Active pressures apply behind retaining structures that can move forward such that the soil begins to shear and the shear forces support some of the weight of the backfill. The excavation of any kind of trenches or cuts such as basements for buildings or cuts for road works or railways may require some kind of support if the soil or rock is not of sufficient strength to stand safely on its own. They compute the passive pressure coefficient for the self-weight of the soil, any surface surcharge and for cohesion separately and if all three are acting. The soil behind the wall will mobilise the active earth pressure and the soil in front of the wall will mobilise part of the passive pressure. The force causing the sliding are assumed to be the active earth pressure behind the wall, and the resisting forces are those due to shearing along the base of the sliding block.