Crawler Crane on Fresh Trench Backfill
A 90-ton crawler crane is being set up for a 22,000 lb pick at a 70 ft radius. The crane is on timber mats over a 4 ft deep trench that was backfilled three days earlier. The operator's level shows the crane 1.2 degrees out of level side-to-side, and one track has settled about 2 inches into the mats. The load chart states that capacities are based on the crane being level within 0.5 degree and supported on firm, drained, compacted soil. The planned lift is at 78 percent of the chart capacity. The site foreman wants to 'just block up the low side with a couple of 6x6s' and proceed because the concrete truck is waiting and the backfill 'looks solid.' No soil compaction records or geotechnical report are available. The crane's boom is 120 ft long.
1. Should the crane be operated? Explain the controlling factors.
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No. The crane is outside the manufacturer's level tolerance: 1.2 degrees versus the 0.5 degree limit stated in the load chart. When the crane is out of level, the load chart no longer represents the crane's capacity, and the boom, swing, and track loads are redistributed. The 78 percent figure is therefore meaningless as a safety margin.
The track settlement is also a warning. Two inches of settlement over fresh backfill suggests the subsurface is yielding, not that the mats need a small shim. The backfill is only three days old, no compaction records exist, and the trench is 4 ft deep. A 90-ton crane at a 70 ft radius imposes large ground bearing pressures. The combination of out-of-level condition and uncertain support means the setup is not stable enough for the lift.
2. Evaluate the foreman's proposal to block the low side with 6x6s.
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The proposal treats a symptom, not the cause. Blocking the low track might make the bubble look better, but it does not verify that the backfill can carry the crane, and it can create a hard point or a void under the track. The crane could settle further or shift as the load swings. The 6x6s are also unlikely to be sized for the point loads involved.
A proper fix requires evaluating the ground support: remove or improve the unsuitable backfill, compact it in controlled lifts, verify compaction, use engineered crane mats or a designed foundation, and then re-level the crane within the manufacturer's tolerance. A qualified person or geotechnical engineer should determine the bearing capacity. The crane should not be operated until the support and level conditions meet the load chart requirements.
3. Outline the setup checks that should be completed before this lift.
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Before setup, the planner should verify the load weight and radius, confirm the crane configuration and load chart, and check the ground for voids, trenches, utilities, and recent backfill. Ground bearing capacity should be documented, not guessed from appearance. Mats should be sized and placed to distribute track loads, with full support under the tracks and no bridging over soft spots.
After setup, a qualified person should confirm the crane is level within the manufacturer's tolerance in both directions, check track tension and structural pins, and verify the boom and rigging. The operator should recheck level after the first load is applied and monitor for settlement. The lift plan should include a stop condition for any new settlement or out-of-level reading. Only then should the 22,000 lb pick be considered.