CRITICAL ROCK HAZARD RATING COMPONENTS FOR SEDIMENTARY SLOPES IN COLORADO

The Colorado Rockfall Hazard Rating System was recently modified to reduce the subjectivity of several of the rated parameters and to include additional geologic and climatic factors shown in the recent literature to contribute to rockfall. In all, 350 slopes were rated at various locations throughout the state, of which 74 were predominantly in sedimentary rock. Eighteen factors were scored to generate a total hazard score, and three additional factors contributed to a risk or exposure score. The data used in the hazard score was analyzed to identify and rank the dominating factors that contribute to the overall rating, using univariate least squares regression, multivariate ordinal logistic regression, and multivariate stepwise regression. This analysis showed that a subset of the hazard factors could be used to predict the total hazard score for sedimentary rock slopes. The critical factors included launching features, rockfall frequency, number of discontinuity sets, interbedding, slope height, discontinuity weathering, annual precipitation, and aperture; and the resulting regression equations had R2 of 76% when only the cut portion of the slope is rated and R2 of 75% when the total slope is rated. While not as detailed as the full Rockfall Hazard Rating System, 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.