Pick-and-Carry Over a Backfilled Utility Trench
You are running a 90-ton crawler crane on outriggers-free crawler tracks, lifting a 14,000 lb prefabricated skid from a flatbed to a foundation pad 40 ft away. The swing path crosses a 6 ft wide, 8 ft deep utility trench backfilled three weeks ago with native spoil. The site foreman says the trench was compacted and points to fresh track ruts near the edge. Ground is firm clay with a thin topsoil layer. The crane's load chart for this radius and boom length allows 22,000 lb. You notice the near track sits about 5 ft from the trench edge and the soil there looks slightly spongy underfoot.
1. How do you evaluate whether the ground bearing under the tracks is adequate before making the pick?
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First, compare demand to capacity. The crane's gross weight plus the lifted load is carried over the track contact area, but the governing case is the near-track reaction during swing, which can approach or exceed half the total weight. With a 22,000 lb charted capacity against a 14,000 lb load you have margin in the chart, but chart margin says nothing about ground pressure.
Estimate track bearing pressure: take the crane's operating weight plus load, divide by the effective bearing area of both tracks, then apply a dynamic/factor allowance (commonly 1.2-1.5 for pick-and-carry or swinging). Compare that to what the soil can take. Three-week-old backfill over an 8 ft trench is not competent bearing material regardless of what the foreman says; native spoil backfill is typically loose to medium dense at best and the trench acts as a void the track can punch into.
Decision: do not pick from that position. Either relocate the crane so no track is within a distance at least equal to the trench depth (8 ft) from the edge, or have the trench area assessed and matted/compacted by a competent person. Spongy soil under the near track is a direct warning of settlement.
2. The foreman wants you to just swing fast to minimize time over the trench. What is your response and why?
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Refuse. Swing speed and swing dynamics increase the overturning moment and the dynamic track loading; centrifugal and deceleration forces add to the near-track reaction exactly when that track is over the weakest ground. 'Minimize time over the trench' treats a bearing failure as a timing problem when it is a static and dynamic load problem.
Also, the crane does not stop bearing on the ground when the boom swings away; the crane's own weight remains distributed over the tracks. If the near track is on a trench void, the machine can settle or tip even with the load over the far side.
The correct control is to eliminate the hazard: reposition, mat, or bridge the trench, then re-verify radius and capacity from the new position. Document the refusal and the reason.