Lifting a Manhole Ring with the Bucket
You need to place a 600 kg precast concrete manhole ring into a 1.5 m deep excavation. The site has no dedicated lifting attachment, but the excavator has a lifting eye on the bucket and a certified chain sling in the site store. The machine is a 21-tonne 360° on outriggers, positioned 3 m from the excavation edge on compacted fill. The load radius is 5.5 m at the bucket eye. The load chart for the machine at that radius and configuration shows a rated capacity of 3,200 kg. The ground beneath the outriggers is firm but one outrigger sits over a backfilled service trench. The banksman is available, and the excavation is 1.5 m deep with vertical sides in firm clay.
1. Assess whether this lift is acceptable and identify the controls required before lifting.
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Assessment: The lift is within the machine's rated capacity at 5.5 m (3,200 kg vs 600 kg load - about 19% of capacity), which is comfortable. However, capacity is only one factor.
Controls required:
- Lifting attachment - a bucket lifting eye is not a rated lifting point unless the manufacturer certifies it and the bucket is designed for it. Confirm the eye's SWL and that the bucket/quick-hitch is rated for lifting. If not, use a certified lifting attachment or a machine with a proper lifting point.
- Lifting plan - a lift of this type needs a written plan (load weight, radius, capacity, ground, personnel, exclusion zone). The chain sling must be certified, in date, and correctly rated for the load and configuration.
- Ground bearing - one outrigger over a backfilled service trench is a settlement/tipping risk. The chart assumes firm, level ground. Either move the machine, or excavate and properly compact the backfill, or spread the load with a properly sized pad. Re-check the outrigger after loading.
- Exclusion zone - no one in the slew area or under the load; the banksman controls the area and signals.
- Slew and travel - no travel with a suspended load unless the machine and plan permit it; keep the load low and slew slowly.
- Excavation edge - keep the machine back from the edge (3 m is marginal for a 1.5 m vertical cut in firm clay); check for surcharge loading and edge stability.
2. During the lift the near-side outrigger sinks 100 mm. What do you do and why?
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Stop the lift immediately. Lower the load to the ground under control - do not slew or retract with the load suspended.
- Do not try to 'chase' the sinkage by extending the outrigger or adding packing under load - this can cause a sudden tip.
- Once the load is grounded and the machine is stable, retract the boom, shut down, and reassess.
- Investigate - the outrigger is over a backfilled service trench; the backfill has consolidated under load. The ground bearing capacity is not what the chart assumed.
- Remedy - relocate the machine to sound ground, or excavate and re-compact the backfill, or use a properly designed load-spreading pad sized for the outrigger load. Re-level and re-check all outriggers before recommencing.
- Re-plan - the lift plan must be revised to reflect the actual ground conditions; consider a smaller radius or a different lifting arrangement.
Why: A sinking outrigger means the machine's stability envelope is changing in real time. Continuing the lift risks a tip-over with a suspended load, which is a fatality scenario. The rated capacity chart is only valid for the ground conditions it assumes.