The sides of a trench may look firm, but looks can be misleading. Excavations and trenches collapse for the following reasons.
- The sides are not supported or supports are not installed properly.
- Vehicles operate too close to the edge (for example dumpers, excavators or other large vehicles).
- Materials and spoil are stored too close to the edge.
- The ground dries out, shrinks and collapses.
- Heavy rain weakens the ground and the sides. Groundwater in an excavation also weakens the sides.
- The excavation is too close and undermines or weakens nearby walls and structures, causing them to collapse.
Keep vehicles a safe distance away from the edge of an excavation. A truck carrying 6m3 of concrete weighs approximately 26 tonnes.
There are many dangers when working within excavations. To work safely the following hazards may need to be considered.
- Falls of materials, people, plant and vehicles into the excavation.
- Making foundations of nearby structures weak, including scaffolds and other temporary works structures.
- Accidental or deliberate contact with underground services.
- Water or other fluids entering the trench, standing water and pumping out.
- The presence of naturally occurring gases (such as hydrogen sulphide (H2S)).
- The build-up of gases that are heavier than air (such as H2S and liquefied petroleum gas (LPG)).