NASA/JPL contract No. 960983: 1997-2000

Identification and Mapping of Expansive Clay Soils in the Western US: Using Field Spectrometry and AVIRIS Data

Principal Investigators

Sabine Chabrillat, CSES/CIRES
Alexander F. H. Goetz, CSES/CIRES
Harold W. Olsen, Colorado School of Mines
Lisa Krosley, Colorado School of Mines
David C. Noe, Colorado Geological Survey

Project

Swelling soils are a major geologic hazard, and expansive clays and clay-shales cause extensive damage world-wide every year. Current engineering practice for characterization of expansive clays involves time-consuming and expensive standard engineering tests for determination of swelling potential, and x-ray diffraction analyses for mineralogical identification. W propose an alternate identification technique using reflectance spectrometry.

One of the worst cases of swelling soils damage is illustrated with the current situation in the Front Range Urban Corridor in Colorado which is underlain by Cretaceous clay-shales, including the Pierre Shale (figure 1, Hart, 1974). The sedimentary bedrock strata are generally flat-lying, except near the foothills of the Rocky Mountains where they have been uplifted into steeply-dipping strata. Expansive clays in the Pierre Shale and adjacent formations along the Front Range are the ones responsible for the damage. The hazard is most severe in areas where these units dip steeply due to differential movement of adjacent beds and has been termed "heaving bedrock". The differential movement has been attributed to the abundances and composition of swelling clays (Gill et al., 1996). The three most important groups of clay minerals are smectite, illite and kaolinite. Smectite (including montmorillonite) has the greatest swelling potential. Fundamental clay properties that control the degree to which the clay minerals swell are percent smectite, cation-exchange-capacity, and exchangeable sodium percentage.

Figure 1. Geological map of the Colorado Front Range (Hart, 1974).

Different types of clays can be identified spectroscopically thanks to their characteristic absorption bands around 2.2um. Kaolinite shows a doublet band, montmorillonite a single band, and illite have additional bands at 2.35 and 2.45 um that we can see in figure 2. A high water content can be detected through deeper and wider H2O absorption bands, located mainly at 1.4 and 1.9 um. Reflectance spectra of montmorillonites exhibit shifts in the absorption band at 2.2. um depending on the calcium content (Clark et al., 1990). The position of the band varies from 2204 nm to 2214 nm, with the center wavelength increasing with increasing calcium content. Sodium montmorillonites have an absorption band centered at 2204 nm and thereby could be distinguished from calcium-rich montmorillonite.

Figure 2. Reflectance spectra of different types of clay.