Outrigger Set-Up on a Compacted Fill Pad
You are setting up a 250 t all-terrain crane to lift a 38 t chiller onto a new warehouse roof. The site engineer has nominated a pad of 400 mm compacted crushed rock over a former drainage trench. The crane's load chart for the required 26 m radius and 42 m boom gives 52 t capacity with full outrigger extension and 48 t with the outriggers at mid-extension. The outrigger floats are 900 mm square. The pad's edge is 3.5 m from the nearest outrigger float centre. A site worker tells you the trench was 'mostly backfilled with rubble and some clay' and that a small excavator sank about 200 mm there last winter. The chiller lift must be completed today because the roof is open and rain is forecast tonight.
1. What ground-bearing information do you need before you authorise this set-up, and how does the 3.5 m pad edge affect your decision?
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The nominated pad tells you almost nothing about bearing capacity; you need evidence, not assurances. Ask for the geotechnical report or proof-rolling records for the fill, the trench location and depth, the compaction method and test results, and the position of any services. A former trench filled with rubble and clay is a variable, low-strength zone, and the worker's report of a 200 mm sinkage under a small excavator is a red flag for voids and soft spots beneath the crust.
Calculate the load actually imposed. With 52 t on the hook plus boom and superstructure, each loaded outrigger may carry well over 20 t, and the float pressure over 900 mm square is high. The 3.5 m edge distance is less than the typical influence zone for a shallow pad; a loaded float near an edge can punch through or rotate the fill into the trench. Do not accept the pad as-is. Options: relocate the crane so all floats sit well clear of the trench (a common rule is at least the trench depth or 1.5 times the float width, whichever is greater), bridge or excavate and recompact the trench under engineering direction, or use steel mats or a purpose-built crane pad designed for the float pressures. If none can be verified before the lift, the lift does not proceed - the rain forecast is a commercial pressure, not a technical justification.
2. The supervisor suggests using mid-extension outriggers to reduce the footprint and stay further from the pad edge. Evaluate this proposal.
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Mid-extension reduces the footprint but also reduces capacity: 48 t versus 52 t at the same radius. That still exceeds the 38 t load, so on paper the lift is feasible, but the margin has shrunk and the load chart must be read for the exact configuration, including whether the chart permits the required slew arc and whether the manufacturer imposes a reduced wind or travel restriction in that mode.
More importantly, mid-extension does not solve the ground problem. The total crane weight is spread over a smaller base, so float pressures rise, and the float nearest the trench may now be closer to the edge rather than further from it - the geometry has to be redrawn, not assumed. Reducing the footprint to gain edge clearance is only valid if the resulting pressures are checked against verified bearing capacity. The correct sequence is: verify ground, select configuration from the chart, check float pressures, then confirm edge clearance. Choosing a configuration first and back-filling the justification is how outrigger punch-through incidents happen.