An Approach of Uncertainty Analysis in Assessing Rock Mass Quality

In many occations considerable discrepancies have been found between predicted and actual rock mass conditions, resulting significant cost and time overrun. Finding innovative solutions for quantifying geological uncertainties and assessing risk are therefore key for cost effective and optimum tunnelling. In this paper, an approach of uncertainty analysis has been proposed to evaluate the quality of rock mass using Q-system of rock mass classification. For carrying out uncertainty analysis a geological uncertainty analysis model concept has been established and the software program @Risk has been used. The data from two headrace tunnel from Nepal – namely Khimti and Modi Khola where Q-system of rock mass classification was used to quantify the quality of rock mass are being used. These two tunnels may be categorized as projects of considerably large and relatively small deviation with respect to predicted and actually incountered rock masses. The degree of correlation between simulated results achieved by the @Risk model and values actually measured in the tunnel have been discussed and the sensitiveness and effect of variations in the value of each input parameter have been highlighted. It has been concluded that the proposed uncertainty analysis approach gives promising results.