Crane Lift Beside an Excavation on Suspect Ground
At 09:30 in August, a 50 t all-terrain crane is rigged on a refinery laydown area in Kuwait. The crane has a 12 m boom, no jib, and all four outriggers fully extended on 600 mm pads. The lift is a 7.8 t skid from a trailer to a foundation 6 m away. The load chart for this configuration shows 9.6 t at the required 5 m radius. One rear outrigger sits on 300 mm of compacted fill beside a 2.5 m deep excavation; the other three are on cracked concrete. A light fuel film is visible under the crane. Ambient temperature is 49°C and the wind is 28 km/h gusting to 40 km/h, with dust reducing visibility. The load will land behind a 3 m temporary wall, so the operator cannot see it. The signaller has a hand-held radio, but its battery indicator is flashing. The rigger proposes a single choked sling and a tag line.
1. Assess the outrigger foundations and weather. Would you allow the lift to proceed, and what actions are required before any lift?
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I would not allow the lift in the current setup. The rear outrigger on 300 mm of fill beside a 2.5 m excavation is the critical defect: the fill can consolidate or shear, and the excavation edge reduces lateral support. The outrigger should be outside the excavation zone of influence, commonly at least 1.5 times the excavation depth unless a competent engineer specifies otherwise, so 2.5 m depth means roughly 3.75 m setback. The crane must be repositioned or the ground engineered with a designed mat or steel plate, not a random timber offcut.
The fuel film indicates contaminated ground and a possible hydraulic leak; both reduce friction and create a slip/fire risk. Clean it up and find the source before loading the outrigger. Cracked concrete may hide voids, so use pads and check for movement during preload.
Wind at 28 km/h gusting to 40 km/h may exceed the manufacturer's permitted wind speed for a solid skid; check the load chart and manual. If the limit is 10 m/s (36 km/h), the gusts already exceed it. Stop, lower the boom, and wait. Heat alone is not a automatic stop, but monitor hydraulic temperature and tyre condition. Decision: no lift until ground, leak, and wind are controlled.
2. The load chart shows 9.6 t at 5 m radius for the configuration, and the load is 7.8 t. Calculate utilisation and explain how radius and configuration changes affect the decision.
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Nominal utilisation is 7.8 ÷ 9.6 = 0.813, or 81.3%. That is already high for a lift beside an excavation. The 7.8 t is only the skid; the crane's gross load must include the hook block, slings, shackles and any tag-line download. If rigging adds 0.2 t, gross load is 8.0 t, giving 83.3%.
The chart value applies only at the actual radius and configuration. The radius must be measured at the boom tip with the load in the air, not estimated from the trailer position. If the actual radius is 5.5 m or 6 m, capacity falls quickly and the lift may be outside the chart. Fully extended outriggers and the stated boom length must match the chart; a different outrigger spread or boom length requires a different chart.
Even if 81% is below the site's maximum permitted utilisation, the margin does not cover ground settlement, wind, or a radius increase. I would require the actual radius to be confirmed and marked, gross load calculated, and the lift kept within the site's imposed limit. If the radius cannot be held at 5 m, re-rate the lift or use a larger crane.
3. What communication and blind-lift controls must be in place before the load goes behind the wall?
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A blind lift needs a planned communication chain, not an improvised radio call. The operator cannot see the landing point, so a dedicated signaller must be stationed where the load and the landing area are visible, with a clear view of the operator or a second relay signaller. The radio must be tested before the lift, with a charged battery and a spare. Agreed hand signals are still needed as a backup.
The pre-lift briefing must confirm: who gives signals; that anyone may give an emergency stop; that the operator stops immediately if communication is lost; the tag-line position; and the exclusion zone behind the wall. The signaller must not stand under the load or inside the swing radius. A tag line can control rotation but must not be used to drag the load.
With a flashing battery indicator, I would not start the blind lift. Replace or recharge the battery and run a radio check. If radio contact fails during the lift, the operator stops and holds, then lowers in a controlled way only under a pre-agreed contingency. Moving the crane to a position where the operator can see the landing point is a better control than relying on a marginal radio.