The bearing pressure of foundation soils
Soils are measured for their allowable bearing pressure. The allowable bearing pressure is the soil's ability to carry the load of a building and its contents without excessive settlement.
For one and two-storey buildings, the Building Code of Australia (BCA) requires a minimum bearing pressure of only:
- 100 kPa for under strip and pad footing systems
- 50 kPa for under slab footings.
Few building failures are caused by insufficient soil bearing capacity even when footings are unevenly loaded. This is because the loads imposed on foundations soils by houses, even those with solid masonry walls, are small compared to the bearing potential of natural firm soil.
Critically low soil bearing pressure is only found in alluvial soil (mud or silt), wet sand or poorly compacted fill. There are also rare cases of soils that have adequate bearing potential in dry weather, but which change to low bearing potential in wet weather.