Outrigger mats and ground pressure: meaning and why it matters

Boom Angle editorial teamSources checked 25 Oct 20263 min read

Ground pressure is the force on the soil divided by the area carrying it. An outrigger float concentrates a large force on a small area. A mat or crib under the float spreads the same force over more ground, so the pressure falls to a level the soil can hold.

Two side views of an outrigger float pushing on soil. On the left the float sits on the ground alone with a few arrows showing high pressure. On the right it sits on a wide mat with many arrows spreading the same force — Same force on the float. A larger bearing area means lower pressure on the soil.

The arithmetic

Pressure equals force divided by area. If an outrigger float puts 40,000 lb on 4 square feet of soil, the pressure is 10,000 lb per square foot. Put a mat under it and the same 40,000 lb on 16 square feet is 2,500 lb per square foot. The force did not change. The area did.

Going the other way, the area a mat needs is the float load divided by the pressure the ground can take. If the soil can carry 2,000 lb per square foot, which is an invented figure for this example, then 40,000 lb needs at least 20 square feet, or about 4.5 ft by 4.5 ft. The outrigger pad calculator does this step and shows the working.

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Operations

Which action creates side loading on a crane boom?

Hoisting a load vertically and slowlyDragging a load with the hoist line toward a spot not under the boom tipBooming down with no load on the hookSetting the load down on cribbing directly under the boom tip after a slow vertical hoist

Choose an answer to begin

Where the numbers come from

The float load. It comes from the crane manufacturer's data for the setup and the load, not from the weight of the lift alone. The loaded-side float carries a large share of the crane's own weight as well as the load.
The allowable ground pressure. It comes from someone who knows the soil. A guess is not a number.
The mat's size and stiffness. A mat that bends under the float spreads the force less than a stiff one.

OSHA 29 CFR 1926.1402 covers ground conditions and who is responsible for them. Read the section itself for the requirements, and use the crane manufacturer's instructions for the support the crane needs.

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Every answer comes with a full explanation.

What to watch

Mats centered under the float, not hanging off one side.
Ground that looks dry but sits on fill, a trench, buried utilities or a slope.
Rain, which can soften soil that carried the crane the day before.
Settling during the lift. Level can change as the ground gives.

Common mix-ups

Bigger mat always fixes it. It helps bearing pressure, but weak soil under a deep layer or a void can still fail.
Mats make the crane more stable. They spread force to the ground. They do not change the chart.
Using the crane's weight alone. The float load includes the lifted load and the boom's effect.
Mixing units. Pounds and square feet, or kips and square inches, must match the allowable figure's units.

On the NCCCO written exam

Core tasks 1.1 and 1.2 cover ground stability, mats and cribbing, in Site, about 22% of the exam under the 01/25 handbook. Expect a short pressure problem done by hand, or a question about what to check before setting up on soft ground.

Practice it

Run your own float loads in the outrigger pad calculator. Read outrigger setup for the beams and jacks above the mat, and tipping axis for why the loaded float carries so much. The crane terms glossary lists the set.

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Sources

Show the list

29 CFR 1926.1402: Ground conditions, OSHA.
29 CFR 1926.1404: Assembly/Disassembly, general requirements, OSHA.
Mobile Crane Operator Candidate Handbook, CCO, Core outline tasks and weights (01/25 revision). Task numbers on this page follow it.

Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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