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Environmental and Engineering Geoscience

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Classifying and Assessing the Geologic Contribution to Rockfall Hazard

¹ Department of Earth & Planetary Sciences, University of Tennessee–Knoxville, Knoxville, TN 37996
² Department of Civil & Environmental Engineering, Virginia Tech, 200 Patton Hall, Mail Code 0105, Blacksburg, VA 24061
³ Department of Civil & Environmental Engineering, University of Tennessee–Knoxville, Knoxville, TN 37996
⁴ Geotechnical Engineering Section, Tennessee Department of Transportation, 6601 Centennial Blvd., Nashville, TN 37243

Rockfalls from roadcuts are a major hazard and pose problems for transportation agencies across the country. In the context of rockfall hazard management, however, no consensus exists about the role of geology in assessing rockfall hazard. This study investigates the geologic contribution to rockfall hazard through application of rockfall hazard rating systems to roadcuts in Tennessee and through additional data collection to reveal correlations between hazard characteristics and geologic attributes. The geologic character of 80 roadcuts in central and eastern Tennessee was evaluated using the Tennessee Rockfall Hazard Rating System (RHRS), which is a revision of the National Highway Institute (NHI) RHRS. Scores for both RHRSs were compared to evaluate whether the improved reproducibility of scoring for the Tennessee RHRS yielded unintended losses of scoring accuracy and sensitivity. Additional geologic attribute data beyond those used in the RHRS system were collected to determine with logistic regression analysis whether relationships among the geologic attributes, rockfall type, and block size exist. Results indicate the revised geologic component of Tennessee's RHRS is more informative and permits description of a wider spectrum of geologic conditions than does the NHI version. Logistic regression analysis indicates rockfall type correlates to lithologic variation and the number of discontinuity sets; and block size correlates to structurally controlled rockfall, lithologic variation, mechanical layer thickness, and number of discontinuity sets. Consequently, roadcuts containing potential rockfall modes with two or more discontinuity sets, no lithologic variation, and mechanical thicknesses that exceed 1.0 m are expected to have greater Geologic Character scores.

Key Words: Rockfall Hazard • Rockfall Modes • Geologic Character • Logistic Regression