Outrigger Mat and Chart Margin at an Excavation Edge
A 12,000 lb equipment skid must be set 18 ft beyond the edge of a 12 ft deep excavation. The crane is positioned with outriggers on compacted fill; the calculated maximum outrigger reaction is 18,000 lb. Each outrigger float is 24 in by 24 in. The geotechnical report gives an allowable bearing pressure of 2,000 psf for the fill, but the site supervisor wants to use 4 ft by 4 ft timber mats. The load chart at the planned 35 ft radius is 15,000 lb, and rigging weighs 1,500 lb. Wind is 12 mph. The pick must be made over the excavation because the access road is blocked.
1. Does the planned lift fit the load chart? Show the comparison and state any caveat.
Read the worked answer
Net load = 12,000 lb + 1,500 lb = 13,500 lb.
13,500 / 15,000 = 0.90, or 90% of the chart value.
It fits numerically, but only 1,500 lb of margin remains. Before proceeding, confirm the 15,000 lb value is for the actual boom length, radius, quadrant, outrigger configuration, and is not a gross rating requiring deductions. At 12 mph wind, verify the chart wind limit and the load shape. Treat this as a tight lift: no personnel under the load, no shock loading, and stop if the radius increases.
2. Calculate the bearing pressure with and without the proposed mat and decide whether the 4 ft by 4 ft mat is adequate.
Read the worked answer
Float area = 24 in × 24 in = 576 in² = 4 ft².
Without mat: 18,000 lb / 4 ft² = 4,500 psf, which exceeds 2,000 psf.
Required area = 18,000 / 2,000 = 9 ft².
A 4 ft × 4 ft mat = 16 ft², so pressure = 18,000 / 16 = 1,125 psf, below 2,000 psf. The mat is adequate only if it is centered under the float, bears on uniform compacted fill, and is structurally strong enough not to punch through or bend. Verify mat condition and keep the outrigger fully extended; otherwise reposition or use a larger crane.
3. List the planning controls you would require before the pick.
Read the worked answer
- Barricade the excavation edge and keep personnel out of the swing path.
- Use properly sized, structural mats under every outrigger; verify level and uniform bearing.
- Confirm the load chart for the exact boom length, radius, quadrant, and outrigger setup.
- Control the load with tag lines; do not let anyone stand between the load and the excavation.
- Establish a single signal person and a stop command understood by the operator.
- Monitor wind and radius; stop if the load drifts outward or the outrigger settles.
- Consider repositioning the crane to reduce radius and avoid lifting directly over the excavation.