Outrigger Mats and Ground Bearing Pressure: Preparing a Site for a Crane

A crane can be perfectly sized, inspected and operated, and still tip because the ground under one outrigger gives way. A significant share of crane incidents start with the ground rather than the machine, and the ground can be checked before the crane arrives. This guide explains ground bearing pressure in plain terms, shows how outrigger mats reduce it, and lists the ground conditions on UAE sites that need checking before a crane sets up.

What Ground Bearing Pressure Means

When a crane lifts, its weight, the counterweight and the load are carried by the outriggers. As the crane slews, most of that weight can shift onto one or two of them. Ground bearing pressure is that outrigger force divided by the area it rests on. The smaller the area, the higher the pressure. A bare outrigger pad is small, so the pressure under it can be far more than many surfaces can carry.

A Simple Worked Example

Suppose the lift plan shows a maximum outrigger load of 30 tonnes. On a 1.2 m by 1.2 m mat, the area is 1.44 square metres. 30 tonnes is about 294 kilonewtons, so the pressure is about 204 kilonewtons per square metre (kPa). Double the mat to 2.0 m by 2.0 m, an area of 4 square metres, and the pressure falls to about 74 kPa. The crane and the load have not changed. Only the area has.

The other half of the check is what the ground can safely carry, known as the allowable bearing pressure. That figure comes from a ground investigation, a geotechnical engineer or, for slabs and podiums, a structural engineer. If the pressure under the mat is higher than the allowable bearing pressure, you need a bigger mat, a different set-up position, or ground improvement.

Choosing Outrigger Mats

• ✓Size: chosen from the outrigger load and the ground's allowable bearing pressure, not by habit
• ✓Material: timber, steel or composite mats, each rated for a maximum load
• ✓Condition: no cracks, rot or bending, and a level surface under the full mat
• ✓Placement: the outrigger pad centred on the mat, and the mat bearing evenly with no voids underneath
• ✓Stacking: mats should not be stacked loosely to gain height unless the arrangement is designed for it

Ground Conditions to Watch on UAE Sites

Loose Sand and Open Desert Plots

Dry sand looks firm until it is loaded at a point. On new plots in areas such as Al Wathba and Al Shamkha, a compacted pad or large mats are normally needed under each outrigger.

Coastal Ground and Reclaimed Land

Coastal areas, quaysides and reclaimed land can have a firm surface over weaker layers, and salt flats (sabkha) can lose strength when wet. Port and quayside lifts at KIZAD and Khalifa Port or Mina Zayed should use the operator's ground data or a quay load limit.

Backfill, Trenches and Buried Services

Recently backfilled trenches, manholes, drainage lines and cable routes are weak points that are easy to miss under a finished surface. Check service drawings and walk the set-up area before the crane arrives.

Interlock Paving, Kerbs and Slabs

Interlock paving can move or sink under an outrigger, and what lies underneath is often unknown. Kerbs and slab edges should never sit under an outrigger. Basement roofs, podium decks and suspended slabs need a structural engineer's confirmation before any crane stands on them.

A Ground Check Before Mobilization

• ✓Mark the planned crane position and all four outrigger points on a site drawing
• ✓Get the maximum outrigger load from the lift plan for that crane and configuration
• ✓Confirm the allowable bearing pressure, or ask for a ground assessment
• ✓Check service drawings for anything buried near the outriggers
• ✓Prepare a level, compacted pad where the ground is loose or uneven
• ✓Agree the mat sizes and who supplies them

Seven Lift checks the outrigger loads against the ground for crane jobs and supplies outrigger mats where the ground needs them. For heavier lifts, see 100 ton crane rental, or read our lift plan checklist for the rest of the planning.