Two-Metre Trench, No Shoring: Entry Decision on a Strip Footing
A residential extension requires a 2.4 m deep strip footing trench along a 14 m length. The subsoil is loose sandy silt with seepage at about 1.6 m. Excavated spoil has been heaped within 0.5 m of the trench edge, and a small backhoe is parked 2 m away. The site supervisor wants two masons to enter immediately to clean the bottom and set out the footing, because the concrete mixer is due in 30 minutes. No shoring, benching or access ladder is in place. Workers have 1.2 m high visibility vests and helmets, but the trench has not been inspected since it was dug yesterday evening. A worker reports a small crack running parallel to the edge, about 3 m long. The trench is open on one side to a public footpath.
1. Should the masons be allowed to enter the trench? Identify the immediate hazards and give the sequence of controls that must be in place before entry.
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Entry should not be permitted. The trench is over 1.5 m deep in loose sandy silt, with seepage at 1.6 m and a tension crack parallel to the edge. The spoil heap and backhoe add surcharge close to the crest, and there is no shoring, benching or safe access. The likely failure mode is a sudden side collapse, giving little warning; the crack shows the edge is already moving. A 30-minute concrete delivery is not a reason to accept that risk.
Control sequence: stop work and barricade the area; move the backhoe and spoil at least 1 m from the edge, preferably further for a 2.4 m depth; divert surface water and dewater the trench; install designed shoring or benching using trench boxes or close-sheeted poling boards; provide secured ladders; have a competent person inspect and record the condition before anyone enters. Entry should occur only after these controls, with a lookout and emergency plan in place. Monitoring the crack alone is not an adequate control.
2. Describe the safe access, egress and inspection requirements for this 14 m long trench, including who may inspect and how often.
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Access should be by secured ladders that extend about 1 m above ground level, with one ladder near each end of the 14 m trench so that no worker has to travel more than roughly 7.5 m to reach one. Ladders must be tied off, placed on firm footing and kept clear of the shoring. Workers must not climb the shoring or jump into the trench. A safe walkway and edge protection should separate the trench from the public footpath.
Inspection must be by a competent person who understands excavation behaviour and the shoring system. It should be done before the first entry of each shift, after rain or seepage increase, after any vehicle or plant movement near the edge, and after any crack movement. The inspection should be recorded, including the date, time, shoring condition, seepage and action taken. If the crack widens, seepage becomes muddy or shoring moves, workers should be evacuated and the trench re-engineered before re-entry.
3. The trench must remain open for three days. Outline a protection and monitoring plan for that period.
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The plan should be based on a shoring design suitable for loose silt, not on ad hoc boards. Install continuous edge protection and warning signs on the footpath side, and keep the area fenced. Maintain dewatering so that water does not soften the trench bottom or wash out fines. Keep all spoil and plant outside a defined exclusion zone, and provide a firm crossing if workers must pass.
Monitoring should include tell-tales across the crack, survey pins at the crest and a daily recorded inspection by the competent person. Any movement, increased seepage or loosening of shoring should trigger evacuation and review. Concrete should be placed as soon as the footing is ready, and the trench should be backfilled and shoring removed in a planned sequence. A named responsible person should control entry, and an emergency rescue plan should be briefed to the crew.