Modified rock mass classification system for preliminary design of rock slopes

Abstract

Considering lacking of suitable system in characterizing heavily jointed rock masses in slopes, attempts were made in this study to propose a modified rock mass classification system for preliminary design of rock slope. For this aim, the modified Geological Strength Index (GSI) was employed as a basis, and the relevant geotechnical data were collected from different rock slope sites in IRAN. Among the studied sites, 8 sites were selected and their stability was analyzed by means of the CLARA scientific software. To establish the proposed classification system, besides the GSI, five more parameters as: uniaxial compressive strength, rock type (lithology), slope excavation method, groundwater condition and earthquake force, were considered, and their relative effects on stability of fractured rock slopes were studied. By analyzing sensitivity of stability of the studied rock slopes to the above five parameters as well as GSI value of the rock masses, a logical basis was provided for determining the proposed rating system values. Then the weight of each parameter was defined by statistical analysis of the sensitivity analyses results. In the proposed rock classification system, a rating is allocated to each parameter and an overall rating for the rock mass is arrived at by summing the rating of all parameters. This overall rating is denoted as Slope Stability Rating; SSR. The interpretation of this new rating system (SSR) for the stability of the rock slopes has been achieved by preparing a number of design charts for different safety factor values. Finally, by comparing the actual stability condition of the studied rock slopes with the stability evaluating results using SSR system, the accuracy and reliability of the presented design charts were approved.