screw jacks

Worker protection

Sloping prevents trench failure by removing the weight (of the soil) that can lead to trench instability. Sloping, including benching (sloping done in a series of steps), requires a wide opening at the top of a trench. The angle of a slope depends on the soil and environment, but slopes range from 0.75 horizontal: 1 vertical to 1.5 horizontal: 1 vertical. The slope of 1.5 horizontal: 1 vertical is set back 1.5 m on each side at the top for each meter of depth. Even the slightest slope is beneficial. However, the width requirements of slopes often make this approach impracticable on construction sites.

Shoring can be used for all conditions. A shore consists of an upright on each side of a trench, with braces in between (see figure 93.5 . Shores help prevent trench wall collapse by exerting outward forces on a trench wall. Skip shores consist of vertical uprights and cross braces with soil arching between; they are used in clays, the most cohesive soils. Shores must be no more than 2 m apart from each other. Greater distances between cross braces can be achieved by using wales (or walings) to hold the uprights in place (see figure 93.6). Close sheeting is used in granular and weaker cohesive soils; the trench walls are covered entirely with sheeting (see figure 93.7). Sheeting can be made of wood, metal or fiberglass; steel trench sheets are common. Tight sheeting is used when flowing or seeping water is encountered. Tight sheeting prevents water from eroding and bringing soil particles into a trench. A shoring system must always be kept tight against the soil to prevent collapse. Braces can be of wood or of screw, hydraulic or pneumatic jacks. Wales can be of wood or metal.
http://www.cdc.gov/elcosh/docs/d0200/d000279/ilochap93.html