Outrigger Settlement Over a Buried Storm Drain
A 40 t all-terrain mobile crane is lifting a 6.2 t precast manhole ring from a lorry into a trench. The crane is configured with 8 t counterweight, a 28 m boom and a 12 m radius. The load chart gives 7.8 t capacity at that radius on fully extended outriggers with proper ground support. The near-side rear outrigger pad sits on compacted fill over a 300 mm storm drain buried about 0.8 m deep. During the lift, the rear outrigger settles about 25 mm. The chassis tilts visibly and the suspended load swings about 200 mm. The banksman shouts to continue because the load is only 1 m above the trench and two workers are guiding it by hand. The operator can see that the outrigger pad has pressed a shallow depression into the fill.
1. What are the operator's immediate actions, and why is 'only 1 m to go' the wrong reasoning?
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The operator should stop hoisting and slewing at once, hold the load, and lower it to a safe lay-down area away from the trench and the two workers. The outrigger must stay fully extended; it must not be retracted or adjusted while a load is suspended. The area around the settled outrigger should be barricaded and the lifting supervisor called. The 25 mm settlement matters because it tilts the chassis, changes the effective radius and redistributes load onto that outrigger. The 7.8 t chart value assumes a level, properly supported crane. The actual radius may now be greater than 12 m, and the outrigger load includes crane weight and counterweight, not just one quarter of 6.2 t. A buried drain is a void: the pad can crush the pipe and cause sudden collapse. Being only 1 m above the trench is more dangerous, not less, because workers are close and there is no time to react if the crane lurches.
2. How should the lift be re-established, and what should be calculated before lifting again?
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The drain should be exposed or surveyed so its position and condition are known. The crane should be relocated so no outrigger loads the drain, or the drain should be protected by an engineered solution such as a reinforced concrete slab or certified steel spreader mats sized for the outrigger load. The ground must be re-compacted and levelled, and the outrigger floats must bear fully. The actual boom length, radius and outrigger configuration should be re-measured, then the load chart and outrigger load chart checked for the new position. The rigging and load weight should be confirmed. A test lift of about 100 mm should be made, with a levelling staff or dial gauge on the chassis and outrigger to detect movement. A competent person should approve the ground support and the amended lift plan before the manhole ring is lifted again.
3. What records and checks should follow the re-setup?
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The amended lift plan should record the new crane position, radius, boom length, outrigger spread, load weight, chart capacity and the ground support design. The ground assessment and mat or slab calculation should be attached. The pre-use inspection of the crane, including outrigger beams, floats, tyres and level indicators, should be completed. The test-lift result, including any settlement reading, should be recorded. The operator, rigger and lifting supervisor should sign on, and the two workers should be briefed on the exclusion zone and hand-signal limits. If the drain was damaged, the repair should be verified before loading resumes. The original near-miss should also be reported under the site's safety system, because the first set-up was approved without checking the buried drain.