Gantry Crane on Soft Ground Beside a Trench
A rail-mounted gantry crane is being used in a yard to place precast concrete panels into a 3 m deep service trench. The crane runs on temporary timber bearers laid over compacted fill. Overnight rain has softened the ground, and the operator notices the crane's near-side wheels have sunk about 40 mm into the fill, with the rail beginning to deflect. The panel being lifted weighs 6 t and is currently suspended 1.5 m above the trench, with two workers in the trench guiding it by tag line. The banks of the trench are near-vertical and show minor slumping. The operator has 20 minutes left on shift and the supervisor wants the panel set before the crew leaves.
1. Identify the hazards and assess the risk of continuing the lift as described.
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Hazards:
- Ground bearing failure. 40 mm settlement and rail deflection indicate the fill is losing bearing capacity under the crane's wheel loads. A rail-mounted gantry relies on level, stable track; deflection changes geometry and can lead to derailment or overturning.
- Suspended load over people. Two workers are in the trench under/near a 6 t panel. A dropped load or trench collapse would be fatal.
- Trench instability. Near-vertical banks with slumping are a collapse risk; vibration and surcharge from the crane and panel add load to the trench edge.
- Surcharge and set-back. The crane's track is presumably close to the trench; the crane and panel weight reduce the factor of safety of the trench wall.
- Time pressure. End-of-shift urgency pushes toward skipping checks.
Risk is high: severity is fatal/major, likelihood is elevated by the observed settlement and slumping. The lift should stop until the ground and trench are reassessed.
2. What immediate actions should the operator take, and who should authorise any restart?
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Immediate actions:
- Stop and lower safely. Land the panel on stable ground outside the trench zone, not in the trench. Do not leave it suspended.
- Clear the trench. Remove the two workers from the trench and establish an exclusion zone around the crane track and trench edge.
- Isolate. Park the crane, apply rail clamps/brakes, and isolate power so it cannot be moved while the ground is assessed.
- Report. Notify the supervisor and the person with management or control of the site; record the event.
Restart authority:
- A competent geotechnical or civil person must assess the fill and rail bearing, and a competent person must inspect the crane and track for damage or misalignment.
- The trench must be reassessed (e.g. by a competent person) and, if people must enter, supported or battered to an approved system.
- The lift plan must be revised with a verified bearing surface (e.g. engineered matting/steel plates, improved drainage) and a revised set-back from the trench.
- Only the site's responsible person should authorise restart after those reviews, not the shift supervisor alone. The 20-minute deadline is not a reason to proceed.