definition is that a trench is any narrow excavation made below the surface of the ground that is deeper than it is wide. They typically are no wider than 15 feet and no deeper than 20 feet and are often dug to install sewer and water lines and other underground utilities at commercial and residential sites. Because of their depth and the myriad ways that different soil types, external vibrations and weather can affect their stability, trenches can collapse without warning, offering little time to escape. One cubic yard of soil can weigh 3,000 pounds, about the weight of a Mini Cooper or other small car. In a collapse, the soil can move rapidly, depending on the depth of the trench and other variables.