Which of the following best describes a 'cave-in'?

A cave-in refers to the sudden collapse of trench walls, which can pose serious safety risks to workers who are within or near the trench. When the walls of a trench lose structural integrity—often due to factors like soil type, moisture, or improper support—the soil can give way, leading to a hazardous situation. This definition emphasizes the unexpected and dangerous nature of a cave-in, which can trap or injure workers.

The other options present scenarios that do not align with the definition of a cave-in. Limited excavations and planned movements of soil are both controlled activities that avoid the risk associated with cave-ins. Additionally, when all workers safely exit a trench, it indicates a situation where no collapse has occurred, further reinforcing that option does not describe the nature of a cave-in. Understanding these distinctions is crucial for recognizing the risks associated with excavation work and ensuring proper safety measures are in place.