Laboratory assessment of coarse granular road foundation materials

A new UK ‘performance based specification’ for road foundations potentially allows any material that is ‘fit for purpose’ and encourages the wider use of marginal and recycled/secondary materials. The specification provides guidance with respect to a target stiffness, density achieved and maximum allowable rutting depth under construction traffic (to avoid damage to the subgrade during construction). Full-scale field trial sections are currently used and proposed for large schemes to assure as the suitability of proposed foundation materials. However, it is prudent to develop a routine, economical and laboratory-scale means to assess the performance and suitability of the foundation materials before any full-scale trials are undertaken. This paper presents the findings of recent research work at Loughborough University with regard to the development of a large-scale laboratory assessment test aimed specifically for granular materials. A series of test results on four granular materials are presented in detail, with an emphasis on their stiffness behavior, with some field data for comparison and preliminary validation of the laboratory method. The effects of a soft and rigid base condition, and wetting and drying of the material is shown to have a significant effect on the measured values of both stiffness and strength for the samples tested. However, there appears a reasonable relationship between the laboratory results for the soft base condition and the field data. Several recommendations for further work are made and practical observations and comments for the construction and measurement of these materials on site within a performance specification framework.