UNBOUND ROADS TRAFFICKED BY HEAVILY LOADED TYPES WITH LOW INFLATION PRESSURE

The paper describes tests undertaken to investigate the behaviour of u nbound roads subjected to heavy wheel loads using low tyre inflation p ressures. It is hoped that the research will aid in the development of procedures for the design of unbound roads used by vehicles equipped with central tyre inflation (CTI) systems, Full scale trafficking test s were carried out in the Transport Research Laboratory's Pavement Tes t Facility at Crowthorne, Berkshire, Test roads consisted of a single layer of granular material placed on a Gault clay subgrade, The roads had base thicknesses of 200, 350 and 560 mm. Wheel loads of 44 and 80 kN were used, in combination with tyre inflation pressures of 345 and 690 kPa. Up to 10 000 passes were made on each of six test road sectio ns. In phase I of the tests, rut depths were observed and falling weig ht deflectometer (FWD) deflections were recorded as a function of the number of passes, In phase II, shallow dynamic subgrade strains were r ecorded. Analyses of the rutting, FWD tests and shallow dynamic subgra de strain behaviour are presented, leading to the general conclusion t hat the benefits of reduced tyre inflation pressure traffic seen in fi eld implementations of CTI systems are derived more from the behaviour of the granular base than the subgrade.