On Joint Roughness: Measurements and Use in Rock Mass Characterization

The paper focuses on the methods commonly used for measuring and quantifying the joint roughness in the current rock mass characterization practice. Among the different parameters traditionally employed to express roughness, major relevance seems to be gained by the “Joint Roughness factor” (jR) applied in the Palmström’s rock mass characterization index system, having this factor also been integrated in some quantitative methods recently proposed for estimating the geological strength index of a rock mass. Therefore, a new quantitative approach for a more objective estimation of this factor has been fine-tuned and is presented in the paper. The new method has been specifically designed to assist engineering geologists in gaining consistent values of jR on the base of conventional routine field measurements. It can suitably integrate and support the traditional semi-qualitative Palmström’s classification of joint roughness, mostly related to more subjective descriptions, to estimate the most representative values of jR to be considered for rock mass characterization. In its original definition, the factor jR is evaluated for two-dimensional joint profiles by the product of a small-scale roughness factor (the joint smoothness or unevenness factor, j_s) with a large-scale roughness factor (the joint waviness factor, j_w). In order to reduce the subjectivity on the estimate of these partial factors, a simple analytic equation relating them with some parameters traditionally used to quantify 2D joint roughness is here proposed, providing a continuous gradation among the different ratings of the jR scale and also facilitating the implementation of the factor in probabilistic approaches for managing its inherent uncertainty and variability. The presented method applies best for characterizing hard rock masses well-exposed after blasting (e.g., in dam foundation or quarry slopes) or favourably outcropping on natural rocky slopes, where joint surfaces can generally be easily attained and analysed.