Effects of Hydrologic Processes on Vertical Displacements in the Critical Zone

Abstract:

Vertical displacements in unconsolidated material respond to local changes in pressure and temperature, as well as loads applied at the ground surface and changes in water content in shallow soils. Our objective is to use these vertical displacements as a hydrologic monitoring tool, so it is important to understand the various processes that contribute to displacements in order to properly interpret field observations. Several years ago we developed a simple, high resolution, vertical extensometer, called a DEL-X, that is capable of resolving displacements of less than 10 nm in soils. The spatial sampling region of the instrument is approximately 2x its depth. Recent applications have been in the range of 5 to 10 m, so the averaging region extends over many hundreds of square meters. Five DEL-X instruments were deployed at a field site near Clemson, SC at depths of 3m, and 6m within saprolite soil derived from biotite gneiss. Signals from the co-located extensometers are remarkably similar, demonstrating reproducibility of the technique. Rainfall is associated with compression, and ET is associated with extension at 6-m depth. Deep infiltration and lateral surface flow may also contribute to unloading. The loading factor is approximately 200 nm of compression per 1 mm of water load change by either rainfall or ET, and this value is consistent with the elastic modulus of the soil at the site. The displacements include a strong signal with an annual period and an amplitude of approximately 50 microns. The peak compression occurs in January and peak extension is in May for all the 6-m-deep DEL-Xs. These displacements are strongly correlated to the height of the water table, which rises in the spring and drops throughout the fall. Temperature changes also affect displacements over an annual period, but the effect is secondary to changes in pore pressure at 6-m-depth, which is slightly above the water table. Data processing techniques have been developed to resolve these different components of the displacement signal.