Reservoir aggregate

Gravel for underdrains and underground infiltration structures should be 50 mm, uniformly-graded, washed stone that has a void ratio of 0.4 (40%).

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)