What Lies Beneath

I’m now fairly certain the contour survey and soil tests for our land now have been completed. I believe this to be the case because, on Friday, I received a request from Orbit Homes for a copy of the developer’s Compaction report. (A request which I duly have passed on to the agent.) I was told that the compaction report is required because there is up to 2 metres of fill on the block. 

The original engineering drawings for the development indicated that going clockwise from the northwest corner, the levels of fill were to be : 0.68m, 1.30m, 1.48m and 0.95m. Given the quoted 2m figure it seems these levels may have increased in practice. Soil compaction occurs when force is applied to soil such that it increases the soil density due to the displacement of air. This is in contrast to soil consolidation which is due to the displacement of water.

Fill should be placed and compacted in thin layers. This provides for consistent compaction at all levels. Under these circumstances the total depth of the fill is no longer a significant factor. AS2870 provides guidance on the specification, execution and control testing of earth works and associated site preparation works for residential developments. It specifies inspection and testing protocols which provide a greater level of assurance that the compaction has been undertaken in a satisfactory manner.

For a block such as ours, with a significant degree of fill, the compaction report is an important document because it has a direct impact the house foundation design requirements. Fill that has been placed and compacted in Layers with Level 1 certification is commonly regarded as suitable for supporting single or double storey houses.