Outrigger on a Septic Tank and a Cracked Float
A boom truck is set on a newly paved driveway to set a 4,500 lb sign. The operator extends outriggers fully and places 16 in by 16 in steel floats under each. The right rear float sits over the edge of a concrete septic tank; the left rear float is on undisturbed soil. The bubble level shows the truck is 2 degrees out of level side-to-side; the operator says the outriggers will settle in. The driveway slopes 4 percent toward the street. The crane's estimated right rear outrigger reaction with this pick is 18,000 lb. The soil report says the fill over the tank and the native soil can be assumed to have 2,000 psf bearing capacity, but the tank itself is not rated for crane loads. A crack is visible in the right rear outrigger pad. The operator plans to lift and then use the outrigger controls to level the truck while the load is suspended.
1. Calculate the bearing pressure on the 16 in by 16 in float and compare it with the stated soil capacity.
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The float area is 16 x 16 = 256 square inches, which is 256/144 = 1.78 square feet. The average pressure is 18,000 lb / 1.78 sq ft = about 10,125 psf. That is roughly five times the stated 2,000 psf bearing capacity, so the float is far too small even on competent soil. To stay at 2,000 psf, the required area would be 18,000/2,000 = 9 sq ft, or about a 3 ft by 3 ft mat, and that is only for uniform bearing; eccentric loading and the septic tank make the actual condition worse. The tank is not rated for crane loads, so a mat over it is not an acceptable fix without an engineered bridge. The cracked float is also a defect and should not be used. The crane must be repositioned or a larger load-spreading system designed before the lift.
2. What should be done about the septic tank and the out-of-level condition?
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No outrigger should bear on the septic tank. The crane must be moved so all floats are on competent, verified ground, or an engineered bridge must be installed with the tank owner's and a qualified engineer's approval. The 2-degree side-to-side out-of-level condition must be corrected before lifting; that is not something to fix by letting the outriggers settle. The manufacturer's tolerated slope must be checked, and the truck should be leveled with proper cribbing or repositioned so the outriggers can be fully extended and loaded evenly. Using the outrigger controls to level the truck while a load is suspended is unsafe because it changes the geometry and stability under load. The cracked pad must be replaced or removed from service, and the outrigger beams must be checked for full extension and pin engagement.
3. During the lift, the left front outrigger sinks 2 inches and the load swings. What immediate actions should be taken?
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The operator should stop the lift immediately, hold the load, and lower it to a safe location if that can be done without further movement toward people or the drop-off. Continuing the swing while an outrigger is sinking changes the level, radius, and stability, and the chart no longer represents the setup. Do not send anyone under the suspended load to place cribbing or to chase the outrigger. Once the load is grounded and the area is secured, the outrigger can be relieved, the ground investigated, and the crane repositioned on proper mats or a different location. The outrigger, beam, pad, and frame should be inspected for damage before reuse. The lift plan and chart must be rechecked because the new position will probably change the radius. If the load cannot be lowered, the area should be evacuated and a competent person should direct the recovery.