Outrigger Reaction Over Fill Next to an Open Trench
A 110-ton hydraulic truck crane is set on ground that was built up with 4 ft of compacted structural fill over natural clay. The geotechnical report gives an allowable bearing pressure of 2,000 psf and notes the fill was never engineered for concentrated loads. For the planned pick - a 26,000 lb air-handling unit at a 28 ft radius with 95 ft of main boom, outriggers fully extended - the manufacturer's outrigger reaction chart shows 62,000 lb on the right rear float. The float pad is 24 in by 24 in. That outrigger sits 6 ft from the edge of an unshored, 9 ft deep utility trench excavated yesterday and scheduled to stay open another week. Four 6 ft by 6 ft timber crane mats are available, and the crew proposes to set one under the float. A one-call locate has not been completed. The crane is level as it sits and the operator is in the cab waiting for your go-ahead.
1. Compute the bearing pressure under the right rear outrigger with the float pad alone and with the proposed 6 ft by 6 ft mat. Does the mat make the setup acceptable?
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Float pad alone: 24 in x 24 in = 4.0 sq ft. 62,000 lb / 4.0 sq ft = 15,500 psf, roughly 7.75 times the 2,000 psf allowable. That is an immediate reject.
With the 6 ft x 6 ft mat: 36 sq ft. 62,000 / 36 = 1,722 psf, or about 86 percent of allowable. On paper it passes, but with almost no margin and with three unverified assumptions baked in: that the mat spreads load over its full area, that the fill is uniform under the whole mat, and that the mat itself can carry a 62,000 lb point load without crushing or bending.
So the answer is no. The bearing check is only one of several failure modes, and the one the arithmetic ignores is the worst: the outrigger is 6 ft from the toe of a 9 ft unshored excavation. Lateral support on that side is gone, and the failure surface from a heavily loaded float can daylight into the trench. A passing bearing number does not fix an edge-stability problem, and it certainly does not clear the missing utility locate.
2. If the crane cannot be relocated, what has to be true before you sign the lift plan, and who has to be involved?
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You need a documented engineering basis, not a crew judgement. In sequence:
- Complete the one-call locate and pothole any utilities under or near the mat path. A mat bridging a void turns a bearing problem into a punch-through.
- Establish the required setback from the trench. Common practice is to keep the outrigger outside an influence line drawn up from the toe - at least the trench depth, and 1.5 times depth (13.5 ft here) where the excavation is unshored. If the setback cannot be met, the trench must be sloped, benched, or shored to a design by a registered engineer.
- Have the geotechnical engineer confirm that the fill still provides the assumed bearing capacity this close to an unsupported edge, and state the reduced value if it does not.
- Have a qualified person design the load-spreading mat or crib for a 62,000 lb point load, including mat thickness, and verify the mat does not create an unacceptable slope.
- Re-verify the 62,000 lb reaction against the final radius and configuration, re-check level in both axes after matting, and revise the lift plan. The lift director and the site competent person both sign; the engineer signs the ground and mat design.