Trench Entry to Trim the Base
A 2.6 m deep, 1.2 m wide drainage trench has been open for three days on a housing project in Johor. The ground is sandy clay. It rained heavily the previous night and about 150 mm of water stands at the base. Spoil from the dig is heaped 0.8 m from the trench edge and is roughly 1.5 m high. A 20-tonne excavator is tracking parallel to the trench about 6 m away, and a water pipe marker is visible 1.5 m from the trench side. There is no shoring, trench box or ladder. The workers have been entering at the end, climbing down the battered slope. The foreman wants two workers to go in and trim the base by hand before the concrete pour tomorrow. One worker says the trench 'looked fine yesterday'. No excavation permit has been issued, and no one has checked for underground services.
1. Is entry acceptable if the workers are quick and wear boots and helmets? Justify with the hazards and the control hierarchy.
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No. Quick entry does not reduce the collapse risk; a trench can fail in seconds under its own weight, water, spoil surcharge or vibration. Here the depth is 2.6 m, well beyond the depth at which support or safe battering is needed. The sandy clay, 150 mm of water, 1.5 m spoil heap only 0.8 m from the edge and a 20-tonne excavator 6 m away all raise the load on the trench walls. The water pipe marker 1.5 m away means services are close and may be damaged. No shoring, trench box or ladder is present. Boots and helmets do not protect against burial. The correct first action is to keep people out, barricade the edge, and treat entry as a planned task with an excavation permit, not a quick job.
2. Set out the steps that must be completed before anyone enters, and the checks needed while work is ongoing.
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Before entry: (1) locate and mark services with a cable and pipe locator and trial holes; (2) stop the excavator and establish an exclusion zone so plant does not track near the edge; (3) set spoil back at least 1 m from the edge and reduce its height; (4) dewater and control run-off; (5) install a designed support system - shoring or a trench box - or batter the sides to a safe angle for the soil; (6) provide a ladder for access close to the work; (7) issue an excavation permit and brief the crew; (8) have the trench checked by a competent person at the start of each shift and after rain. While inside: keep the excavator away, monitor for cracking or bulging, have a banksman, and stop work if water rises or soil moves.
3. What should happen if a side starts to slump while a worker is inside?
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Everyone should get out immediately by the nearest ladder and the area should be barricaded. Do not send people back in to rescue a buried worker without shoring - secondary collapse is common. Call the emergency services and the site emergency team, isolate plant, and if the person is partly buried, support the sides before any digging. Hand-dig around the casualty, not with an excavator. Keep the public away and preserve the scene for investigation.