Selection of Optimal Threshold of Generalised Rock Quality Designation Based on Modified Blockiness Index

Abstract

Rock quality desig nation(RQD)isacritical index for quantifying the degree of rock massjointing;it is widely used fore valuating the qualities and stabilities of engineering rock masses. However, the use of traditional RQD may yield inaccurate assessments becauseonlycorepieceslongerthan100mmarecounted.ToenhancetheutilityofRQD,generalisedRQDwasintroduced.Based on the modified blockiness index (MBi), the determination of the optimal threshold of generalised RQD was performed. In this work,35typesofhypotheticalthree-dimensionaljointnetworkmodelswereconstructed,andtheirgeneralisedRQDvalues(with differentthresholds)andMBi valuesweremeasured.ThecorrelationbetweenthestandardratingsofMBi andRQDwasassessed; basedonthiscorrelation,thetheoreticalRQDvaluesofthe35modelswerederived.Thereasonablethresholdsofthegeneralised RQD were determine daccording to the theoretical RQD values,and the optimalthres hold of generalised RQD was obtaine dusing the variation coefficient and anisotropy index of the jointing degree. The discrepancy between the results produced using traditional and generalised RQDs was discussed. Finally, an actual case study was conducted, and the results indicate that the generalised RQD associated withth eoptimal thres hold determined in this study can properly quantify the degree of jointing of a given rock mass.