Ground Bearing and Rigging Check Beside an Unshored Excavation
A 90 t crawler crane will lift a 4.6 t generator from a delivery truck and set it on a foundation 9 m from the track edge. The crane weighs 76 t with counterweights; the tracks sit on compacted fill soaked by rain. Each track is 6.0 m long and 0.9 m wide. The geotechnical report gives an allowable bearing pressure of 120 kPa for the fill, and the lift plan applies a 1.25 dynamic factor. The foundation excavation is 10 m from the crane centre and 2.8 m deep with near-vertical, unshored sides. The rigger plans two 2.5 t single-leg nylon slings choked at 45° from horizontal, no tag lines, and two workers to guide the load by hand at the foundation edge. The swing path passes within 4 m of a 13.8 kV overhead line. The only flat area is 12 m from the excavation edge, and the site is congested.
1. Calculate the average ground bearing pressure with the load and 0.2 t of rigging, applying the 1.25 dynamic factor. Is the fill acceptable? What else must be checked before setup?
Read the worked answer
Total vertical load = 76 + 4.6 + 0.2 = 80.8 t. 80.8 t x 9.81 = 792.6 kN. Track contact area = 2 x 6.0 x 0.9 = 10.8 m2. Average pressure = 792.6 / 10.8 = 73.4 kPa. With 1.25 dynamic factor = 91.7 kPa, below the 120 kPa allowable.
That does not make the setup automatically acceptable. The calculation is an average; track pressure is higher under the idler and drive end and during swing, and the fill is soaked. Use crane mats or cribbing sized to spread the load, and have a competent person check the fill, drainage, and any voids. Also check level, track alignment, and the excavation: the crane centre is 10 m from a 2.8 m unshored cut, so the track edge is about 7 m away; confirm the excavation is stable and barricaded. The geotechnical allowable should be verified for the wet condition, not just the report's dry value.
2. Check the sling plan. What is the tension per leg at 45°, and what is wrong with the exclusion zone?
Read the worked answer
Use the load plus rigging: W = 4.6 + 0.2 = 4.8 t. For two legs at 45°, tension per leg = W / (2 sin 45°) = 4.8 / 1.414 = 3.39 t. Each sling is rated 2.5 t, so each leg is overloaded by about 36%. A choked hitch also reduces the sling's vertical capacity; check the manufacturer's choke factor, which makes it worse.
The rigging plan is therefore not acceptable. Use slings with a WLL above 3.4 t at the actual angle, or use four legs or reconfigure so the leg tension is within the WLL. The exclusion zone is also wrong: two workers guiding the load by hand puts them under or near the load and in a pinch zone. Barricade the set-down area, use tag lines from outside the radius, and keep hands off the load. The load must not be suspended over anyone.
3. What controls are required for the 13.8 kV line and the unshored excavation before the lift?
Read the worked answer
Overhead line: 4 m from the swing path is close for a 13.8 kV line because the load and boom can sway. Verify the utility owner's minimum approach distance and the company lift plan. Preferred controls: de-energize and isolate the line, or relocate the crane or lift so no part of the crane, load, or tag line can approach the line. If the line stays energized, use goalposts, a warning line, a dedicated spotter, and a hard stop before the limit; do not rely on the operator alone.
Excavation: the unshored 2.8 m cut is a cave-in and ground-loss hazard. Have a competent person inspect it, barricade the edge, and keep the crane, mats, and load outside the influence zone. If the crane must sit near the edge, use shoring or benching and a geotechnical check. Do not place mats bridging the excavation. No lift until both controls are in place.