Ground bearing pressure

Divide the load on the outrigger by the area of the pad or mat under it. A 22.5 t outrigger load on a 1.2 m square mat, 1.44 m², gives about 15.6 t/m², or 153 kN/m². Compare that with the allowable bearing pressure from the ground assessment.

At least the outrigger load divided by the allowable bearing pressure. 40 t on ground assessed at 15 t/m² needs 2.7 m² or more, so a mat at least 1.65 m square. The mat must also be stiff enough to spread the load to its edges.

It depends on the ground and what lies under it, so it comes from a competent person's assessment of the site. Without one, the CPA's table of safe outrigger pad loads gives cautious figures for common surfaces and soils, up to 25 t on one outrigger.

Often, but a concrete slab or hardstanding is a structure, not ground, so what it can carry has to come from whoever designed it or a competent person who has assessed it. That is why the tool asks for the slab's own allowable pressure rather than giving a figure for concrete.

What a slab can carry depends on its thickness, the strength of its concrete, its reinforcement (if any), the sub-base and ground under it, and how close the outrigger sits to a joint, an edge, a drain or a service duct, where a slab is at its weakest. A slab can fail by cracking or by the pad punching through, often with little warning.

Mats spread the load over more of the slab and help on any surface. A suspended slab, such as a car park deck or a floor over a basement, needs a structural engineer to check it before a crane sets up on it. See our guide to BS 7121-3 and our guide to outrigger mats.

The load spreads out as it goes down, so the pressure falls with depth, but a bigger mat sends it deeper. Hardcore or a road over soft clay is a common case. If the ground assessment or a trial pit shows a weaker layer, enter its depth and what it can carry, and the tool checks the pressure that reaches it too. Voids, cellars, buried services and backfilled trenches need the site's competent person.

The customer in control of the site, under CPA guidance, on crane hire and contract lifts alike. The crane supplier gives the outrigger loads, and the appointed person plans the lift with the site's ground assessment.

The ground types come from Table E1 of the CPA’s guide Ground conditions for construction plant. The table gives a safe load for some common pad sizes, worked out by a standard method for foundations (Brinch-Hansen’s bearing capacity method, as in BS 8004), with a factor of safety of 3. The tool uses the same method for your mat’s own size and shape, so you are not limited to the sizes in the table.

• Clay can carry the same pressure whatever the size of the mat: about 16.7 t/m² on firm or stiff clay and about 55 t/m² on very stiff clay. Long, narrow mats carry a little less, about 15% less for a mat twice as long as it is wide, because the clay can squeeze out more easily along the long sides.
• Roads and car parks are treated as firm clay under the surfacing (250 mm on major roads, 100 mm on car parks and minor roads). The surfacing spreads the load out before it reaches the clay, which helps a small pad more than a big one.
• Sand, gravel and loose fill carry more per square metre under a wider mat, because a wider mat presses on a bigger, deeper block of ground. So on these the shape and size of the mat change the area you need.

The method stops at 25 t on one outrigger, and it does not cover soft clay, silt or peat, which need an engineer. It also assumes the mat is stiff enough to spread the load evenly over its whole area.

Clay fails by squeezing out from under the mat, and its strength does not change with depth near the surface, so the pressure it can carry is the same for a small pad or a big mat. Sand and gravel get their strength from friction, which grows with the weight of ground pressing down. A wider mat spreads into a deeper, heavier block of ground, so each square metre of it can carry more. That is why, on sand or gravel, a wider mat can need less area than a narrow one for the same load.

Shape matters too. On clay a long, narrow mat carries less per square metre than a square one (by the CPA's figures a 2:1 rectangle carries about 15% less), so the longer the rectangle, the more area it needs. Under a road or car park the surfacing spreads the load by the same distance all round, which helps a small or narrow mat more than a big one.

The ground types and their safe loads are from Table E1 of Ground conditions for construction plant, written for the Strategic Forum for Construction and published by the CPA. The rule of thumb for estimating an outrigger load is from the CPA's brief guide to ground conditions, which gives it as a last resort. Planning lifts on mobile cranes, including the ground, is covered by BS 7121-3.

Both measure pressure. One tonne is about 9.81 kN, so 1 t/m² is about 9.81 kN/m², and 100 kN/m² is about 10.2 t/m². Ground assessments usually give kN/m², and crane makers often give outrigger loads in tonnes.