BACK-ANALYSIS OF HOEK-BROWN CRITERION: ROCK SLIDE CASE IN MANADO

The discontinuity of geological structure on tuff layer of rock slopes in Manado triggered a phenomenon of rock slope instability. Mohr-Coulomb criterion which is commonly used as a method of analysis is not suitable to solve the slope stability problem because this criterion does not consider the condition of the geological structure as one of its factors. This paper analyzed the rock slope stability by accommodate the condition of the geological structure using Hoek-Brown criterion. Geological structure effect in Hoek-Brown criterion was realized in the form of GSI (Geological strength index). Poor geological structure will reduce GSI. In this paper, GSI of tuff has been determined by discontinuities of mapping and back analysis. The results from discontinuities mapping revealed that the rock structures were very blocky to blocky and surface conditions were very poor to poor. GSI value ranges between 20-30. From back analysis with shear strength reduction, GSI value was 20 and UCS value was 230 kN. The results are quite close with the field observation.