Ground Pressure Calculator

Ground Pressure Calculator finds the pressure a machine puts on the ground through its tracks, pads or outriggers. It also checks the figure against a ground limit and sizes a mat.

Ground Pressure Calculator for Tracks, Pads and Mats

Ground pressure is the load a machine puts on the surface, divided by the area that touches it. A heavy machine on wide tracks can press on the ground less than a person walking. The Ground Pressure Calculator works that figure out for the four ways a load usually meets the ground.

Choose tracks, rectangular pads or mats, round pads or feet, or a contact area you already know. Then enter the load and the size. The result is the pressure in psi, a check against the ground's limit, and the mat size that would bring the pressure down to that limit.

How Do You Calculate Ground Pressure?

Divide the force on the ground by the contact area. In US units the force is the weight in pounds. Pounds over square inches gives psi, and pounds over square feet gives psf. One psi equals 144 psf.

$$P=\frac{F}{A}$$

The default entries in the Ground Pressure Calculator are a 45,000 lb machine on two tracks, each with 24 in shoes and 10 ft of track on the ground. Each track covers 2 ft by 10 ft, or 20 ft², so the two cover 40 ft². Dividing 45,000 by 40 gives 1,125 psf, which is 7.81 psi.

The number of contacts matters as much as their size. By default the load is shared by all of them, so each track carries 22,500 lb. If your figure is already the load on one contact, change Load Entered Is to On each contact under Load Share and Ground Limit.

Excavator Ground Pressure From a Spec Sheet

For a tracked machine, take the operating weight and the shoe width from the spec sheet. The length to enter is the track that sits flat on the ground, which is roughly the distance between the front idler and the rear sprocket centers. It is shorter than the overall track length.

Caterpillar's sheet for the 320 GC excavator lists 49,400 lb on 31 in triple grouser shoes and a ground pressure of 5.1 psi. Enter that weight in the Ground Pressure Calculator with 31 in shoes and 12 ft of track on the ground, and the result is 5.53 psi.

The gap has a reason. Makers follow ISO 16754, which the ISO summary says allows for the track sinking slightly into soft soil and gaining support area. The same summary says the value is only for comparing machine models. On firm ground the flat-footprint figure is the safer one to plan with.

Shoe width is the easy way to change the result. The same 49,400 lb machine on 24 in shoes would press at 7.15 psi. That is why machines built for soft ground carry wider shoes than the standard ones.

Crane Ground Pressure Under an Outrigger

An outrigger is the hardest case, because the whole load on that corner lands on one small float. The load to use is the maximum outrigger reaction. DICA, a maker of outrigger pads, stresses that this is not the load being lifted and not the crane's rated capacity. It comes from the crane maker's data or from lift planning software.

The pad area follows from that load and the allowable ground bearing pressure for the site. Divide the first by the second, then round up to a pad size that is actually made.

$$A_{pad}=\frac{F_{outrigger}}{P_{allowable}}$$

DICA's example is a 100,000 lb outrigger load on ground rated for 3,500 psf, which needs 29 ft². Set the Ground Pressure Calculator to Rectangular pads or mats, one contact, load on each contact, and a 3,500 psf limit. It returns 28.57 ft², a 5.35 ft square mat or a 6.03 ft round one.

The same card shows what happens without the right pad. That load on a 4 ft by 4 ft pad is 6,250 psf, which is 179% of the limit. Federal rule 29 CFR 1926.1402 requires ground that is firm, drained and graded, with blocking, mats or cribbing where needed to give the support the crane maker specifies.

Ground Pressure of Vehicles on Tires

A tire is different from a track because it flattens until it carries its load. Its ground pressure is roughly equal to its inflation pressure, a rule given in Wong's Theory of Ground Vehicles. The contact patch is therefore the wheel load divided by the tire pressure.

$$A=\frac{F}{P_{tire}}$$

A pickup with 1,500 lb on each wheel at 35 psi has a patch of about 43 in² per tire. Choose Known contact area, enter four contacts of 43 in², and a 6,000 lb total. The Ground Pressure Calculator returns about 35 psi, which is roughly 5,000 psf.

That is far above any soil value in the list, and it explains why wheeled machines rut soft ground that a tracked machine crosses cleanly. A tracked excavator at 5 psi presses about one seventh as hard as that pickup tire.

Ground Limits and What They Mean

The first card compares the pressure with an allowable ground pressure. The presets are the presumptive values in IBC Table 1806.2, and the chart under the results shows the area each contact needs on every ground type. For the default machine with 22,500 lb per track, the figures are these.

Each 20 ft² track is larger than every figure in that column, so this machine is within the limit on all five. On sand it uses 56% of the 2,000 psf limit and could weigh up to 80,000 lb on the same tracks.

Those code values were written for building foundations on undisturbed soil. Fill, topsoil, wet ground and ground over buried voids can be much weaker. For crane work the site's controlling entity is responsible for the ground conditions, so use its figure when one is given. To check a footing against the same table, use the Bearing Pressure Calculator.

The Ground Pressure Calculator result is also an average for a level, still machine. Lifting over a corner, digging, or working on a slope shifts weight to one side and raises the pressure there. Treat the average as the lowest pressure the ground will see, not the highest.

Ground Pressure Calculator in Metric

Switch Units to metric for loads in kilograms, tonnes or kilonewtons and sizes in millimeters or meters. A 20,000 kg machine on two tracks with 600 mm shoes and 3 m on the ground presses at 54.5 kPa. The last card restates any result in the other systems.