LABORATORY MEASUREMENTS OF CAPILLARY RISE IN 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 1.5 cm in fine gravel to 100 cm in “very fine sand” up to 750 cm in “fine silt”. We found these numbers for finer materials unbelievable. Our research goal is to measure capillary rise in sands and silts, and use our data to identify believable equations and values in the literature for sands, silts and clays.

Uniform sand grains 0.4-0.7 mm in diameter were carefully packed into two-inch diameter glass columns. These were placed into clear tanks with water level held constant. The average height of capillary rise observed above the constant water level was 13.5 cm, similar to Fetter’s 15 cm for a similar size sand. 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 a finer uniform 0.3-0.6 mm sand is currently being measured. Capillary rise in silt with average grain size below 40 microns was also measured after the silt was mixed with equal volumes of the 0.4-0.7 sand to make the column packing work better. The columns are up to 264 cm high. In several silt column experiments, the silt cracked. All of the tests above were repeated using kerosene instead of water; in two silt columns, the capillary rise of kerosene was 134 cm and there was no cracking. This value for kerosene scales to a water capillary rise of 228 cm.

We hope to identify equations for calculating capillary rise that come close to data values we believe. We are focusing on the equation Fetter used as well as the Polubarinova-Kochina (1952) (P-K) equation which is: hc = 0.45 ((1 – n) / n)/ d10 , with n = porosity, hc = capillary rise and d10 = effective grain diameter (hc and d10 in cm). Our tests with water in sand averaged hc = 13.5 cm for 5 tests; the P-K value is 14.8 cm. The capillary rise in silt calculated using P-K was 181 cm; Fetter’s value would be 188 cm. We hope to add water to a 25% silt, 75% sand column without cracking to measure capillary rise and compare with these values.