Outrigger on Fill
You are setting up a 20 t slewing mobile crane to lift air-conditioning plant onto a roof. The site is a former car park: 150 mm of compacted crushed rock over what the site plan calls 'controlled fill', with a stormwater line shown running diagonally beneath the hardstand. Your outrigger spread is 6.5 m by 6.5 m. The lift radius is 9 m and the load is 2.8 t, well inside the chart. The site foreman says the fill has been there 15 years and 'nothing has ever moved'. The stormwater line is 900 mm below surface and the plan does not give its diameter. There is a light drizzle and the ground is softening at the surface.
1. Identify the ground-condition hazards and rank them by consequence.
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The dominant hazard is outrigger punch-through or differential settlement over the buried stormwater line. A 900 mm deep pipe beneath a pad concentrates load on a void; if the pipe crushes, one outrigger drops, the crane lists, and the load moment changes instantly - potentially beyond the chart with the load in the air. Second is general bearing failure of the fill: 15 years of age is not proof of compaction, and the plan does not state the fill's engineering properties or its bearing capacity. Third is surface softening from the drizzle, which reduces the friction and bearing of the pad and can let it walk or tilt. Fourth is the unknown pipe diameter - a large-diameter pipe is a bigger void, a small one may be a lesser risk but still a void. Ranked by consequence: pipe collapse with load suspended (dropped load, crane overturn, injury) > bearing failure of fill > pad settlement from softening.
2. What would you do before and during setup to manage these hazards?
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Before setup: obtain the pipe's diameter, material, and cover from the asset owner or a survey; if it cannot be confirmed, treat the zone as a void. Use load-spreading mats or steel plates sized so the outrigger pressure on the fill is within its allowable bearing pressure - calculate pad pressure from the crane's outrigger reaction for this configuration and radius, not from the load alone. If the pipe lies within the load-spread zone, relocate the crane so no outrigger is over it, or bridge it with a designed spreader approved by an engineer. During setup: extend outriggers fully, level the crane with the bubble, and check that all pads bear evenly - a pad that takes up early or stays loose signals uneven ground. Re-check levels after the first lift and after any rain. Keep the load low and the slew slow until you are satisfied the ground is stable.
3. Halfway through the lift the drizzle becomes steady rain and you notice one outrigger pad has sunk about 40 mm. What now?
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Stop the lift. Lower the load to the ground if you can do so without slewing over the affected side; if the load is already over the roof, hold position and get the load down at the nearest safe point before doing anything else. Do not retract or adjust that outrigger with a load on the hook. Once the load is grounded, take the load off the crane, then re-level: the pad must be re-packed and re-matting reviewed, because 40 mm of settlement in soft ground under rain usually means more is coming. Re-check all four pads, re-level, and re-verify that the crane is within the chart at the actual radius. If the ground cannot be made stable in the rain, suspend the lift. Document the settlement and the rainfall - the lift plan's assumptions about ground bearing have been shown to be wrong and need review before the next lift.