final report of the working party on the slippage of rolled-asphalt wearing course

a working party was set up under the auspices of the department of the environment, the county surveyors' society and the asphalt and coated macadam association to determine the causes of failure of a number of rolled-asphalt wearing courses by sliding relative to the basecourse; failures normally occurred shortly after the completion of the construction of new roads. most of these failures were reported in the eastern part of the country from nottinghamshire northwards as far as aberdeen in scotland. after considering the incomplete and sometimes apparently inconsistent evidence from 56 slippage failures an interim report was published in 1976 with the aim of initiating action that would help to reduce the incidence of failures. to identify with greater authority the factors affecting the risk of slippage and their interaction, proposals were made for further research. a programme of full-scale testing under controlled conditions has since been undertaken by the transport and road research laboratory and the results published in lr813, together with some data from road construction sites. a slippage failure is associated with the presence of a plane of weakness at or near the bottom of the wearing course. the present report confirms the important part played by the stiffness of the road structure in the development of such a plane; it recommends that the stiffness should be improved by making every effort to protect the subgrade during construction from the effects of moisture and from undue disturbance by construction traffic and by ensuring the structural adequacy of the unbound granular materials in the pavement. the rolling of the asphalt wearing course at too high a temperature is another factor that increases the risk of developing a plane of weakness, and a range of temperatures during laying is recommended. whenever possible, wearing courses should not be laid on basecourses at temperatures close to freezing point.(a)