The Trench, the Rain and the Neighbour's Fence
You are a plumber on a residential site in the Blue Mountains. A 1.8 m deep, 12 m long trench has been excavated along the side boundary for a sewer connection. The soil is sandy clay and it rained heavily overnight; the trench walls are damp and one side shows vertical cracks and a bulge about halfway along. The spoil has been stockpiled 0.5 m from the trench edge, and a neighbour's timber fence runs 1 m from the other edge. There is no shoring, no battering and no trench box. A ladder sits in the trench but its top extends only 300 mm above ground. Two workers are in the trench laying pipe. The excavator operator is waiting to backfill and says the trench 'held up fine yesterday'.
1. Assess the condition of the trench and decide what must happen before anyone goes back in.
Read the worked answer
The trench is not safe to enter, and the two workers inside should be brought out now.
What the evidence tells you
- Depth: 1.8 m exceeds the 1.5 m threshold at which a trench must have adequate shoring, benching, battering or other support to prevent collapse.
- Soil and water: sandy clay that has been soaked by heavy overnight rain is far weaker than dry material. Water increases pore pressure and reduces the soil's shear strength, and it lubricates failure planes.
- Cracks and a bulge: these are active signs of instability - the wall is already moving. A bulge in a trench wall is a precursor to collapse, not a cosmetic defect.
- Spoil at 0.5 m: stockpiling spoil close to the edge adds surcharge load directly above the failing wall and increases the risk of collapse.
- Neighbour's fence at 1 m: the fence adds load and is itself at risk; its footing may be undermined.
- Ladder only 300 mm above ground: the ladder does not provide a safe, compliant means of escape - it should extend at least 1 m above the landing.
- No shoring, battering or trench box: there is no collapse protection at all.
What must happen before re-entry
- Evacuate and barricade the trench; post signage and keep everyone, including the excavator, clear of the edge.
- Stop the excavator from working near the edge; its weight and vibration add load.
- Move the spoil back to at least the distance specified in the SWMS (commonly 1 m or more from the edge) or off site.
- Inspect and reassess after the rain, by a competent person, and record it. Do not rely on 'it held up yesterday'.
- Install collapse protection - a trench box or shoring designed for the depth and soil, or batter the walls to a safe angle - and dewater the trench.
- Provide compliant access - a ladder extending at least 1 m above the surface, secured, with a clear landing.
- Check the fence and neighbour's property for undermining and support as needed.
Only after those controls are in place and the competent person signs off should anyone re-enter.
2. Who has duties here, and what should be done about the two workers who were in the trench?
Read the worked answer
Duties
- The PCBU (the plumbing business and the principal contractor) has the primary duty to ensure health and safety so far as is reasonably practicable, including providing and maintaining safe systems of work, plant and structures, adequate information, training and supervision, and a safe workplace. For construction work, a safe work method statement (SWMS) is required for high-risk work, and excavation deeper than 1.5 m is high-risk construction work under the WHS Regulations.
- The excavator operator has a duty as a worker to take reasonable care and to follow safe systems, including not stockpiling spoil at the edge and not working near an unstable trench.
- The supervisor/leading hand must ensure the SWMS is followed, the trench is inspected and signed off, and no one enters an unprotected excavation.
- Each worker in the trench has a duty to take reasonable care for their own safety and to report hazards - including refusing to enter an unshored trench.
- The principal contractor must manage the site and coordinate with the neighbour's property owner where the works affect their land.
The two workers
- Get them out and check them for injury and for signs of exposure or distress; provide first aid if needed.
- Do not blame them. They were directed into an unprotected trench by the system of work. The failure is the PCBU's, not theirs.
- Record the event in the site incident register, including the trench condition, the time, who was present, and the corrective actions.
- Determine notifiability. A collapse or a serious injury would be a notifiable incident requiring immediate notice to SafeWork NSW; an evacuation and near miss should still be recorded and investigated.
- Consult and retrain. Brief the crew on trench safety, the SWMS, the spoil placement rule, access requirements and their right to stop unsafe work.
- Review the SWMS with the crew and the competent person before work resumes.
The exam point: an unshored 1.8 m trench in wet sandy clay with visible wall movement is a foreseeable collapse waiting to happen, and the duty to stop work sits with everyone on site, not just the safety officer.