The amount of support required for vertical or near-vertical excavations in partially cemented or moist fine grained soils is significantly influenced by the “apparent” cohesion of the soil. This is a parameter that is often difficult to determine and its value is influenced by a number of factors.

A theoretical evaluation was carried out to quantify the influence of cohesion on the overall factor of safety for various face heights. The dangers of relying too heavily on cohesion are highlighted and backed up with simple probability analyses which show that slopes designed to a given factor of safety have a higher probability of failure when the stability of the slope is derived largely from cohesion. Practical recommendations are provided to assist in lateral support designs.