Investigation of the geologic controls on rockfall hazard potential in Tennessee

Rockfall 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 role of geology through two approaches: (1) Eighty roadcuts in central and eastern Tennessee were evaluated with the geologic character component of the Tennessee Rockfall Hazard Rating System (RHRS), which is a revision of the geologic component of the National Highway Institute (NHI) RHRS. Scores for both RHRS's were compared to evaluate improved reproducibility, accuracy, and sensitivity of scoring for the Tennessee RHRS. (2) Collecting additional geologic attribute data beyond the RHRS system to determine if the geologic attributes correlate to rockfall type, potential abundance, and block size as identified with the RHRS. Logistic regression analysis was performed to investigate potential relationships between geologic attributes and rockfall type, block size, and rockfall mode abundance. Results indicate the revised geologic component of the RHRS is more informative and permits description of a wider spectrum of geologic conditions than the NHI version. Logistic regression analysis indicates rockfall type is predicted by lithologic variation and the number of discontinuity sets; and block size is predicted by structurally controlled rockfall, lithologic variation, mechanical layering thickness, and the number 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. Additionally, nearly half of all potential rockfall modes are expected to have low block size scores.