Outrigger Pad on Filled Ground
A self-erecting tower crane is being positioned on a residential site where an old swimming pool was backfilled two years ago. The ground is a mix of clay and loose fill. The crane's manual gives a maximum outrigger load of 95 kN per leg. The rigger proposes using 600 mm × 600 mm steel pads on the firm-looking surface. A geotechnical report for the block is available but only covers the original soil, not the fill. The crane will sit 3 m from the edge of the excavation for the new house, and the site foreman wants to start lifting the same afternoon. It has rained heavily for two days. The crane's base is a four-point outrigger system with hydraulic legs, and the ground under one leg already shows a slight depression after the crane was levelled.
1. Calculate the bearing pressure under the proposed pads and judge whether the setup is acceptable.
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Bearing pressure equals load divided by pad area. A 600 mm × 600 mm pad has an area of 0.36 m². At 95 kN per leg, pressure is 95 / 0.36 = 264 kPa. That is a high bearing pressure for loose fill and saturated clay, which may have allowable capacities well below that, often in the range of 50–150 kPa depending on compaction and moisture. The calculation also assumes the load is spread uniformly over the full pad, which is optimistic if the pad is not fully bearing or if the ground is uneven. The depression under one leg is direct evidence that the ground is yielding, and the crane is no longer level. The proposal should be rejected. Options include larger pads or crane mats to reduce pressure, a designed foundation, or relocation to ground verified by a geotechnical assessment that includes the fill. The 3 m distance from the excavation edge is also a concern because the excavation reduces lateral support and the fill may be weakest there. The foreman's afternoon start is not achievable safely.
2. Describe the checks and controls you would require before the crane is used.
Read the worked answer
First, obtain a geotechnical assessment of the actual ground under each outrigger, including the fill and the zone near the excavation. Second, calculate the required pad size from the assessed allowable bearing pressure with a suitable factor of safety, and specify mats or a designed base. Third, ensure the crane is set up level, with all outriggers fully extended on stable bearing, and re-check level after loading. Fourth, keep the crane far enough from the excavation edge, or provide shoring or a designed crane base, because the edge reduces support. Fifth, inspect after rain and after any sign of movement, and stop work if the crane goes out of level or a pad sinks. Sixth, record the setup, the assessment and the pad design, and have the setup checked by a competent person before lifting. The depression already observed should be investigated and the leg re-supported before any further load is applied.