Effects of Unbound Granular Materials Gradation Parameters on the Drainage Quality of Pavement Structures

Drainage quality, as defined by AASHTO 1993, is affected by parameters related to subsurface
materials properties as well as parameters related to roadway geometry. Hydraulic conductivity
(permeability) of unbound granular materials (UGM) used in base and subbase layers
construction is one of the major properties that influence drainage quality. This study
investigates the variations in hydraulic conductivity and drainage quality resulting from modifying
UGM gradation parameters. The considered UGM gradation parameters were porosity, fines
content, and effective size of the blend. Field and laboratory testing of hydraulic conductivity
were performed in order to quantify the benefits gained from basing UGM blends on
performance related parameters. The test results were also used to investigate the reliability of
the estimated hydraulic conductivity from the Moulton prediction model.

Several dense-graded UGM gradations of gravel were evaluated in this study. Permeability field
testing was carried out on those gradations in multiple highway construction projects throughout
Manitoba. The field testing utilized the double ring infiltrometer test for measuring the in-situ
hydraulic conductivity of compacted base layers. In addition, UGM samples from each
construction project were collected for further laboratory testing of hydraulic conductivity using
the rigid-wall permeameter. Results from field and laboratory testing were used to provide
performance-based range of values for drainage quality corresponding to the range of UGM
gradation parameters investigated in this study. The measured hydraulic conductivity values
were compared to values and prediction models reported in the literature for dense-graded
UGM. Moulton’s hydraulic conductivity prediction model was found to provide an approximation
of hydraulic conductivity values of the studied materials.