Matting Decision on Engineered Fill Over a Storm Sewer
A 100-ton class crawler crane, 180,000 lb operating weight, is setting precast wall panels with 110 ft of boom. The heaviest pick is 20,000 lb at 40 ft radius plus 2,000 lb of rigging. The crane has 36 in. track shoes and 24 ft of track on ground per side. The geotechnical report allows 2,000 psf on the recently placed 4 ft of fill, and the site was proof-rolled. The operator will swing the panels 90 degrees over the right track to reach the deck. Cross-slope at the setup point measures 1.5%. A 60 in. storm sewer crosses the site about 30 ft from the crane, beneath the fill. The superintendent says the fill was tested and no mats are needed.
1. Compute the ground bearing pressure and decide whether mats are required.
Read the worked answer
Total weight on the ground = 180,000 + 20,000 + 2,000 = 202,000 lb
Track contact area = 24 ft x 3 ft (36 in.) = 72 sq ft per track, 144 sq ft total.
Average pressure = 202,000 / 144 = 1,403 psf (~9.7 psi). That looks fine against 2,000 psf - but the average is the wrong number to use.
- A crawler crane is not symmetrically loaded. Weight is biased toward the boom side, and with the panels swung 90 degrees over the right track that track momentarily carries roughly 70-75% of the total.
- 0.72 x 202,000 = 145,440 lb / 72 sq ft = ~2,020 psf on the right track - at or above the allowable, with zero margin for swing acceleration, boom deflection, or wind.
- The 2,000 psf allowable is a soil value, not a crane value; the crane's own manual may require a lower bearing pressure, and the manufacturer's floating/blocking instructions govern.
Call: require crane mats or a designed pad under both tracks, sized to spread the peak track load to well under 2,000 psf (target roughly half the allowable). Do not accept the average-pressure argument.
Second issue: a 3 ft wide track spreads load roughly 2 times its width below grade, so the stress bulb reaches 5-6 ft down - straight into the 4 ft fill and the sewer trench backfill. A proof-roll tests the surface only. Confirm the buried sewer and its backfill with the geotechnical engineer or bridge/protect it before the crane walks or sets up over it.
2. What do you do about the measured 1.5% cross-slope, and how does it affect the lift?
Read the worked answer
- Load charts assume the crane is level (usually within 1% in both directions, per the manufacturer - verify the actual number in the operator's manual). 1.5% exceeds a typical limit.
- On a crawler crane the effect is not just cosmetic: the boom is tipped out of plumb, which shifts load into the boom's weak axis, changes the true radius, and lets the swing brake drift toward the low side.
- Action: level the setup pad (cut/fill or shimmed timber mats), then re-verify with a level on the machine's level indicator, not by eye and not from the fill's design grade.
- Leveling must be done before the boom is assembled, and the level condition must hold for the whole pour/set sequence - not just at the start of the shift.
- Slope also matters separately for the travel path; a crane that is level for lifting may still not be permitted to walk across that slope with a load.
3. During the pre-lift inspection, the load moment indicator's boom angle reads 3 degrees different from a digital inclinometer placed on the boom, and the anti-two-block alarm sounds intermittently. What is your call?
Read the worked answer
Do not make the lift.
- On a lattice boom crane, the LMI computes radius from boom angle and boom length. A 3 degree angle error at 110 ft of boom is roughly 5-6 ft of radius error - the difference between two chart columns, or more, at long radius. The crane would be operating against a capacity number that does not describe reality.
- The intermittent anti-two-block alarm means the two-block protection is unreliable. That is a rated operational aid, and it cannot be bypassed, disabled, or 'worked around' by the operator. If the device prevents the lift, the crane comes out of service until it is repaired.
- Sequence: lower/remove the load, tag the crane out of service, notify the competent/qualified person, have the LMI recalibrated and function-tested per the manufacturer's procedure, document it, and re-verify boom angle against an independent instrument before the next pick.
- Do not 'add a few percent' to the chart as compensation for a known instrument error.