Nearshore deep excavation and associated problems in cohesionless soil

摘要

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