Introduction

The shear strength of an unsaturated soil comprises three components: cohesion, frictional strength, and suction strength.  Suction strength arises from the negative pore-water pressure acting on the soil grains, which increases the effective stress.  This component is often ignored is geotechnical design, however, it can be important in certain classes of problems including rainfall-induced instability or back analysis of overly steepened slopes. This example describes how to include suction strength in a SLOPE/W analysis and the effect it has on the calculated factor of safety.