Supporting slab: Maximum compacted hardfill depths under NZS 3604:2011

Under NZS 3604:2011, the maximum depth of compacted hardfill under a slab on grade is 600 mm. Hardfill must be compacted in 150 mm maximum thickness layers. Where the hardfill depth is more than 600 mm, specific engineering design is required. 

In one case recently, suitable bearing was found at a depth of 650 mm. However, the depth of hardfill required to ensure the finished floor level to finished ground level met minimum ground clearance requirements exceeded 600 mm. This took the design outside the scope of NZS 3604:2011. Options to allow the design to proceed may include:

• carrying out additional soil testing to determine actual bearing capacity of the soil
• specifically engineering the fill design (the limitation on fill depth in NZS 3604:2011 is based on the weight of the fill and the bearing capacity of the soil)
• making up the height with a specifically engineered lightweight concrete. 

From BRANZ Guideline, December 2016