Quantification of rock mass weathering using spectral imaging

The degree of weathering, a key parameter for evaluating rock mass strength, has traditionally been assessed based on visual inspection by engineers. In an effort to reduce the human bias associated with this approach, a study was conducted to investigate the potential for using spectral imaging to quantify weathering. This entailed developing a portable, rapid method for narrow-band multispectral (NBMS) remote sensing using a spectral index classification algorithm, applying this algorithm to detect weathered features, and then quantifying the degree of weathering based on the percentages of weathered and aperture areas. A case study was conducted on Mt. Lemmon in southern Arizona and spectral images were collected from rock slopes using a visible and near-infrared (VNIR) hyperspectral camera. A two-band ratio approach was used to delineate key areas. Wavelength ratios of 601 nm to 550 nm and 993 nm to 450 nm, were used to delineate weathered and aperture areas respectively on the rock mass. The weathering degree at the test site was then quantified using thematic images. This entailed assessing the percentages of the weathered (22.5%) and aperture (12.5%) areas in the thematic image and using them in a modified Geological Strength Index (GSI) evaluation. The weathering rating (R_w) was classified as ‘slight’ and scored as ‘5’ based on the percentage of weathered and aperture areas, and the GSI was determined to be 43. This study successfully demonstrated the potential for using spectral information to quantify rock mass weathering, as well as for using the calculated weathering degree to estimate the GSI.