Assessment of permeability of granular drainage layer considering particle size and air void distribution

Granular drainage layers are utilized in the permeable pavement structure to temporarily store the infiltrated water and to gradually penetrate it into the underlying layer or the subsurface drainage system. The selected particle size distribution (PSD) of aggregate and consequently the size of air voids distributed between particles inherently affect the drainage capability of drainage course. In the present study, the effect of distribution of pores between aggregate along with the general properties of characterized gradation curve on the permeability of this granular porous media is assessed by establishment of various PSDs. For this purpose, large-scale constant head permeability test is carried out on distinct gradations of coarse-sized aggregate derived from three different rock types. A well-established analytical procedure is utilized to determine the constriction size distribution characterizing the interconnectivity of pores. A probabilistic approach is employed to estimate the distribution of size of air voids based on the captured two-dimensional images from prepared specimens. Obtained results confirm nonlinear relationship between the flow velocity and the applied hydraulic gradient especially for specimens comprised of more uniform size range. Also, the developed relationship based on the power law between the hydraulic conductivity and the characteristic pore size can effectively describe the permeability of coarse-sized aggregate used as drainage layer. Furthermore, establishment of gradation with higher uniformity coefficient leads to smaller-sized pores between particles and consequently lower hydraulic conductivity coefficient. Finally, the extended regression model based on the characterized properties can effectively predict the permeability of aggregate specimen.