Finite Element Analysis For Stability Of Multi-braced Excavation In Soft Soil With Strength Parameter Reduction

In recent years, with the development of FEM software and computational technology, the strength reduction method on slope stability analysis was becoming more and more popular. However, the method brought much trouble when it was employed to analyse the stability of deep excavations. In this paper, the numerical software Plaxis and strength parameter reduction method were chosen to analyse the stability of multi-braced excavations in typical soft soil in the eastern coastal areas of China. The main work was shown as follows:(1) The main failure modes in soft soil excavations and the methods on stability analysis were introduced. The main problems of using strength parameter reduction method to analyze the stability of excavations were summarized.(2) The FEM software Plaxis was used to analyze overall stability of soft soil excavations. Some widely uesd soil constitutive models were introduced. Especially, the conversion on the safety factor obtained by Mohr-Coulomb criterion and Drucker-Prager criterion was discussed(3) Based on a typical multi-braced soft clay excavation model, Mohr-Coulomb criterion and Hardening-Soil criterion were employed to simulate soil behaviors in overall stability analysis through strength parameter reduction. After discussing characteristic soil nodes in convergence judgment, the nodes near excavation surface in front of the retaining wall were selected as the reasonable points. The numerical simulation showed that the safety factors obtained by Hardening-Soil criterion agreed well with the result ganied by the limit equilibrium method, while the safety factors obtained by Mohr-Coulomb criterion were distinctly larger. Then the influences of many factors, such as the width of pit, the thickness of soft soils beneath the base of excavation, the embedment depth and the stiffness of retaining wall on overall stability were investigated thouroughly. The safety factors obtained by the two constitutive models were also compared.(4) Finite element numerical model of typical cross-section in Pengbu Station of HangZhou Metro was established. After comparing the numerical horizontal deformations and the observed data in excavation, several important stiffness parameters in Hardening Soil criterion were selected appropriately. It was shown that the safety factors predicted by Hardening-Soil criterion agreed with the results obtained by the limit equilibrium method, which verified the accuracy of analysis process.(5) Finite element numerical model of the 2rd North excavation of Xianghu Station of HangZhou Metro was established. The excavation case was known by all for it had losen its stability on 15, November,2008. Based on the analytical results, the probably reason of the accident were discussed in detail.