Contents preface (VII) introduction 1—37

15.4.5. Factor of Safety
In most of the stability analysis methods for embankment dams, it is assumed that a slope might fail by a mass of soil sliding on a failure surface. At the moment of failure, the shear strength of the soil is fully mobilised on the entire failure surface and the overall slope as well as each of its parts are in the static equilibrium. For stable slopes, the shear strength mobilised under equilibrium conditions is less than the available shear strength and this is conventionally expressed in terms of a factor of safety, F defined as (14).

F =	shear strength available	(15.32)
shear strength required for stability

The acceptable value of factor of safety for different conditions of stability of an embankment dam depends on method of analysis, site conditions, size of the dam, functions of the reservoir, and so forth.