Almost all types of hydraulic structures such as levees, floodwalls, weirs, barrages, dams, reservoirs, irrigation and power generation canals, etc., are provided with a sheet pile cut-off wall to reduce the exit gradient at the downstream toe, and to prevent the chances of sand boiling and piping.

In most cases, the design of a sheet pile cut-off wall is produced taking into consideration the average seepage force, as suggested by Terzaghi [12]. As a result, the thrust acting on the sheet pile cut-off wall is determined by subtracting the passive thrust from the combined active thrust, added to the unbalanced water pressure around the cut-off wall. In this approach, the submerged unit weight of the soil, reduced by the average seepage pressure per unit volume of the mass of soil, is adopted as the unit weight of the soil for the passive earth pressure. A downstream failure wedge with an angle of

Equation 1

with the horizontal has been considered for computation of the coefficient of the passive earth pressure, where

Equation 2

is the effective angle of internal friction of the foundation soil. In the present paper, a rigid sheet pile cut-off wall under a flood wall (rigid base) has been considered for analysis and design. Exact seepage forces have been computed, and active and passive earth pressures acting on a rigid cut-off wall have been determined. A difference of about 12% has been found between the passive thrusts for a rigid sheet pile cut-off wall of normal length, computed using Terzaghi’s method, and the method presented within this paper.