Outrigger Setup Over a Backfilled Utility Trench
An 80-ton rough-terrain crane is set up in a newly graveled lot to set precast vaults. The lot sits over a 6 ft deep sanitary sewer trench backfilled eight months ago. The crane weighs 92,000 lb; the load chart lists a maximum outrigger reaction of 46,000 lb at the planned 25 ft radius. Floats are 24 in square. The outriggers are fully extended. The crew has 4 ft x 4 ft timber mats. After preloading the outrigger nearest the trench, the float sinks about 1.5 in and the gravel shows cracks running parallel to the trench. The site superintendent says the fill was compacted and points out that a loaded dump truck crossed the area yesterday. A light rain fell overnight. The operator must decide whether to set up, what cribbing to use, and what to tell the crew.
1. Calculate the ground bearing pressure under the loaded outrigger using the 4 ft x 4 ft mats, and state whether it meets a presumed allowable bearing pressure of 2,000 psf. Show your work.
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Pad area = 4 ft × 4 ft = 16 ft². Pressure = 46,000 lb ÷ 16 ft² = 2,875 lb/ft² (psf). That is 44% above the assumed 2,000 psf allowable. The 24 in × 24 in float alone would impose 46,000 ÷ 4 = 11,500 psf, so the mat is essential and must be in full contact with competent soil. To get to 2,000 psf, the effective bearing area would need to be at least 46,000 ÷ 2,000 = 23 ft², or about 4.8 ft square, before any factor of safety. The calculation is only a check: if the soil under the mat is loose backfill, the mat can bridge and concentrate load, so the actual pressure can be higher than the average.
2. What field evidence indicates the trench backfill is not capable of supporting the crane, and what should the operator do before setup?
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The 1.5 in sink under preload is settlement, not normal mat compression. Cracks parallel to the trench show a failure plane or differential movement in the backfill. Eight-month-old fill over a 6 ft trench, plus overnight rain, means the soil may be soft and compressible. A visual pass by a dump truck does not prove capacity. The operator should stop, keep the outrigger back from the trench edge, and have a competent person or geotechnical engineer evaluate the location. Options are to relocate the crane to undisturbed ground, use an engineered crane mat or steel plate sized for the actual reaction, or bridge the trench only with a designed structure. Preload each outrigger and monitor for movement before lifting.
3. The superintendent says a loaded dump truck crossed the area, so the ground is fine. How should the operator respond?
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A truck crossing is a rolling load of short duration; an outrigger is a sustained, concentrated load that also vibrates and shifts as the boom swings. The truck may have crossed a stronger spot, or the fill may have settled after it passed. The operator should not accept the analogy as proof. Document the refusal and the observed settlement and cracks, ask for the trench location and backfill records, and require verification by the competent person. If the superintendent insists, escalate to the lift director or safety manager. The operator's obligation is to not set up on ground that cannot support the load.