EXCAVATION

What's the difference between a trench and an excavation? An excavation is a hole left in the ground as the result of removing material. A trench is an excavation in which the depth exceeds the width.
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An unstable trench or excavation can collapse, killing or injuring workers by suffocation or crushing when a worker is buried by falling soil. Trench stability is affected by a number of factors such as:
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soil type
weather
moisture
vibration
surcharge (excessive weight near the trench)
time
existing foundations, and
previous excavation
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The risk of a cave-in is not the only hazard in trenching. Injuries and deaths are also related to six other major areas:

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personal protective equipment
utilities underground and overhead
materials handling and housekeeping
heavy equipment
traffic control
confined spaces.
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Before excavating, the gas, electrical, and other services in the area must be accurately located and marked. If the service poses a hazard, it must be shut off and disconnected. Over half of all powerline contacts involve buried cable.