It is commonly assumed by Geotechnical Engineers that sand has no cohesion. However the sands in Perth usually contain a trace to some fines (material finer than 0.075mm). Provided that the sand is moist (that is not fully dry or saturated) then soil suction creates an apparent cohesion. It is this apparent cohesion that allows shallow trenches in sand in Perth (say less than 1 m deep) to stand with nearly vertical sides for several days. If there were no apparent cohesion, the sides of the trench would immediately collapse back to a slope approximating the friction angle of the sand. For a slope cut in sand the likelihood of a slope failure can be assessed by calculating a factor of safety for a wedge of sand sliding into the excavation as illustrated in Figure 1. The factor of safety is the ratio of the forces resisting movement and the forces driving movement. The driving force is caused by gravity and surcharge. The resisting force is provided by the friction and cohesion in the soil. A spread sheet to perform these calculations has been developed.
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