According to BS6031 Code of practice for earthworks, a trench is any excavation whose length greatly exceeds it width. They sometimes qualify under the Confined Spaces Regulations.
Incidents in trenches are not as common as falls from stepladders, but are often fatal. A typical small trench collapse will contain 800kg of soil, which is equivalent of 8 people sitting on your chest, you won’t be able to breathe and you won’t be able to dig yourself out.
There are two basic types of collapse slough and slab. Slough collapse is where a bowl shaped section of wall detaches, usually at the top of the excavation, due to pressure or vibration from above or at the bottom where they hydrostatic pressure is greatest. Slab collapse is where an entire wall topples over into the trench; these can be catastrophic and occur without warning.
Extra material added to the normal ground level can dramatically increase the chances of collapse. Spoil tips, machinery, people etc need to be kept back from the top of the trench. Spoil tips should be at least 5 metres back. There are two ways to prevent trench collapse – you shore up the sides or you slope them at a shallow angle. Shoring a trench involves two things – lining the walls to support them evenly and bracing that lining so it can’t move. In a narrow trench you can brace one side against the other, but the problem comes when a trench is full of pipes and cables. Whatever you use, shores must reach the full height of the trench and must be in contact with the soil, with any voids behind filled in – the sudden impact from soil collapsing across a void can be enough to crush the shore.
For all piling and shoring systems a competent temporary works designer should be appointed, so the chain of responsibility is clear to everyone, and their designs must never be changed on site without consultation.
Further information from the HSE can be found here.