EXC/General/TBT-EXC-002

Trench Collapse Prevention

Excavations › General › Trench Collapse Prevention

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Trench collapses are one of the most lethal hazards on construction sites — burial in soil is almost always fatal.
One cubic metre of soil weighs approximately one tonne, enough to crush a person's chest and cause suffocation.
All trenches must be assessed for collapse risk — factors include depth, soil type, water, vibration, and surcharge loads.
Trenches deeper than 1.2 metres generally require formal support such as trench sheets, shoring, or trench boxes.
Battering back excavation sides to a safe angle is an alternative to shoring where space and ground conditions allow.
A competent person must inspect the trench at the start of each shift and after any event affecting stability.
Spoil, materials, and plant must be kept at least 1.5 metres back from the trench edge to reduce surcharge loading.
Water ingress from rain, groundwater, or burst mains weakens trench walls rapidly and dramatically increases collapse risk.
Ground conditions can change within hours — clay that appears stable in dry weather can collapse after rainfall.
Emergency rescue from a trench collapse is extremely difficult — prevention through proper support is the only reliable approach.

Prevent burial alive	A person buried under collapsing soil has only minutes to live — proper trench support prevents burial.
Rapid deterioration	Ground conditions change quickly with weather, vibration, and loading — what was safe yesterday may collapse today.
Legal duty	CDM 2015 and HSE guidance HSG185 require competent inspection and adequate support for all excavations.

Don't

Assess the trench for collapse risk considering depth, soil type, water, and surcharge loads.
Install trench sheets, shoring, or trench boxes for all trenches deeper than 1.2 metres.
Have a competent person inspect the trench before each shift and after any adverse event.
Keep spoil, materials, and plant at least 1.5 metres from the trench edge at all times.
Monitor for water ingress and arrange dewatering before it weakens the trench walls.
Use stop blocks to prevent vehicles and plant tracking along the trench edge.
Provide safe ladder access within the trench at regular intervals for emergency egress.
Batter trench sides to a safe angle where space and ground conditions allow.
Re-inspect trenches after heavy rain, frost, or any vibration from nearby plant.
Brief all workers on the signs of imminent collapse — cracking, bulging, and water seepage.

DON'T enter any unsupported trench deeper than 1.2 metres under any circumstances.
DON'T assume soil is stable — conditions change rapidly with weather and loading.
DON'T skip trench inspections — they must happen every shift and after any adverse event.
DON'T place spoil, materials, or plant within 1.5 metres of the trench edge.
DON'T ignore water in the trench — it dramatically increases the risk of wall collapse.
DON'T drive vehicles or plant along unsupported trench edges without stop blocks.
DON'T work in a trench without a safe means of escape within easy reach.
DON'T remove trench support until the trench is being backfilled in a controlled sequence.
DON'T continue working if you see cracks, bulging walls, or sudden water ingress.
DON'T attempt to rescue someone from a collapsed trench by digging with your hands.

See also: Trench Sheeting & Shoring | Excavation Inspection Requirements

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