Trench Entry for an 18 m Sewer Connection
A contractor has to connect a school's new toilet block to the municipal sewer manhole 18 m away. The trench is 0.6 m wide and 1.6 m deep at the manhole end, tapering to 1.1 m at the building. The ground is loose sandy silt backfilled two years ago, and it is the third day of monsoon rain; about 100 mm of water stands in the trench bottom. Two workers are on site with one wooden ladder, no shoring and no pump. Excavated spoil is heaped about 0.8 m from the trench edge, and the hired JCB that dug the trench is parked 2 m away with its engine running. The supervisor wants the line laid and the trench backfilled before evening so that the school can reopen on Monday. One worker says he has done deeper trenches many times and offers to go down alone to set the pipe, while a 110 mm PVC line with a 1 in 60 fall is planned.
1. List the hazards in this trench and, for each, state the control that must be in place before anyone enters, including the numbers you would insist on.
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- Collapse - 1.6 m in loose sandy silt is past the 1.5 m depth at which shoring becomes mandatory in such soil. Provide a trench box or close-sheet shoring to full depth, with the top frame not more than 0.5 m below ground, before entry.
- Water - 100 mm of standing water softens the base and raises collapse risk. Dewater with an attended pump before and during work, and keep a sump at the low end.
- Spoil surcharge - spoil at 0.8 m sits inside the zone of influence. Move it to at least 1 m from the edge (1.5 m is safer) or cart it away.
- Vibration - an idling JCB 2 m away adds dynamic load. Switch it off or move it beyond 3 m.
- Access - one ladder for two workers is not acceptable. Provide a ladder every 7.5 m of trench, extending 1 m above ground.
- Atmosphere - connecting to a live manhole can release H2S and methane. Test the air before entry and ventilate.
- Solo entry - never permitted. A standby person with harness and rope stays at the top and keeps visual contact.
2. Write the sequence of operations for the day, from dewatering to backfill, and name the one check that must be signed off before entry.
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Sequence: (1) barricade and sign the trench, set the ladder; (2) pump out the standing water and dig a small sump; (3) install the trench box or shoring and check the frames; (4) move spoil to at least 1 m and stop the JCB; (5) test the atmosphere at the manhole end and ventilate; (6) entry only with a standby person, helmet and boots; (7) lay the 110 mm pipe to a 1 in 60 fall, checked with sight rails and a boning rod, jointing with solvent cement and allowing the stated cure time; (8) test the line, then backfill in 150 mm layers with sand or fine soil around the pipe and compact each layer; (9) remove shoring as backfill rises, never pulling it out from the bottom. The signed check is the shoring inspection and gas test record: depth, soil type, shoring type, date and the name of the competent person must be on paper before the first worker descends.
3. A section of trench collapses while a worker is inside. Explain what the two workers outside must and must not do.
Read the worked answer
Must: raise the alarm and call the fire service and ambulance (112) immediately; switch off and move the JCB to stop vibration; clear the spoil heap back from the edge; keep other people away; establish voice contact and keep talking to the casualty; supply air if a blower is available; shore or place a trench box before anyone reaches the casualty, and lift from the side using boards or the ladder while standing on stable ground; treat for crush injury after release - keep the casualty warm, still and nil by mouth, and hand over to the medical team. Must not: jump into the trench (the most common cause of a second fatality, because the collapsed face and the spoil heap are still unstable), dig with an excavator bucket near the casualty, pull the person by the arms, or restart the JCB. After the rescue, preserve the scene and report the incident in writing.