Ground Pressure
Ground pressure is found by dividing the force on the leg by the contact area, and it determines whether the leg will sink. A 500 kg load on an area of 100 cm² gives roughly 0.49 MPa, while the same load spread over 400 cm² gives roughly 0.12 MPa. If the bearing capacity of the ground is exceeded, the leg sinks.
The calculation is straightforward: a mass of 500 kg produces a weight of about 4905 N. When this force acts on an area of 100 cm² (0.01 m²) the pressure is about 490 kPa, and when it is spread over 400 cm² (0.04 m²) it is about 123 kPa. Quadrupling the area reduces the pressure to a quarter.
- Concrete and asphalt: carry these pressures without trouble in cold weather; hot asphalt softens and takes an imprint.
- Compacted gravel: generally sufficient.
- Wet soil, grass and sand: the bearing capacity is markedly lower; without a plate, sinking should be expected.
In practice you can control two variables: distributing the load more evenly over the four legs and enlarging the contact area. The second is far more effective, because pressure is inversely proportional to area. A sinking leg is not only a comfort problem; a leg that sinks during the day spoils the angle and creates twist in the body.
Context in which this term is used: How to choose levelling leg capacity