Granular Pavements: Design with understanding

The following outlines a method for the estimation of subgrade cover of unbound granular pavements that is fundamentally different to that of Austroads (2012). The proposed method, which is concerned with single wheel loads ≥ 25kN, is linked to a rudimentary model of the response of granular materials and pavements to repetitive loads. The pavement’s response is determined primarily by the ratio of the subgrade’s undrained shear strength to the subgrade’s peak shear stress. The nature of this response, which will range from elastic to plastic, determines the nature and scale of potential threats to a wheel path’s performance and longevity. Whereas the current method is focussed on the long term cumulative effect of traffic, the behavioural models and reasoning behind the proposed method indicate that short term increases in loads and moisture, unless properly accounted for, are likely to constitute a serious threat to longevity. The paper demonstrates that Austroads (2012) is not equipped to differentiate between the cover requirements for the inner and outer wheel paths or to identify the threat to a pavement’s serviceability resulting from short term heavy loads, from upward trends in wheel loads over time and from short-term reductions in subgrade shear strength.