Slope stability in temporary excavations

Most construction sites require excavation, and the stability of the sides must be considered in relation to safety and economy. Where possible, it is usually most economical to form excavations in open cut. The essence is to determine the steepest practicable slope. Several factors influence slope behaviour: (a) the nature and variability of the soil; (b) the groundwater conditions; (c) the loading close to the crest; (d) the time the excavation will remain open; and (e) the consequences of failure. Factors (a) and (b) require adequate information from site investigation, and (c)–(e) require good liaison with the construction planning team. Construction is generally straightforward, although it must be coordinated with other processes (especially groundwater control). Slopes in rock require similar considerations, but the behaviour of rock slopes tends to be dominated by the discontinuities in the rock, as the blocks of intact rock are comparatively strong. The important characteristics of rock slope discontinuities are their inclination relative to the cut slope (noting that there may be more than one set of discontinuities), frequency, persistence, openness and smoothness. Safety is a paramount consideration: unsupported trenches are examples of steep cut slopes and collapses are a major cause of accidents on construction sites.