Abstract：In practice of soil slope engineering，most research work on earth pressures employs the same assumption that the failure surface in the backfilled clay is a plane. In fact，a number of laboratory and field tests have proved that the failure surface is actually a curving surface. Based on the vertical differential element method and the variational principle with movable boundary，a differential equation for the failure surface is deduced. Two cases of the equation are discussed as follows：(1) no lateral shear force exists at the interface between two elements，and the back of the retaining wall is frictionless. The failure surface is a plane，identical to Coulomb¢s theory. (2) lateral shear force exists，and the back of the wall is rough. The failure surface is a curving surface. The results show that the proposed approach can predict the failure pattern of the backfilled clay，which is in good agreement with those of the laboratory and field tests to certain degree. Finally，the effects of the properties of backfilled clay and the smoothness of the retaining wall on the angle of rupture are discussed.
