This report covers the permeability of structural backfills, such as Class 6N and 6P materials as defined in Table 6/1 of the SHW (MCHW1). Five materials were investigated using a large diameter permeameter in the laboratory. The fill materials were typical of those frequently encountered in highway construction. The particle size distribution of the materials was systematically modified to allow permeability to be determined for a range of uniformity coefficients ranging from about 10 up to 100. The data were analysed to determine relations between permeability and uniformity coefficient. Whilst good correlations were obtained for each individual material, it was found that overall correlations covering all the materials tested were poor. Alternative descriptions of particle size distributions were also tried, but these did not lead to any improvement. Finally, the data are discussed in terms of the current requirements in MCHW and DMRB. (A)

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