Stabilising the expansive subgrade of a permeable car park

A very large car park is being built for a commercial enterprise in Sydney, Australia. The owners wanted to trial the design of a permeable car park which captures surface water for subsurface storage and sustainable use within the premises of the commercial enterprise. As is well known, Sydney is underlain by extensive residual soil which exhibits shrink-swell and other characteristics potentially damaging to pavements. Thus the concept of a permeable car park, while having many benefits, is contrary to the fundamental principle of pavement design which is to divert water away from the pavement structure, much more from a potentially expansive subgrade with poor drainage. These challenges demand a careful assessment of the subgrade in a fashion more rigorous than that required of a typical subgrade, and of any potential treatment method above that used in normal practice. This paper describes the characterization of the in-situ subgrade material undertaken by the authors to understand more clearly what the implications of a permeable car park with underground water storage may cause. Based on this understanding, the authors then propose a treatment method to produce a stabilized subgrade for the car park pavement, one that would achieve the desired properties of strength, volume stability and low permeability.