Trench support

Frequently asked questions

Does a trench box hold the ground up?

No, and that is the most consequential misunderstanding about it. A trench box — a shield, a drag box — is two side panels held apart by struts, placed in the trench so that people work inside it. It is designed to withstand a collapse, not to prevent one: the ground outside can move, and the box's job is that the people inside are not in the way when it does. SHORING is the different thing that does prevent it, holding the faces in place with hydraulic props or sheets pressed against the ground so the soil never moves at all. The practical distinctions follow directly. A box protects only the space inside it, so working outside the box in an unsupported length is unprotected regardless of the box being there. A box does not stop ground movement, so it gives no protection to a service, a road or a neighbouring foundation beside the trench. And a box is dragged along as work proceeds, which is what makes it fast and what makes the unprotected moment while it moves worth planning for.

Why does the required slope depend on the soil rather than the depth?

Because what fails is the ground's ability to stand at an angle, and that is a property of the material. Cohesive clay can stand near vertical for a while; granular sand cannot stand much steeper than its angle of repose at all; and previously excavated ground — a backfilled service trench, made ground, anything that has been disturbed — behaves worse than either, because its structure was destroyed once already. So classification comes first and the permitted slope follows from it, which is why American regulation defines soil types with field and laboratory tests and publishes a maximum slope for each. Three factors make real ground worse than its classification suggests, and all three are visible on site: WATER, which removes apparent cohesion and adds weight; VIBRATION from plant or traffic alongside; and SURCHARGE, which is anything stacked near the edge — spoil, materials, a parked machine. Spoil heaped at the lip of a trench is the commonest of these and the easiest to move.

What makes a shallow trench dangerous?

The weight, and the fact that a person buried to the waist cannot get out or be pulled out. A cubic metre of soil weighs somewhere over a tonne and a half; a collapse does not need to cover someone to kill them, because the pressure on the chest of material up to waist height is enough to stop breathing, and the victim is held in place by friction against their whole lower body. Rescue then takes far longer than the time available, and attempting it without supporting the trench has killed rescuers repeatedly, which is why regulations require support well below the depth at which people intuitively start worrying. Two further properties make it worse than it looks: collapses are sudden and give no useful warning, and a trench that has stood open safely for days can fail after rain, after a machine passes, or as the ground dries and cracks. Time standing open is not evidence of stability, which is why inspection is required at the start of every shift rather than once at the start of the job.