Numerical studies of seepage failure of sand within a cofferdam

Urban development often requires the construction of deep excavations. These might be for deep basements, cut-and-cover tunnels, underground parking or underground transportation systems. The design of deep excavations is often dominated by the water flow around sheet piles or propped walls. The seepage flow influences the stability of the walls where bulk heave, piping or failure by reduction of the earth pressure may occur. Several methods of calculating the stability against seepage failure of the soil have been proposed in the literature, leading sometimes to great differences on the hydraulic head loss inducing failure. In this paper, the FLAC-2D Code using an explicit finite difference method is used to analyse seepage failure of the sandy soil within a cofferdam subjected to an upward seepage flow. Based on this study, the conditions for seepage failure occuring by boiling or heaving of the soil behind sheet piles are clearly identified.