Their purpose

Class P sites are problematic. The low allowable bearing pressures of such sites cannot support the loads imposed by a house structure via the footings. The building would suffer damage.

Blinding concrete is used to transfer the loads from the house to the good bearing. This avoids the need to increase the total footing depth to reach the underlying good bearing soil.

Most problem sites contain a problem area on the top layers of soil only. Often, good bearing soil is located at a specific depth below the natural ground level. The depth at which this good bearing soil is located will determine the structural footing design and the cost of the footing construction.

When good bearing soil or strata cannot be located within 1.5 metres of the NGL or surface, pier and beam construction is usually considered. Pier and beam footings are only considered for low bearing soil sites. They require engineering design.

Diagram of a slab laid on top of a number of poles.