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Rock engineers and practitioners use regression equations/models between uniaxial compressive strength, UCS and point load index, II(50) to estimate UCS when there is difficulty in direct determination of UCS from laboratory tests. II(50) has been reported as an indirect measure of the compressive strength of rocks. However, there are many equations in the literature relating II(50) to UCS for all rock types. Selecting the appropriate model for UCS estimation in a particular site/deposit becomes problematic. This is because UCS of rocks, like other geomechanical properties are products of different geological processes that rocks are subjected to, which makes them to be variable even within a formation or deposit. This study presents an approach for selecting regression equation for estimating UCS from II(50), using only a limited number of II(50) data obtained from a specific deposit or site. The approach works by comparing the occurrence probability of each model for a given set of observation (i.e. II(50)) data obtained from a site/deposit. The most appropriate model is the model with the highest occurrence probability for the given set of observation data. This approach is different from previous works that need both UCS and II(50) data to draw comparison; instead it selects the appropriate model using the II(50) data only. This is important because the time when there is need for selection and use of regression model is when the UCS data are not available. The selected model is then used in a Bayesian framework to integrate the prior knowledge about UCS with the limited number of site-specific II(50) data available for probabilistic characterization of UCS. This is achieved by using Markov Chain Monte Carlo (MCMC) simulation to generate UCS samples from resulting posterior PDF of the Bayesian framework. Statistical analyses are then performed on the UCS samples to obtain its mean, standard deviation and full probabilistic distribution.

BAYESIAN SELECTION OF REGRESSION MODEL FOR PROBABILISTIC CHARACTERIZATION OF ROCK UNIAXIAL COMPRESSIVE STRENGTH