Laboratory Measurements of Capillary Rise in Fine Sands and Silts.

Literature values for the height of capillary rise in fine sands, silts, and clays are contradictory. The late C.W. Fetter (Applied Hydrogeology, 3rd ed., 1994) claimed that the height of capillary rise varies from 100 cm in fine sand up to 750 cm in fine silt. We found these numbers unbelievable. Our research goal is to measure capillary rise in sands and silts, and use our data to identify equations and values in the literature for sands, silts and clays that are believable.

Uniform sand grains 0.45-0.55 mm in diameter were carefully packed into two-inch diameter glass columns. These were placed into clear tanks with water level held constant. The height of tension-saturated rise observed above the constant water level was 7.9 cm, and the average height of damp sand above that was 11.6 cm, for a total of 19.5 cm, only about one-fifth of Fetter’s 100 cm. The sand was also treated with a water-repellent spray to test capillary rise in a hydrophobic porous medium. This was done to show the effects that wettability has on capillary rise. The capillary fringe was observed to be below the free water level in the tank. In four hydrophobic sand columns, the average depression of the saturated zone was 5.75 cm. Capillary rise in silts with average grain size below 40 microns was also measured. The columns are up to 264 cm high. In three silt columns, the average capillary rise was near 155 cm, about one-fifth of Fetter’s 750 cm. All of the above tests will be repeated using kerosene instead of water.