LABORATORY TESTING OF CAPILLARY RISE IN FINE-GRAINED SANDS AND SILTS

Data for the height of capillary rise in fine-grained sands, silts, and clays, is inconclusive. The literature gives a wide range of values and equations. One source (Applied Hydrogeology, 3^rd ed., p. 182) claims that the height of capillary rise varies from 100 cm in fine-grained sand up to 750 cm in fine silt. We found these numbers unbelievable, and asked the late C.W. Fetter to put better numbers in his 4^th ed. Instead, he deleted the table in the 4^th ed. The equation which generated his numbers was used by Adamski et al. (2005)* to explain free product accumulations of up to 520 cm in wells.

Our research goal is to measure capillary rise in fine-grained sediments, and use our data to identify in the literature believable equations and values for sands, silts and clays.

Uniform sand grains 0.45-0.55 mm in diameter were carefully packed into 5-cm diameter glass columns. These were placed into a clear water tank, with water kept at a constant level of 12.7 cm. The height of tension-saturated capillary rise observed above the constant water level was 7.5 cm, and the height of damp unsaturated sand above that was 8.9 to 12.7 cm.

The sand was also treated with a water-repellent spray (Kiwi Camp Dry) to test capillary rise in a hydrophobic porous medium. This was done to show the effects the contact angle has on capillary rise. The capillary fringe in our first hydrophobic sand column was below the free water level in the tank as expected. This test will be repeated.

Measurements of capillary rise in silts averaging below 40 microns are now underway in the same glass columns and water tank. Capillary rise reached the tops of the columns, which were 142 cm high. Some columns were extended by 122 cm. The silt compacted and cracked when wet, leaving gaps. Later tests used silt mixed with equal volumes of fine sand. There was much less compaction and cracking, and no gaps in these experiments. The presence of sand in the mixture will have little effect on capillary rise, because silt fills in between the sand grains and capillary rise is determined by the properties of the smallest interconnected pores.

*Adamski, M., V. Kremesec, R. Kolhatkar, C. Pearson and B. Rowan, 2005. LNAPL in Fine-Grained Soils: Conceptualization of Saturation, Distribution, Recovery, and Their Modeling, Ground Water Monitoring & Remediation, 25, no.1, 100-112.