Site Preparation, Ground Conditions and Outrigger Set-up
Ground bearing is the hidden failure
Most crane collapses on large machines trace to ground or set-up failure. You need to know the maximum outrigger or track load, which the manufacturer publishes, and compare it with the ground's bearing capacity. Soft fill, recently excavated trenches, service pits, drains, basements and uncompacted fill are classic traps. Obtain geotechnical advice where bearing capacity is uncertain.
Spread the load
Outrigger pads or crane mats increase the contact area and reduce pressure. Pressure equals force divided by area. Doubling the pad area halves the pressure, but only if the pad is stiff enough to spread load evenly and the ground beneath is uniform. Check pad condition, avoid bridging across voids, and ensure pads sit flat, not on a slope or edge.
Set-up essentials
Level the crane within the manufacturer's tolerance; out-of-level changes radius and load distribution. Fully extend outriggers as specified, fit pins and locks, and confirm the outrigger beams are not obstructed. Keep clearances from excavations, trenches and embankments, allowing for the angle of repose. Provide adequate access, hardstand, drainage and lighting. Establish exclusion zones and barricades before erecting.
Traps
Questions test the assumption that "the pad is big enough" without checking the ground. Another trap is setting up near a trench with outriggers close to the edge, or on a slope where the bubble is only just inside the line. Also watch for hidden services and for the effect of rain softening the ground overnight.
Worked example
An outrigger load is 60 t on a 1.2 m by 1.2 m pad, giving 1.44 square metres and about 41.7 tonnes per square metre (roughly 409 kPa). If the ground is rated at 200 kPa, you need at least double the area, so use a 1.7 m by 1.7 m mat or better, and verify the ground.