# Ground Pressure

Ground pressure is the pressure exerted on the ground by the tires or tracks of a motorized vehicle, and is one measure of its potential mobility, especially over soft ground. Ground pressure is measured in pascals (Pa) which corresponds to the EES unit of pounds per square inch (psi). Average ground pressure can be calculated using the standard formula for average pressure: P = F/A. In an idealized case, i.e. a static, uniform net force normal to level ground at Earth sea level, this is simply the object’s weight divided by contact area. The ground pressure of motorized vehicles is often compared to the ground pressure of a human foot, which can be 60 – 80 kPa while walking or as much as 13 MPa for a person in spike heels.

Increasing the size of the contact area on the ground (the footprint) in relation to the weight decreases the ground pressure. Ground pressure of 14 Pa (2 psi) or less is recommended for fragile ecosystems like marshes. Decreasing the ground pressure increases the flotation, allowing easier passage of the body over soft terrain. This is often observed in activities like snowshoeing.

## Example: Average ground contact for a Hitachi EX 200 (20 ton)

Calculate average ground contact area (footprint):
A = length x width
A = 132.1″ x 23.6″ =3115.2 x 2 for two tracks = 6230.4

Calculate average ground contact pressure:
P = 43651.5 lbs divided over the area 6230.4″ = 7 lbs per square inch
P = 7 psi

## OR

Over 4 Bog mats
A = length x width
A = 197″ x 39.6= 7801.2 x 4 for 4 mats = 31204.8