Trench shoring cages are used to support a wall when someone enters a trench with a wall greater than 1.2m (1.2m.=4ft) deep except when the trench is cut in sound and stable rock or sloped to within 1.2m of the bottom of the trench. If the trench is more than 1.2 m deep, the employer must ensure that either the trench is sloped at a 45 degree angle from the horizontal or has trench shoring cages installed. If the trench is cut in sandy soil or if the material is wet, the angle slope should be shallower as sandy and wet soils collapse very easily.
An engineer must certify the design of the shoring cages and include information on the depth at which the trench cage can be used and the manner in which the trench cage is to be installed, used, maintained and dismantled. A nameplate has to be permanently attached to trench shoring cages. The nameplate must be in a location visible for inspection, and identify the engineer that certified the design of the trench shoring cage and the depth at which the cage may be used. If trench shoring cages has been changed in any way that may affect the structural integrity of the cage, it must be certified by an engineer. The trench shoring cage should rest as close as possible to the bottom of the trench. In no case can the cage rest above the bottom of the trench more than the design maximum height, or 900mm, whichever is less. (900mm»3ft)
When to Use Trench Shoring Cages
When the walls or crests of a trench are cut in rock, the employer must ensure that rock bolts, wire mesh, or equivalent protection supports the walls. If mobile equipment is used near the edge of a trench, the employer must ensure that the trench shoring cages for the trench is adequate to support the increased load. When working in a trench and concerned about a nearby utility pole becoming unstable, the employer must ensure that a utility pole, building or other structure is provided support or removed if it can become unstable due to trenching activity. The employer must ensure water is not a hazard to anyone working in the trench. Examples of hazards due to water in the trench are electrical equipment in the trench or weakening of the soil and support walls. It is advised that before a person enters a trench, if they can be exposed to a hazardous substance or to an oxygen rich atmosphere in the trench, the employer must ensure testing is performed to determine the concentration of oxygen and any hazardous gas and adequate precautions are taken to reduce the risk of injury to that person. All temporary excavations greater than 1.2m in depth (1.2m.4ft) require fences, guards or barricades that can prevent a person from falling into the excavation.