Outriggers Over a Fresh Trench, Boom Near a 25 kV Line
You are setting a 9,500 lb air-handling unit on a one-story roof with a 40-ton truck crane. The only setup area is a gravel lot over a utility trench backfilled three weeks ago; site drawings show the trench is 6 ft wide and its near edge is 14 ft from where the crane's center of rotation will be. Each outrigger float is 24 in. square and you plan 4 ft × 4 ft timber mats. The chart for this configuration gives a rear outrigger reaction of 34,000 lb. No soil borings exist. A 25 kV overhead line runs parallel to the building, 22 ft horizontally from the crane center. Your working radius is 30 ft. Wind is 15 mph.
1. Check the ground. Will the planned mats work, and what does the trench position tell you?
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Bearing pressure with the planned mat. 4 ft × 4 ft = 16 sq ft. 34,000 lb ÷ 16 sq ft = 2,125 lb per sq ft (psf).
That is at the top of what anyone would credit to undocumented fill, and there is no soil information to justify even that. Backfill three weeks old has not consolidated; a trench backfill can settle under load and can also fail by punching through the trench walls. Prudent allowable values for unknown fill with a crane on it are commonly taken at roughly 1,000-2,000 psf, and a job like this should assume the low end without a geotechnical report.
Sizing up. To get to about 1,000 psf I need about 34 sq ft per float, for example 6 ft × 6 ft mats (36 sq ft → 944 psf). But mat size is not the whole answer.
The trench is the real problem. The rear outrigger sits roughly 12-14 ft from the crane center - directly on or adjacent to the trench edge. A mat cannot bridge a 6 ft void, and the load spreads at roughly 45 degrees into the soil below, so the pressure bulb will reach the trench even if the float does not. The correct fix is to move the crane so no outrigger is within a distance equal to the trench depth or more from the edge, or to have the trench compacted and verified, or to bridge it with engineered mats or a steel plate designed for the outrigger load.
Verdict: the planned setup is not acceptable. Relocate or get geotechnical verification and designed mats; do not rely on 'it looked solid.'
2. Check the power line. Does a 30 ft working radius clear a line 22 ft from the crane center, and what are your options?
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Geometry. The boom tip and the load travel on a circle of radius 30 ft around the crane center. A vertical line whose ground point is 22 ft from the center is approached to within 30 − 22 = 8 ft by that circle. So the closest approach during a swing is about 8 ft horizontally, not 22 ft. With the line at height and the boom elevated, the actual three-dimensional clearance can be somewhat more, but I cannot count on that, and the load, the tag line, or a swung load's overhang closes it further.
Standard. For lines up to 50 kV the minimum approach distance is 10 ft, and 8 ft is inside it. Voltage is also only as good as the utility's word; a 25 kV line may be a 34.5 kV circuit in practice.
Options, in order of preference:
- Have the utility de-energize and visibly ground the line for the duration, with the utility's own confirmation.
- Relocate the crane so the swing path stays outside 10 ft plus a margin - here that means moving the center to at least about 40 ft from the line if the working radius stays at 30 ft, or reducing the working radius.
- If neither is possible, a documented plan with encroachment prevention: a dedicated spotter whose only job is the line, a range limiter or physical barrier (flagged rope or rigid barricade) at the minimum distance, and non-conductive tag lines. This is a plan, not a verbal 'watch the wires.'
I would also confirm the line's owner and voltage in writing, call 811 for the undergrounds feeding the building, and treat the 15 mph wind as a reason for more clearance, not less.
3. Write the go/no-go for the lift, including who decides and what has to be in place.
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No-go as planned. Two independent disqualifiers: outrigger bearing on unverified backfill adjacent to a 6 ft trench, and a swing path that closes to about 8 ft on a 25 kV line against a 10 ft minimum approach distance.
What has to be true before I would set this unit:
- Crane relocated, or the trench area verified by a geotechnical opinion with mats sized for the actual outrigger reactions (about 34,000 lb on the governing float) to a defensible allowable pressure.
- Line de-energized and grounded by the utility, or a swing path that stays outside the minimum approach distance with a physical barrier or range control and a dedicated line spotter.
- A written lift plan for a 9,500 lb pick at 30 ft radius, with the crane's chart attached, the setup position marked, and the wind limit stated.
- Cribbing under the floats, outriggers fully extended on the correct side of the chart, and a level check of the carrier.
- The site's competent person and the utility's representative aware, and the plan communicated in a pre-lift meeting.
Who decides: the operator decides whether the crane is set up and operated safely, and stops the lift if it is not. The superintendent decides schedule. Those roles do not overlap, and if the schedule pressure continues I document the refusal in writing and escalate to the general contractor and my employer.