Construction projects involving excavation and trenching are among the most hazardous workplace activities. An excavation is defined as any man-made cut, cavity, trench, or depression that is formed by earth removal. The term “trench” is specific to underground excavations that are deeper than it is wide, being no wider than 15 feet. The fatality rate for all types of excavation work is 112% higher than that of general industry (U.S. Occupational Safety and Health Administration). Given this high level of danger, it’s critical that safety precautions and controls be used at all times and that extreme caution and patience be exercised when working in and around pits and excavations.
Types of Excavation Risk
Cave-ins pose the greatest risk and are the most likely type of excavation-related incident to result in fatalities. Other potential hazards associated with excavation include:
• Falls into trenches or excavations
• Tripping over equipment, debris and spoil
• Excavated material or other objects falling on workers
• Exposure to underground services or overhead lines
• Mishandled or poorly placed materials
• Difficulty breathing due to noxious gases or lack of oxygen
• Toxic, irritating, or explosive gases
• Vehicles and mobile equipment
Mitigating the Risk
The two basic methods of protecting workers against cave-ins are sloping and temporary protective structures.
Sloping involves cutting back the trench wall at an angle that is inclined away from the work area of the excavation. The appropriate angle of the slope depends on the soil conditions at the site of excavation.
Temporary protective structures are designed to provide protection from cave-ins, collapse, sliding or rolling materials. Examples of temporary protective structures include shoring, trench boxes, pre-fabricated systems, hydraulic systems, and engineering systems.
Shoring is a system that supports the sides or walls and normally requires the use of aluminum, steel, or wood panels that are supported by screws or hydraulic jacks. Shoring should be done in conjunction with the progression of the excavation. If there is any delay between digging and shoring, no workers should enter the unprotected trench. Trench Boxes are often used in open areas that are away from utilities, roadways, and foundations. Trench boxes can be used to protect workers in cases of cave-ins, but are not a substitute for shoring. If the trench or excavation walls are made of rock, rock bolts or wire mesh can be used to offer additional support.
Excavation Safety Tips:
• Know the location of any underground utility lines
• Wear proper PPE – including protective hardhat, eyewear and footwear
• Keep heavy equipment away from trench edges
• Test for low oxygen and toxic gases.
• Inspect trenches at the start of each shift or if there has been significant rainfall
• Place barriers along the outside perimeter and safety signs at key locations
• Consider consulting with a professional engineer regarding the design and installing of the shoring
• Establish a safe means of access and egress
• Know the location of water sources and what the drainage patterns will be
• Develop an emergency response plan and include provisions for extreme weather, evacuation routes, and communication plans
What NOT to do:
• Do not enter an unprotected trench deeper than 4 feet
• Do not start digging before locating and de-energizing the buried services
• Do not enter a trench before testing the air
• Do not place anything within 1 metre from the trench’s edge
• Do not rely on natural freezing to act as a method of soil stabilization
It’s important to remember that collapses can occur without warning, regardless of the depth. In fact, the vast majority of fatalities occurs at minimal depths when workers fail to appreciate the risks involved. All excavation projects present serious safety risks, but injuries and fatalities resulting from collapses are preventable with proper planning and safety precautions.