Void Ratio and Porosity (e = n/(1 − n))

Both numbers describe the same emptiness, and both are needed because they have different denominators. Void ratio e divides void volume by the volume of solids, which stays constant as a soil compresses: that is why consolidation theory is written in e. Porosity n divides void volume by the total volume, which is what you measure with a sampling tube, so seepage and storage calculations use n. A porosity of 0.40 corresponds to e = 0.40 ÷ 0.60 = 0.667; a void ratio of 0.85 corresponds to n = 0.85 ÷ 1.85 = 0.459.

The trap is the ceiling: porosity can never reach 1 and is typically 0.25–0.50 in sands and 0.40–0.70 in clays, while void ratio has no upper bound at all. A soft marine clay can sit at e = 3 and a Mexico City lacustrine clay at e = 7, meaning seven parts water to one part solid. That is not a typo, it is why the Palacio de Bellas Artes has sunk several metres into the Valley of Mexico since 1904 while the streets around it sank further still. If a "porosity" you have been handed is greater than 1, someone has given you a void ratio.