimensional seepage consolidation model. erefore, R(t), the unloading rate, and Cve, the anti-consolidation coefficient, are introduced, and equation (2) can be modified as z2u(z, t) zu(z, t) Cve zz2 zt − R(t), (3)
Summary
Based on the basic hypotheses of Terzaghi’s one-dimensional consolidation theory, it is considered that, during the loading process, the change in thickness of the saturated clay layer (H) is negligible, and the external forces cause only upward or downward seepage and compression of the soil, while the permeability coefficient does not change with depth. According to equation (5), the excess pore water pressure caused by soil unloading is jointly influenced by R(t), the unloading rate Cve, the anti-consolidation coefficient, and the unloading amount. To study the variation law concerning excess pore water pressure and rebound in soil at different depths and different unloading levels, undisturbed soil samples were cut at each of the three layers of the excavation surface in the middle of the subway station foundation pit excavation for the triaxial unloading test.
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