THE FACTOR OF SAFETY AGAINST UNDRAINED FAILURE OF A SLOPE

Abstract

In the application of the Fellenius, the Bishop and the friction circle methods of slope stability analysis the factor of safety is defined in terms of the failure shear stress that may be mobilized at the existing in-situ value of effective normal stress. No account is taken of the additional pore pressure change (positive or negative) that may occur between the in-situ state and the failure state when the possibility of undrained failure is being examined. Expressions for all three methods in which allowance is made for the development of these additional pore pressures, have been derived. It is claimed that the resulting redefinition of the factor of safety is more logical and more valid than the conventional definition. The use of these expressions is illustrated by their application to the analysis of the failure of the Seven Sisters embankment in Canada.