Most deaths in trenches are due to cave-ins. Other risks include falls, electrocution, falling loads, hazardous oxygen and incidents involving mobile equipment. Before a trench is dug, the contractor must name a competent person who knows the excavation standard, is trained in soli analysis and protective systems, can identify hazards and has the authority to stop work right away. The competent person must inspect the trench before very shift and after anything that might increase hazards, such as a rainstorm or vibrations from nearby equipment. The type of soil is very important in determining the stability of the excavation. Even material that seems , such as soil with a high clay conternt, can cave in if there is accumulated water, it has been previously dug up, or is subject to traffic vibrations.
Choosing the proper protective system depends on soil type and depth. All trenches over five feet deep should be protected. The most common types of protective systems are:
Sloping: removal of soil to eliminate the chance of cave-in
Shoring: a lightweight, portable and easy to install system of wood or metal sheeting and supports.
Shields: heavy metal boxes usually used in deeper trenches more than twenty feet deep.
In addition, any trench that is more than four feet deep must have a ladder or some other means by which workers can quickly exit the trench. Consult with Subpart P of the OSHA construction standards before starting in excavation work.