Excavation, Trenching and Ground Support Safety

About this course.

Because the ground is far heavier and less predictable than most people assume, and a collapse gives almost no warning. A cubic metre of soil can weigh well over a tonne, so even a small fall of ground can pin a worker instantly, crushing the chest so that breathing stops even if the head remains clear. Trenches are also confined spaces in which people work below the surrounding ground, close to unstable faces, near buried services carrying electricity, gas and water, and beneath the reach of plant. The combination of sudden collapse, engulfment, services and traffic makes excavation an activity where ordinary care is not enough — the work has to be actively engineered to be safe before anyone enters the ground.

The central hazard is the fall of ground itself. Undisturbed soil holds together through friction and cohesion, but as soon as an excavation is opened, the face is no longer supported on one side and is free to move. Movement can be triggered by nothing more than the passage of time, a change in the weather, or vibration from nearby plant. When a face fails it does so quickly and without a reliable warning sign, and the material that comes down is dense and heavy. Workers are rarely killed because they are buried completely; they are more often killed or seriously injured because even a partial burial applies enough pressure to the chest and abdomen to prevent breathing. This is why depth alone is a poor guide to danger: people have died in trenches that were barely deeper than their own height.