Modification and statistical analysis of the Colorado Rockfall Hazard Rating System

Abstract

A modified version of the Colorado Rockfall Hazard Rating System (RHRS) was created by adding several geological and climatic factors that have been recently shown to contribute to rockfall. The modified RHRS was then used on slopes distributed throughout the western, mountainous terrain in Colorado, including sites where only cut slopes were rated and sites where both the cut portion and the higher, uncut portion were both judged to contribute to the rockfall hazard. In total, 355 slopes were rated: for crystalline rocks, 147 cut slopes and 77 total slopes, for sedimentary rocks, 43 cut slopes and 31 total slopes, and for block-in-matrix materials, 45 cut slopes and 12 total slopes. Univariate least squares regression was used to find parameters that had a statistically significant influence on the total hazard score. This subset was then analyzed using ordinal logistic regression to rank these parameters against one another to determine which had the most significant influence on the total hazard score. Finally, statistically significant parameters from these regressions were analyzed using stepwise regression to obtain predictive equations to estimate the total hazard score for each of the slope types analyzed based on the scores of a few parameters. The stepwise regression produced equations in which the total hazard scores can be estimated from four of the RHRS parameters, reduced from the original 12 or 18 parameters, depending on slope type, with R 2 values ranging from 68 to 82%. For some cases, equations with more terms and correspondingly higher R 2 values could be applied. Analysis of the equations identified the most important and least important parameters for each type of slope. For all slope types, the presence of launching features and the slope aspect were important, while slope angle, annual freeze/thaw rating, slope water, and friction along discontinuities were not important. While not as detailed as the full RHRS, this approach allows for a more rapid preliminary rating of rock slopes for screening-level evaluation and it helps identify the most basic geologic factors influencing rockfall potential.