Design of a Rigid Cutoff Wall

A sheet pile cutoff wall simply called cutoff wall is used in almost all types of water retaining structures to reduce the exit gradient at the downstream toe and prevent the chances of sand boiling and piping. In 1954, Terzaghi has suggested a simplified approach for the design of cutoff walls considering average seepage force. In Terzaghi’s approach, the unbalanced water pressure around the cutoff wall is coupled with the active earth pressure and the resultant thrust is determined by subtracting the passive thrust from the combined active thrust. The value of passive earth pressure coefficient has been assumed to correspond with a downstream failure wedge having an angle of $(45°-{\overline{\varphi }}_1∕2)$ with the horizontal, where ${\overline{\varphi }}_1=\mathrm{effective}$ angle of internal friction of the foundation soil. In this paper, it is shown that under steady state flow conditions for a flexible levee base, failure does not take place always at this angle. Considering exact seepage forces and applying the laws of equilibrium, active and passive earth pressures acting on a rigid sheet pile cutoff wall on an impervious levee have been determined. The weight of the levee has been considered in the computation of active thrust. The design procedure of a rigid cutoff wall is also explained. There is a difference of approximately 10% between the passive thrusts for a rigid cutoff wall of normal length computed by Terzaghi’s method and by the rigorous method presented herein.