01 Mar Trenching and Shoring

Trenching and Shoring

Construction trenching for buried utilities, pipelines, water transport, and other activities may be hazardous. Trenches are usually deeper than they are wide and the walls may become unstable and collapse on top of workers.

Trench cave-ins occur when dirt, sand, and rocks collapse into the trench. These materials can engulf, injure, or kill workers in the trench. Soil can be very dense and heavy. When it engulfs workers, it can break bones, immobilize and restrict breathing, or suffocate them outright.

First, get training in trenching and shoring procedures. If workers will be entering a trench 5 feet or deeper, you will need to get a permit from the Division of Occupational Safety and Health (DOSH). Call 811 at least 2 days before the excavation. They will contact utilities so that representatives can determine if there are buried pipes or utilities in the planned work area.

Classify the type and quality of the soils on the trenching site according to cohesiveness, grain size, and saturation. Type dense and heavy clay, is the most stable. Type B is silt, sandy loam, or medium clay. Type C is the least stable soil, made of gravel, loamy sand and soft clay. If you are in doubt, treat the soil as if it is Type C.

Use sloping and/or benching techniques for trenches 5 feet or deeper and for all trenches with unstable soils. Trench walls can be angled or sloped downward and/or include flat benches or steps. Sloping and benching proportions should match the soil type. Type A soil requires 3 feet horizontal to 4 feet vertical (3/4:1). Type B requires 4 feet horizontal to 4 feet vertical (1:1). Type C requires 6 feet horizontal to 4 feet vertical (1-1/2:1). Benching cannot be used for Type C soil because it is too unstable.

Shoring uses a system of wall sheets, wales and cross braces created by an engineer to hold up and stabilize the trench walls. Trench boxes can also be used to hold up walls. Trench boxes can be stacked for deep trenches, but they cannot be used to raise the height of the trench wall. No workers are allowed in the trench when shoring or trench boxes are being installed.

Workers injured or killed in trenches often had a false sense of security and made incorrect assumptions about the working conditions and safety, prior to the accident. Weather and heavy operating equipment can cause soils to destabilize. Trenches can have hazardous atmospheres or contain pipelines and electric lines, so take proper precautions. Trenches and the surrounding soils need to be inspected daily by a competent person to ensure that they are stable and safe. Assumptions can lead to accidents, so be prepared for safety on a trenching site.