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There are many potential hazards when working in excavations and trenches. Probably the most common hazard at any work site is the threat of cave-in. A cave-in occurs when walls of an excavation collapse.
Cave-ins can be deadly. Wall failures often occur suddenly, with little or no time for the worker to react. The weight of the soil crushes and twists the body, causing death or serious injury in a matter of minutes. Excavations need not be deep or large to create a life threatening hazard, so every excavation must be taken seriously.
Why do cave-ins occur? Undisturbed soil is kept in place by natural horizontal and vertical forces of the nearby soil. When we dig in the earth, these natural forces are no longer able to hold back the soil left behind. With no support, eventually the laws of gravity take over, and the soil from the excavation walls move downward and inward into the excavation. The result is a cave-in.
Cave-ins are more likely to occur in unprotected excavations where:
- The excavation is dug in unstable soil, or in soil that has been dug in before;
- There is excessive vibration from construction equipment or vehicle traffic around the excavation ;
- Too much weight near the sides of an excavation, most frequently from equipment or the excavated material (spoil pile) too near to the edge;
- Water has collected in the excavation;
- Changes in weather conditions (freezing, melting, sudden heavy rain, etc.)
Although there isn’t much that can be done about the weather, there are ways to help control these conditions. In order to safely and efficiently work around excavations and prevent cave-ins from occurring, always follow the points below:
- Re-route traffic whenever possible, and keeping only the heavy construction equipment needed near the excavation;
- Keeping the spoil pile at least 2 feet back from the edge of the excavation;
- Pumping water out of the excavation before anyone enters it;
- Using protective systems when required.
The threat of a cave-in is not the only safety concern when working around excavations and trenches. Other hazards to be considered include accidental contact with utility lines, crushing and striking hazards posed by mechanized equipment, and hazardous atmospheres . These hazards will be discussed in greater detail in later chapters. All of these hazards, however, can be kept to a minimum with thorough planning. A pre-job survey allows contractors and owners the opportunity to avoid costly changes after the work has begun.