Laboratory Assessment of Coarse Granular Road Foundation Materials

Abstract

A new performance-based specification for road foundations in the United Kingdom 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 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 proposed for large schemes to ensure the suitability of proposed foundation materials. However, it is prudent to develop a routine, economical, and laboratory-scale means of assessing 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 designed specifically for granular materials. A series of tests done on four granular materials is presented in detail, with an emphasis on their stiffness behavior. Some field data are given for comparison, and a preliminary validation of the laboratory method is given as well. The effects of a soft and rigid base condition and of 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 to be 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 are given regarding the construction and measurement of these materials on site within a performance-specification framework.

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