Field reflectance spectrometry for detection of swelling clays at construction sites

We have developed techniques to map the distribution and composition of clay-rich soils with portable field spectrometers on the ground. Spectral reflectance measurements in the 1800–2400-nm region with 10-nm resolution can distinguish smectites, which cause swelling, from illites and kaolinite that do not swell significantly. Illites and smectites are often mixed in the soil and result in varying swell potential. Standard engineering soil tests are too time consuming and costly to be used in areas where there is highly varying swell potential. Therefore, in many instances in regions of steeply dipping sedimentary layers, beds of swelling clay go undetected. We show that it is possible to determine smectite content with a standard cross-validation error of 10% based on partial least-squares analysis of second-derivative reflectance spectra. Loadings show that the 1800–2000- and 2150–2250-nm regions contain the most relevant information for the detection and quantification of smectite content and these correspond to the bound water in the clay lattice interlayer and the Al–OH combination band, respectively. Correlations as high as 87% were obtained with the Seed swell-potential index. The correlations are higher when the samples were dried rather than measured in their moist condition shortly after collection in the field, as in an earlier study. Correlation with other swell indices shows that reflectance spectroscopy would be a reliable indicator that could divide samples into low, medium, and high swell potential. The effect of sample moisture was studied and the results show that the surface of the field samples must first be dried before measurement, in order to obtain a reliable swell potential value from the model. © 2001 John Wiley & Sons, Inc., Field Analyt Chem Technol 5: 143–155, 2001