Idealised configurations of a porous media as a collection of spherical particles: (a) porous media has length (l) and cross-sectional area (a), spheres represent solid particles, and the void volume is equal to the volume of hydraulic conduits under saturated conditions; idealised configurations of seepage flow paths in the porous media under the (b) loosest and (c) densest packing conditions, with a REV shown by the red rectangles and the streamlines represented by the blue arrows