Load Chart Limits on a Slew-Restricted Site
You are operating a 22-tonne hydraulic excavator fitted with a 1.2 m³ bucket and a standard 5.7 m boom / 2.9 m stick. The site is a basement dig inside a shored excavation 18 m wide. The spoil stockpile truck bay sits 9.5 m from the slew centre, and the manufacturer's load chart for that radius at ground level allows 4.8 t with the bucket, or 3.1 t if you fit the ripper attachment. Your excavator's mass is 22.4 t and the chart is based on a firm, level surface. The ground beside the tracking path has been softened by a leaking dewatering line; a plate test shows the bearing capacity has dropped noticeably. The supervisor wants you to lift a 3.4 t precast pit collar from the truck and place it in the trench at 9.5 m radius, using the ripper attachment because the bucket is too wide for the trench. It is 14:20 and the concrete truck arrives at 15:00.
1. Using the load chart figures and site conditions, decide whether the lift can proceed and explain your reasoning.
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The lift cannot proceed as described. The chart allows 3.1 t at 9.5 m with the ripper, but the load is 3.4 t - already over the rated capacity by 0.3 t before any derating. Two further factors make it worse. First, the chart assumes a firm, level surface; the softened ground beside the tracking path reduces bearing capacity, so the machine could settle or tilt under load, which invalidates the chart entirely. Second, a precast collar lifted from a truck involves dynamic effects - the load is not static, and any swing or snatch adds to the effective radius and load. The correct judgement is to stop, not to try a slower lift. Options: reposition the truck closer to reduce radius (checking the chart at the new radius), use a larger machine, or wait for the ground to be remediated and re-plated. Document the refusal and the reason.
2. The supervisor says "just lift it slowly and keep the boom in". Explain why this instruction is unsafe and what you should do.
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Lifting slowly does not change the physics. The load chart is a structural and stability limit, not a speed limit. Keeping the boom in reduces radius but the collar must still be placed at 9.5 m - you cannot shorten the reach without moving the machine or the truck. Slow lifting also does not fix the soft ground, which is the more serious issue: if a track sinks, the machine tilts, the radius increases, and the load moment rises sharply - potentially causing a tip before the operator can react. The correct action is to refuse the lift, explain the chart figure (3.1 t) versus the load (3.4 t), and the ground condition. Offer alternatives: move the truck to a shorter radius, bring in a bigger excavator, or remediate the ground and re-test. Record the refusal in the site diary and notify the supervisor in writing.