Sustainable pavement design methodologies applied in the Main Road 398 Project in Kwazulu-Natal, South Africa: a Greenroads case study

Main Road P398 is a main arterial running from Durban to Ballito along the South African East Coast. The road has seen an influx of traffic following the relocation of the City’s Airport from Isipingo to La Mercy in 2010. The increased traffic lead to an accelerated degradation of the pavement. The KwaZulu-Natal Department of Transport commissioned the rehabilitation of the pavement and an innovative design approach was employed in order to reduce the overall cost, together with the carbon footprint of the rehabilitation works. The initial design was focused on strengthening the pavement using hot mix asphalt (HMA). This design called for between 130mm and 150mm of the existing pavement to be milled off and removed from site. The horizon exposed by milling would then receive one of two treatments (dictated by the condition of the underlying support). The two treatments comprised either insitu recycling with cement to achieve a new 250mm thick C3 subbase layer that would then be overlaid with a 150mm thick HMA base followed by 40mm of HMA surfacing, or directly overlaid with a 200mm thick HMA base followed by 50mm of HMA surfacing. In addition to being an expensive solution, this design posed a challenge for accommodating high traffic volumes, especially where a new cement stabilised subbase was required. Furthermore, with fatigue as the failure criterion for such thick asphalt layers, rehabilitation measures (de-construction) at the end of the service life promised to be extensive. This paper describes the processes followed to address these concerns by adopting a design approach that focused on maximizing the reuse of existing pavement material to provide a balanced pavement with the required structural capacity. In addition to being a thoroughly “green solution”, achieving such an objective by recycling and stabilizing with bitumen offered the lowest construction, maintenance and de-construction costs (i.e. life-cycle costs). This paper also outlines the process involved in registering the project as a pilot Greenroads Project in South Africa.