A mobile crane can be well inside its duty chart and still overturn if the ground under an outrigger gives way. Checking ground bearing pressure is part of every crane set-up.
Outrigger load
The highest outrigger load occurs when the boom is slewed over or close to that outrigger. It depends on the crane's own weight, counterweight, boom configuration, the load and the radius. Crane manufacturers publish outrigger load figures or calculators for their models; these should govern. Rules of thumb are useful only for a first estimate.
Bearing pressure
Pressure = outrigger load ÷ area in contact with the ground
Example: an outrigger load of 400 kN on a 0.6 m diameter float (area 0.28 m²) gives about 1,400 kPa — more than most natural ground can carry. The same load spread by a stiff 1.5 m × 1.5 m mat (2.25 m²) gives about 180 kPa.
Allowable pressure
The pressure applied must not exceed what the ground can safely carry. Without a site investigation, planners use presumptive allowable bearing values for the soil type, reduced where the ground is wet, loose, made up or disturbed. CIRIA C703 Crane stability on site discusses suitable factors of safety for temporary crane support. Where the ground is soft, near an excavation, over buried services or voids, or simply unknown, get a site-specific assessment.
What a mat can and cannot do
- A mat only spreads the load if it is stiff enough. A thin or long timber mat bends and concentrates pressure under the float; the effective area is smaller than the mat.
- Mats must sit level and fully supported, with the float central.
- Mats do not cure a void, a soft layer below the surface or a trench wall. Keep outriggers back from excavations.
- Inspect mats for splits, rot and missing bolts before use.
The wooden mat calculator holds outrigger data for 209 crane models and presumptive bearing values for 23 soil types, and sizes the mat for the chosen crane and ground.
General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.