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Generally speaking, an excavation is a hole in the ground as the result of removing material. A trench is an excavation in which the depth exceeds (is bigger than) the width.
Working in trenches and excavations is hazardous to both the workers who work inside them, and to workers on the surface. The hazards include:
Definitions of soil types vary by jurisdiction in Canada. In addition, some jurisdictions have not defined soil types, but do require preventative measures when an excavation reaches a certain depth or proportion.
When a soil type is defined, the purpose is to try to identify or predict the potential for the soil to move and cause a collapse while the work is being done. Soil types typically use a scale of 1 to 4 where 1 is hard and dense to 4 which is loose, soft, wet or muddy soil, or a scale of A to C where A is hard and solid, and C is soft, sandy, filled or loose.
The soil type is determined by the characteristics of the soil’s consistency, ease of removal, appearance, ability to excavate with hand tools vs. machine, water seepage, whether the soil has been excavated before, etc.
The employer or supervisor is responsible for the work, and must take the necessary steps to identify all the hazards and risks before beginning any work. These steps include to:
In general, trenches that are 1.2 metres (4 feet) deep or greater require a protective system unless the excavation is made entirely in stable rock. The factors to consider include:
There are two basic methods of protecting workers against cave-ins:
Sloping involves cutting back the trench wall at an angle that is inclined away from the work area of the excavation. The angle of slope required depends on the soil conditions. Benching is a similar method to sloping.
Saskatchewan Labour defines a temporary protective structure as “a structure or device in an excavation, trench, tunnel or excavated shaft that is designed to provide protection from cave-ins, collapse, sliding or rolling materials, and includes shoring, trench boxes, trench shields and similar structures.”
The following are some points to consider. Each circumstance will be different, so be sure to adapt the questions to suit your situation.
Temporary protective equipment, such as:
- Is the shoring equipment the right equipment as required for the depth of the trench/excavation and type of soil?
- Is the equipment damaged (e.g., cracked, crushed, split, or bowed)?
- Are there loose or missing cleats?
- Are the struts off level?
- Are the boxes damaged or have defects?
- Are the plates deformed, bent, have holes, or show other damage?
- Are the welds cracked, bent, or distorted?
- Are there missing or missing struts?
- Are trench boxes shifting or settling to one side?
- Are there any visible leaks in hoses or cylinders?
- Are there bent bases?
- Is any equipment cracked, split, broken or cracked?