A crane that is well within its load chart can still overturn if an outrigger punches through the ground. Ground failure is one of the less visible lifting hazards. The crane is set up, the first lifts go well, and then a single outrigger is loaded far more heavily as the boom slews over it. This article explains how to check bearing pressure and size outrigger mats. It is for lift planners, site managers, civil engineers and HSE practitioners.
Outrigger loads are not total weight ÷ 4
A crane's weight and its load are not shared equally between four outriggers. As the crane slews, the load moves over one or two outriggers. The maximum single outrigger load can then be a large share of the crane's total weight plus the lifted load.
Take outrigger loads from the crane manufacturer's data: outrigger load tables, the manufacturer's lift-planning software or the crane's onboard system. The figures depend on the configuration, counterweight, boom length, radius and slew angle. Use the worst case for the planned lift path, not the average.
Pressure = force ÷ area
Ground bearing pressure = outrigger force ÷ bearing area
Units matter:
- 1 tonne (mass) ≈ 9.81 kN (force)
- 1 kPa = 1 kN/m²
Worked example: maximum outrigger load of 40 t on the crane's own float pad, 0.6 m × 0.6 m Force = 40 × 9.81 ≈ 392 kN Area = 0.36 m² Pressure ≈ 392 ÷ 0.36 ≈ 1,090 kPa
That is far more than many natural or compacted soils can safely carry, which is why mats are normally needed.
Sizing the mat
Rearrange the formula to find the bearing area needed:
Required area = outrigger force ÷ allowable bearing pressure
Continuing the example with an allowable bearing pressure of 150 kPa: Required area = 392 ÷ 150 ≈ 2.6 m², for example a mat of about 1.65 m × 1.65 m.
| Mat size | Area | Pressure under 392 kN (rigid mat assumed) |
|---|---|---|
| 0.6 m × 0.6 m (float only) | 0.36 m² | ≈ 1,090 kPa |
| 1.2 m × 1.2 m | 1.44 m² | ≈ 272 kPa |
| 1.5 m × 1.5 m | 2.25 m² | ≈ 174 kPa |
| 1.8 m × 1.8 m | 3.24 m² | ≈ 121 kPa |
This calculation assumes the mat is stiff enough to spread the load evenly over its full area. A thin or flexible mat concentrates pressure under the float, so the effective area is smaller than the mat's footprint. Mat thickness, material and bending capacity must suit the load. For large cranes and critical lifts, a competent engineer should design the mat.
Allowable bearing capacity
The allowable pressure is a property of the ground, not of the crane. Sources, from best to weakest:
- A geotechnical report for the site, with allowable bearing pressures at the relevant depth
- Design data for existing concrete slabs, hardstands or crane pads, including slab thickness, reinforcement and sub-base
- Plate load or other site tests specific to the set-up position
- Presumed values from general tables, used only with caution and a conservative margin
Previously disturbed ground, fill, recent rain and a high water table can all reduce capacity well below the values in published tables.
Hidden hazards below the surface
Before setting up, check each outrigger position for:
- Buried services: pipes, cables, culverts, drains and manholes
- Trenches and excavations, including backfilled ones
- Edges of slabs, retaining walls and embankments, where the load spreads into ground that can move sideways
- Basements, voids and sumps
- Recently placed or poorly compacted fill
Use service drawings, scanning and the site's permit system. Keep outriggers back from excavations by a distance the engineer determines from the depth and soil conditions.
Timber, steel or composite mats
| Mat type | Strengths | Limitations |
|---|---|---|
| Timber (hardwood) | Readily available, tolerant of uneven ground | Absorbs water, degrades, capacity varies with condition |
| Steel | High stiffness and strength, durable | Heavy, needs lifting equipment to place, slippery when wet |
| Composite / engineered polymer | Light, rated by the manufacturer, doesn't absorb water | Must be used within its rated capacity, can deflect under heavy loads |
Whatever the material, inspect the mats before use and place them on level, prepared ground with the float pad central.
Try the calculator
Our ground bearing pressure calculator converts outrigger loads into pressure and checks mat sizes against an allowable bearing pressure. Plan the lift itself with the crane lift calculator.
How Redsafe helps
Redsafe has an in-house ECSA-registered Lifting Machinery Inspector, and its leadership includes a Rigging Manager & Lead LMI with more than 30 years' experience. Lifting supports its mechanical installation work. Talk to us about your lift.
For planning and education only. Outrigger loads must come from the crane manufacturer, and bearing capacity from a competent geotechnical or civil engineer.
T-10 · Free toolGround bearing pressure calculatorPressure under crane outrigger pads and mats, and the mat size needed for the allowable ground bearing pressure.
