What is a common cause of secondary collapse at a trench rescue incident

• A. Water intrusion from sidewall
• B. Heavy equipment inside trench
• C. Undermining soil from vibrations
• D. Personnel at the edge of the trench

Answer: (D)

Secondary collapse happens when an existing trench failure is worsened by a later disturbance to the surrounding soil or loads placed on the trench area. The trench walls are already unstable, and any added weight or movement can push loose soil into motion, causing another segment to fail.

Moving or standing near the trench lip is a particularly common trigger because it directly changes the load distribution on the top of the wall. The weight and vibration from people at the edge can loosen the overhanging soil, initiate a new slide, and propagate a secondary collapse that can trap rescuers or victims. The edge is the weak point where the wall has the least support, so disturbances there are most likely to cause another failure.

Water intrusion, heavy equipment inside the trench, or vibrations from equipment can contribute to instability, but the most frequent single trigger for a secondary collapse is movement or loading at the trench edge.