Before anyone can discuss the protective systems used in trenching, they need to understand what they're protecting against. The largest hazard in trenching operations, but certainly not the only hazard, is the potential for a cave-in. In 1926 Subpart P, the Occupational Safety and Health Administration (OSHA) defines a cave-in as "The separation of a mass of soil or rock material from the side of an excavation or the loss of soil from under a trench shield or supporting system, and its sudden movement into the excavation, either by falling or sliding, in sufficient quantity so that it could entrap, bury, or otherwise an injure and immobilize a person."
During trenching operations, not all soil is created equal. Some types of soil, by its very nature, can remain cohesive and "stick" together better than others. There are three classifications of soil, Type A, Type B, and Type C. Type A soil is a soil with a high clay content and the ability to maintain its form during trenching operations. Type A soil has a unconfined compressive strength, a determination of when the soil will fail while in compression, of more than 1.5 tons per square foot (tsf).
There are some situations where soil cannot be classified as Type A regardless of what the unconfined compressive strength is found to be. For example, soil cannot be classified as Type A if the surrounding ground contains fissures. These fissures are small cracks in the ground showing the potential for a section of soil to separate and fall into the trench. Another situation where soil cannot be classified as Type A is if the area has been subject to vibration through heavy traffic, railroads, pile driving, or even low flying aircraft. These vibrations can work to "work loose" the soil, increasing the potential for a cave-in. Likely the most common reason soil cannot be classified as Type A is soil that has been previously disturbed. Once the soil has been disturbed, it will never be as cohesive as it once was, regardless of the amount of compaction used.