Technical bulletin 31 - a new flexible pavement design procedure

The new pavement design manual will provide pavement thickness designs that are essentially the same as technical bulletin 26 if the same assumptions on design period and traffic loading are made. The new manual however requires consideration be given to the design period for which the facility is required to serve before rehabilitation and also to the repetitions of traffic loading in that design period. In addition to forcing the designer to better consider and evaluate these design parameters the procedures also aim to ensure adequate consideration is given to the factors influencing the choice between asphalt and surface sealed pavements. To ensure adequate performance of asphalt surfacing a strong pavement with low surface deflections compared to that tolerated by a chip sealed surface is required. In this respect the importance of adopting the specified base quality and thickness is stressed. The selection of an appropriate design subgrade CBR remains a most difficult matter and the resulting design pavement thickness is very sensitive to this factor.