Why it matters
Soil is heavy. One cubic metre can weigh well over a tonne. When a trench wall collapses, it buries a worker in seconds, and the weight stops the chest from moving. Rescue is slow and dangerous.
Collapse is not the only killer. Workers hit buried power cables and gas pipes. Excavators strike people at the edge. Vehicles roll in. Fumes from engines and pumps collect at the bottom.
Ground can look stable and still fail. Rain, vibration from plant, a nearby load or a leaking pipe can all change it within hours. Never trust a trench because it held yesterday.
Before you dig
Most excavation hazards are fixed at the planning stage. The ILO construction code lists what to sort out before the first bucket goes in.
- Decide the dig method and the type of support, based on the ground and the depth.
- Have a competent person check ground stability and the effect on nearby buildings, roads and footings.
- Find every buried service: power, telecoms, gas, water and sewers. Use drawings, then detect and mark on site.
- Isolate services where needed. Support and protect any that must stay live.
- Plan where spoil, plant, ramps and vehicle routes will go.
- Check for contaminated ground and other hidden hazards.
Drawings are often wrong or out of date. Detect and mark services on the ground, then hand-dig near them. Do not let the excavator find them for you.
Order of controls
- Eliminate. Can trenchless methods, such as directional drilling or pipe jacking, avoid the open trench? Can more work be done from the surface?
- Substitute. Use precast units or trench boxes to cut time spent inside the trench.
- Engineer. Shoring, sheet piling, trench boxes or safe sloping. Guard-rails and barriers at the edge. Stop blocks for vehicles. Ventilation.
- Administer. Permit-to-work, inspections, exclusion zones around plant, a banksman, and a count of who is in the excavation.
- Personal protection. Helmets, high-visibility clothing and safety boots. None of these protect you from a collapse.
Supporting the sides
Any excavation deeper than 1.5 metres needs support to stop collapse. So does any excavation with undercut banks. Support may be underpinning, sheet piling, bracing, trench boxes or other shoring. The only exception is when a professional engineer has examined the excavation and certified it safe without shoring.
Deeper than 4 metres, a professional engineer must design the shoring. The PE, or another PE the occupier appoints, checks it is built to the design and certifies it safe. Until then, only digging and shoring work may happen inside.
Put in, change or remove support only under a competent person's supervision. Never enter an unsupported section to fit the shoring. Install it from outside or from inside a trench box.
An overhang or a bank cut away at the bottom can fail at any depth. Treat it as needing support, even under 1.5 metres.
Edges, spoil, plant and access
Place spoil and stacked materials well back from the edge. Their weight pushes on the wall and can bring it down. Loose material can also roll in onto workers.
Keep plant and vehicles away from the edge unless the support has been designed to carry them. Use anchored stop blocks or barriers where vehicles tip or reverse near the excavation. Keep people out of the excavator's swing radius. Use a banksman when plant works near people.
Fit guard-rails on open sides deeper than 2 metres. Put up warning notices where people can see them. Provide enough ladders, stairs or ramps so every worker can get out quickly.
Air in the excavation
Exhaust from pumps, generators and plant builds up in deep excavations with poor air flow. Gases can also seep in from the ground. Keep engines out of the excavation and away from the edge. Test the air if there is any doubt. Ventilate if needed.
A deep, narrow excavation with poor air flow may be a confined space. Apply your confined space procedure if it is.
Inspection
A designated person must inspect the excavation and its surroundings after every rainstorm or other event that raises the risk. If it is not safe to enter, they tell the project manager at once. The project manager can stop all work inside and revoke the permit.
Good practice goes further. The ILO code says to check the sides before each shift, and after heavy rain, a fall of ground or damage to supports. Look for cracks at the top, bulging walls, water seepage, loose wedges and bent struts. Record each check.
What the rules require (Singapore)
Part XI of the Workplace Safety and Health (Construction) Regulations 2007 covers excavation. These duties fall mainly on the occupier of the worksite.
- Shoring for excavations deeper than 1.5 m or with undercut banks, unless a PE certifies it safe without (reg 77(2), (3)).
- PE-designed shoring for excavations deeper than 4 m. Only excavation or shoring work inside until a PE certifies it (regs 77(4), (5) and 78).
- All reasonably practicable steps to prevent burial, strikes by plant or dislodged material, falls into the excavation, and exposure to carbon monoxide or other bad air (reg 77(6)).
- Spoil and other loads kept away from the edge (reg 77(7)).
- Guard-rails on open sides deeper than 2 m (reg 77(8)); warning notices (reg 77(9)).
- Inspection by a designated person after every rainstorm or other hazard-increasing event (reg 77(10)).
- Safe access and egress, enough in number and easy to reach (reg 79).
- Permit-to-work for excavation and trenching deeper than 1.5 m. The supervisor applies, the safety assessor inspects, and the project manager issues the permit (regs 10 to 15).
The occupier appoints a project manager and a safety assessor for high-risk construction work. No permit, no digging below 1.5 metres. Rescue work is the only exception.
Common mistakes
- Workers stepping into an unshored trench for a quick look or to clear a pipe.
- Spoil piled right at the edge to save space.
- Excavator or lorry parked at the trench edge.
- Struts removed early to make room for pipe laying.
- No re-inspection after an afternoon downpour.
- One ladder at the far end of a long trench.
- A pump or generator running at the bottom of a deep pit.
Ground changes. Rain, vibration and new loads weaken walls through the day. Check again before anyone goes back in.
Use it on site: the excavation & trenching checklist and the toolbox talk to brief your crew.
Sources
- Workplace Safety and Health (Construction) Regulations 2007, Singapore Statutes Online (AGC).
- Code of practice on safety and health in construction (revised 2022), International Labour Organization.
This is practical guidance, not legal advice. Laws and codes of practice change; always check the current official text and your site's own procedures.
