Assessment of basal heave stability for deep excavation

In this study, PLAXIS finite element software with shear strength reduction method (SSR) will be used and plane strain (2-dimensional) model will be applied to examine the parameters that affect the stability of braced excavation in clays. The parameters considered are thickness of the soft clay layer underlying the hard stratum (T), wall penetration below the base of excavation (D) and the stiffness of the wall (EI). Only deep excavations, with excavation width less than the final excavation depth are considered. There are 45 cases in total which are examined with varied parameters. The cases examined are divided into three main categories: D is small, D/T = 0.5 and wall penetrates 2m into hard stratum. The comparisons of factor of safety between PLAXIS results with the conventional methods such as Bjerrum and Eide (1956) and Eide et al. (1972) method are performed as well. The comparisons are not performed for the case where wall penetrates 2m into the hard stratum. It is because the two conventional methods cannot be applied when the wall penetrates into the hard stratum. The results show that, the factor of safety against basal heave increases with decreasing T, increasing D and EI. The results also show that the effect of D is not really significant unless T is small and EI is large. Furthermore, it is observed that the lateral deflection of the wall increases rapidly when the factor of safety is smaller than 1.6. The result of the comparisons between the three factor of safety methods (i.e. finite element, Bjerrum and Eide (1956) and Eide et al. (1972)) indicate that Bjerrum’s method will generate the most conservative value of factor safety while PLAXIS factor of safety is the highest one. The relationship between Finite Element Factor of Safety and Eide’s Factor of Safety is also established. However, the relationship needs to be further verified with additional data.Bachelor of Engineering (Civil