Stability analysis of revetment constructed on thick, soft ground treated with sand compaction piles

Sand compaction pile (SCP) was developed last century and has been widely applied in various onshore and marine projects to improve the ground properties to meet project performance requirements. Stability is critical for land reclamation projects with sand bund constructed on thick soft clayey ground with SCP treatment. Proper design parameters and approach of SCP are essential to achieve the desired ground improvement performance. However, stability checks for slopes with SCP seem not to be straightforward due to various assumptions. How the SCP-treated ground should be modelled is still quite challenging due to the complexity of the ground behaviour from the interaction between SCP and unimproved soft clay. In this chapter, the stress concentration ratio (SCR) was extensively investigated with various modelling approaches, such as unit cell models, 2D plane strain model and full 3D model. The slope stability analysis for revetment constructed on thick, soft clay treated with SCP was investigated using the limit equilibrium method (LEM) and the finite element method (FEM) with composite/discrete model and the total stress method/effective stress method. The results showed that the SCR reduces with increase of consolidation, working load magnitude, replacement ratio, plastic failure and flexibility of loading pattern. The SCRs obtained from the 3D unit cell model and full 3D model represent the upper and lower boundary, respectively. Results indicated SCR ranges between 2.5 and 6 for a replacement ratio of 50% and between 2.0 and 3.6 for a replacement ratio of 20%. Comparison of results between LEM and FEM indicated that the SLOPE/W discrete model is not suitable for stability checks for revetment with SCP treatment. Comparison of results between full 3D analysis and equivalent 2D plane strain analysis showed that plane strain analysis provided more conservative results. Among all methods and models used for this study, the composite plane strain model with equivalent strength from original undrained shear strength of soft clay and friction angle of SCP provided the quickest yet most reasonable solution while the discrete plane strain model with updated undrained shear strength considering consolidation effect and pile installation effect provided the most conservative solution. However, the undrained shear strength gain due to consolidation and pile installation should be verified with in situ tests and instrumentation during construction.