Foundation Settlement on a Sloped Site
A scaffold is erected on a site with a 5% slope. The base plates are placed directly on compacted gravel, but no sole boards are used. After two days of rain, the scaffold is observed to have settled on one side by 50 mm, causing the standards to be out of plumb by 1/100. The scaffold is 12 m high and is fully decked at three levels. The scaffold is tied to the building at every lift and 4 m horizontally. The competent person must assess whether the scaffold is safe for continued use and what remedial actions are needed.
1. Assess the significance of the settlement and out-of-plumb condition.
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The settlement of 50 mm on one side has caused an out-of-plumb of 1/100 (1%). Common standards (e.g., OSHA 1926.451) require that scaffolds be plumb and level; a typical tolerance is 1/4 inch per 10 feet (approximately 1/500) or as specified by the manufacturer. An out-of-plumb of 1/100 is significantly beyond acceptable limits and indicates foundation failure. The lack of sole boards allowed the base plates to sink into the saturated gravel. The scaffold is at risk of further settlement, racking, or collapse. The ties may be subjected to additional forces. The scaffold should be immediately evacuated and not used until corrected.
2. What remedial actions should be taken to make the scaffold safe?
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The competent person should: (1) immediately evacuate and barricade the scaffold; (2) unload all materials from the platforms; (3) assess the extent of settlement and check all ties and bracing for damage; (4) install proper sole boards (e.g., 50 mm thick, 200 mm wide timber) under all base plates, sized to distribute the load; (5) on the slope, cut steps or use adjustable base jacks to ensure a level foundation; (6) re-level the scaffold using screw jacks, ensuring the out-of-plumb is corrected to within tolerance (e.g., 1/500); (7) verify that all ties are secure and undamaged; (8) if any structural members are bent or damaged, replace them; (9) re-inspect the scaffold before returning it to service; (10) document the incident and remedial work. The scaffold should not be used until it is re-plumbed and stable.