Slabs on fill

Concrete slabs may be constructed on filled sites only where the placement of the fill has been strictly controlled. This is referred to as 'controlled fill'. This means the fill material has been carefully selected and compacted in accordance with the procedure set out in AS 2870.

Typically, coarse clean sandy soils are used for this purpose. They are compacted by mechanical rollers. Appropriate amounts of moisture aid the compaction process. The fill is laid down in layers not exceeding 150 mm in thickness. Before the slab is constructed the minimum bearing pressure of the fill must be tested.

Under no circumstances are slabs to be constructed on uncontrolled fill. Uncontrolled fill consists of material that has not been compacted or has been poorly compacted and contains a high portion of expansive clays. This will result in excessive footing movement.

Diagram of a concrete slab on controlled fill. The top soil is re-used and covers the controlled fill.