Outrigger Setup on a Suspected Fill Slope
You are setting up a 25 t non-slewing pick-and-carry crane on a compacted site beside a new warehouse. The lift plan calls for full outrigger extension on the north side, but the ground there is a 1.2 m deep band of loose fill over a stormwater trench, showing recent tyre ruts and a slight depression after overnight rain. The outrigger pad is 500 mm square. The load chart for the planned 12 m radius with full outriggers allows 4.1 t, and the load weighs 3.6 t. The site supervisor says the area was compacted last week and asks you to proceed because the concrete truck is waiting. You have dunnage, extra pads, and a ground-bearing assessment is not on file.
1. What should you do before any outrigger is deployed, and why does the chart capacity alone not settle the question?
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Stop and treat the setup as unverified. The load chart assumes the crane stands on ground able to carry the outrigger reactions, so a 4.1 t capacity at 12 m is conditional, not absolute. Estimate the reaction: with the load at 3.6 t and the crane's own weight carried through the outriggers, the north-side pads could see well over half the total load, and a 500 mm pad concentrates that force into a small area. Loose fill over a trench can settle or shear, letting the crane tilt and increasing the radius, which further reduces capacity.
Before deployment: get a competent person to assess bearing capacity or obtain geotechnical advice; probe or excavate to confirm the trench location and fill depth; reposition the crane so no outrigger sits over the trench; use large spreader pads or crane mats sized for the calculated reaction; and re-check level after pre-loading each outrigger. If the ground cannot be verified, relocate or use a different lifting method. The waiting truck is not a reason to accept an unassessed foundation.
2. The supervisor produces a photo of a plate compactor on the fill and says that proves it is fine. How do you respond, and what records matter?
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A photo of a plate compactor shows only surface compaction of the top layer; it says nothing about the trench below, the fill's depth, moisture content, or its ability to carry a concentrated outrigger reaction. Compaction of a thin crust over a void can even hide the problem. I would explain that the assessment must address the actual failure mode, which is punching or slope failure beneath the pad, not surface firmness.
Records that matter: the ground-bearing or geotechnical assessment, the crane's outrigger reaction figures from the manufacturer, the pad size and mat design, the setup inspection and level check, and the name of the competent person who signed off. I would record the decision to pause, note the trench location on the lift plan, and only proceed once the foundation is verified and the setup re-inspected. If verification is not possible, I would relocate the crane or change the lift method and document why.