Siting the truck beside a trench on soft fill
You arrive at a residential site to unload roof trusses with a 12 tonne-metre vehicle loading crane mounted on a 4-axle rigid truck. The only access is a strip of recently placed fill, and the spot the builder has marked for you is 2.5 m from the edge of a 1.8 m deep footing trench. The crane's load chart for the planned 6 m radius and boom length gives a capacity of 1 850 kg with the outriggers fully extended. The truss packs weigh 1 400 kg. The truck's plate shows a gross vehicle mass of 26 t and a tare of 15 t. The builder says the fill has been there 'a couple of weeks' and that a concrete truck used the strip yesterday without problems. It rained heavily overnight. You notice the outrigger pads supplied are 300 mm square steel plates, and the ground under the marked position shows tyre ruts about 150 mm deep.
1. Before extending the outriggers, what must you resolve about the ground, and how does the trench affect your set-up decision?
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Two separate problems must be resolved: bearing capacity and edge stability.
Bearing capacity. Fresh fill, saturated by overnight rain, with 150 mm ruts from a heavy vehicle, is not evidence of adequate support. Ruts show the surface has already failed under wheel loads, and outrigger loads are far more concentrated. The 300 mm pads give only 0.09 m² of area; a single outrigger carrying several tonnes would impose well over 100 kPa on that area. I would not accept the builder's assurance: the concrete truck's loads are distributed over tyres, not point-loaded through pads at a set radius. I would require the fill to be assessed or compacted and proven, and use purpose-made pads of at least 0.3 m² each, or crane mats, spread over undisturbed material.
Edge stability. The trench edge is a slope failure hazard. A 1.8 m deep trench in fill has a zone of influence well beyond the lip, and an outrigger 2.5 m back may sit within the wedge that can move. I would not set up with an outrigger load that close to an unsupported edge; I would relocate so outriggers bear on stable, competent ground well clear of the excavation, or have the trench shored/backfilled and the position engineered.
Conclusion. The load is 1 400 kg against 1 850 kg chart capacity - only 76% - but that margin is meaningless if the ground fails. I would stop, relocate or remediate, and re-check the chart for the new radius before lifting.
2. The builder pressures you to lift because the concrete truck 'proved' the ground is fine. Explain how you would handle this, including what you would record.
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I would decline the lift and explain the reasoning in plain terms: the concrete truck proves nothing about point loads from outriggers, and the overnight rain changed the ground conditions after that truck left. A 1 400 kg load at 6 m is within the chart, but the chart assumes a stable, level, adequately supporting surface - that assumption is what has failed, not the crane's capacity.
I would offer practical alternatives rather than a flat refusal: move the truck to a position where outriggers land on competent ground and re-check capacity at the new radius; bring in crane mats or engineered pads; or have the fill compacted and the trench area made good, then reassess. If the load cannot be placed safely from any accessible position, the lift waits.
I would record the decision in the site diary or lift plan: date, time, location, ground description (fill, saturated, 150 mm ruts), trench depth and offset, pad size, planned radius and load, the reason for stopping, who was informed, and the action agreed. I would notify my supervisor and the site controller. Standing firm and documenting is the correct outcome; proceeding under pressure and hoping the ground holds is how outrigger penetration and tip-over incidents occur.