INFLUENCE OF DESICCATION FRACTURES IN HIGH-SMECTITE CLAY SOILS ON GROUNDWATER INFILTRATION RATES

Soils and sediments with high smectite clay contents are generally treated as barriers to infiltration and subsurface fluid flow. However, these clays have a high shrink-swell potential and are subject to extensive cracking when they dehydrate. Clay desiccation fractures can act as conduits for infiltration and can increase the potential risk of contamination of potable water supplies from surface releases. An understanding of infiltration and flow through high-smectite clay soils requires a quantitative understanding of the development of these fractures and their relationship to properties of the soil, such as moisture content.

Desiccation fracture networks in several test plots located on the Edwards Aquifer recharge zone were measured and monitored over a three-month period. Measurements of fracture density, aperture distributions, and fracture depths were measured two times per week using a combination of manual field measurements and digital graphical methods. Soil samples were also taken and analyzed for gravimetric moisture contents. Local weather data, including precipitation and air temperatures, were recorded at the site. Infiltration rates during storm events were measured using tipping-bucket gauges in caves beneath the test plots. Infiltration rates were also measured using ring infiltrometer experiments. Fracture measurements were then correlated to measured infiltration rates to determine relationships between infiltration rates and the extent of fracturing in the soils.