photo credit putevodnik
There are several different types of protective systems that are used to prevent cave-ins. Soil type must be determined before deciding the best method of protective system to use.
If a trench is 5 or more feet deep it must be stabilized by shoring, sloping, or have some protective system to prevent a cave-in (unless the excavation is in 100% stable rock).
SLOPING AND BENCHING
Sloping is a technique that uses the shape of the excavation to prevent cave-ins. OSHA defines the maximum allowable slope as:
“Maximum allowable slope means the steepest incline of an excavation face that is acceptable for the most favorable site conditions as protection against cave-ins, and is expressed as the ratio of horizontal distance to vertical rise (H:V)”
Slope configurations are based on type of soil, depth of the trench, length of time the excavation will be open, and the soil type. OSHA has specific guidelines on slope configurations.
Benching is an excavation technique that creates a series of horizontal steps or “benches” in the side of the excavation. Workers are not allowed to work on the faces of sloped or benched excavations.
SHORING AND SHEETING
Shoring and sheeting are structures that are constructed in place to support the sides of an excavation and help to prevent cave-ins. They can be timber or aluminum hydraulic or pneumatic. The structure of shoring and sheeting can include uprights or sheeting, cross braces, struts and wales.
An engineer must design these protective systems as required by OSHA.
SHIELDING/ TRENCH BOXES
Shielding structures (trench boxes) do not prevent cave-ins, however they are designed to withstand the force of a cave-in and protect workers who are inside the shield/box. They must be used in accordance with the manufacturer’s instructions. No one must be inside or on the shield until it is safely in place. The space between the outside of the shield and the side of the trench must be backfilled to help prevent the shield from moving laterally. Shields/boxes can be used with sloping or benching techniques.
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