The undrainable water in sand

Abstract

It has long been known to soil science that after water-saturated sand has been allowed to drain freely, a small percentage of fluid (2-3%) is still to be found in the sand. This undrainable water, localized on the surface of particles and at the contact points between particles under a negative capillary pressure, is studied here as a function of its surface tension and the apparent specific surface of sand. The influence of the surface tension of water on the amount of undrainable water in quartz sand suggests that most of the fluid is retained by capillary forces which are presumably localized near the points of contact between particles. However, the influence of the apparent specific surface of sand on the amount of undrainable water suggests that some of the fluid forms a layer c. 0.5 µm thick on the rugose surface of the sand particles.