Embankment on Soft and Sensitive Clay Foundation

A test embankment built on a soft sensitive clay foundation has failed at a computed safety factor higher than one, thus confirming the allegation that the field vane test overestimates the shear strength in this type of clay. The increased shear strength values measured by the vane, as compared to the shear strength mobilized at failure, are explained by the affect of the rate of loading (time effect) and by the anisotropy of the clay. However, correcting the measured vane shear strength for these two factors, as recommended by Bjerrum (1973), results in a computed safety factor at failure still higher than one. This is due to the progressive failure phenomenon, which tends to develop in sensitive clays (liquidity index higher than one). Pore pressure and deformation measurements carried out during construction have confirmed the development of this progressive failure. The writers recommend the use of semi-empirical correction factors as functions of the plasticity index, anisotropy, and progressive failure.