Nearshore deep excavation and associated problems in cohesionless soil

Abstract

This paper presents a case study of piping failure resulting in differential settlement of a nearshore diaphragm wall and highlights the mitigation measures undertaken to stabilise the structure. To facilitate a 15 m deep excavation, a diaphragm wall was constructed and a well-point dewatering system was adopted to keep the water table below the base of the excavation. However, during the foundation work, sand boiling was observed accompanied by foundation heaving and uneven settlement of the wall. A study was done based on field observations and numerical analysis to assess the root cause of the problem. Finite-difference analysis confirmed piping failure of the subsoil as the hydraulic gradient exceeded the critical hydraulic gradient near the base of the diaphragm wall. To mitigate the problem, additional well points were installed at certain locations to bring down the hydraulic gradient within safe limits. The construction was then carried out in stages, and standpipes were installed in the foundation and subsoil to limit the excess pore pressure generated below the foundation during construction. Field monitoring showcased that the development of this excess pore pressure could be minimised, and the rectification measures worked well towards increasing the structural stability of the diaphragm wall.