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Estimating Permeability of Untreated Roadway Bases

Laboratory testing and statistical analysis of the resulting data to determine the effects of top size, aggregate gradation, and porosity on the permeability of unbound roadway base layers are described. Three different types of aggregates were tested: crushed limestone, crushed granite, and uncrushed river gravel. Top sizes of 19 mm (0.75 in.) 25 mm (1.0 in.), 38 mm (1.5 in.), and 63 mm (2.5 in.) were used. A total of 90 permeability tests were performed using permeameters of 15.2 cm (6 in.) and 30.5 cm (12 in.) in diameter. A statistical method was used to arrive at a first-order multiple regression equation to predict the coefficient of permeability of untreated, dense-graded or open-graded bases in the range of 0.004 to 0.7 cm/sec (1,000 to 2,000 fpd) using the void ratio of the sample and the percent by weight of materials that pass the 0.6 mm (No. 30) and the 0.075 mm (No. 200) sieves. Predicted permeabilities using the equation were compared to the predicted permeabilities using the Hazen equation and the Moulton nomograph. The comparison indicated that the new equation does a better job of predicting permeabilities in the range of 0.2 to 0.7 cm/sec (500 to 2,000 fpd) than the other prediction tools.