Tower on Soft Ground After Rain
A two-lift access tower (frame scaffold, 1.8 m bays) is erected beside a new warehouse to reach a roof edge at 6.4 m. The base plates sit on 50 mm timber sole boards laid directly on compacted fill that was placed three weeks ago. Overnight rain has left standing water around the base, and the ground under the two windward legs now shows 20 mm of settlement with visible mud pumping when a worker walks past. The tower carries three workers plus hand tools, and a materials hoist is planned for the second lift. The site supervisor wants to add a third lift tomorrow to save time. Wind forecast is 45 km/h gusting.
1. Diagnose the foundation problem and decide whether the tower can remain in service while work continues.
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The evidence points to bearing failure, not cosmetic settlement. Fresh fill placed three weeks ago has not been proven compacted, standing water is softening the surface, and mud pumping under load means the soil is being displaced plastically. Differential settlement of 20 mm across the base already tilts the tower and redistributes leg loads. A tower is only as stable as its base, so the correct decision is to stop work on and around the tower, barricade it, and treat it as out of service.
Reasoning to state: sole boards spread load but cannot fix weak bearing; 50 mm timber on saturated fill is inadequate. Settlement plus pumping is a failure symptom, not a tolerance issue. Adding a third lift increases both dead load and wind sail area on a foundation already yielding, so it must be refused. Before reuse, the ground must be drained, the fill excavated or proven by assessment, and a proper base such as a designed pad or hardstanding provided, with the tower re-plumbed and re-levelled.
2. List the immediate controls you would put in place and the checks required before the tower is returned to service.
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Immediate controls: isolate and sign the area, prohibit access and loading, record the settlement with photos and measurements, notify the scaffold designer or competent person, and check whether the tower needs partial dismantling to remove load. Do not attempt to jack or pack legs under load without direction.
Before return to service: verify ground bearing capacity for the actual leg loads including wind uplift and overturning; install drainage so water cannot pond at the base; use adequate sole boards or a designed foundation; re-erect or re-level so the tower is plumb within tolerance; confirm ties, bracing, guardrails and toe boards; re-inspect and re-tag. Also re-check the wind loading for the planned height and the materials hoist, since both change the loads the foundation must carry.