A new perspective for the directivity of Rock Quality Designation (RQD) and an anisotropy index of jointing degree for rock masses

Abstract

The Rock Quality Designation (RQD) index is defined as the percentage of the borehole core or scanline that consists of intact lengths ≥0.1 m, and has been widely used as an engineering measure of the jointing degree of a rock mass. The directivity of RQD, which means the value of the RQD obtained from drilling cores or scanlines with different orientations can be different for a given site, is usually regarded as a drawback by researchers and engineers. This study presents a new perspective for the directivity of RQD: the directivity of RQD should be regarded as an advantage of the index because it essentially reflects the anisotropy of rock masses. Because a smaller value of the RQD index implies a better reproduction of the different jointing degrees of different rock masses, its minimum value is selected as the representative RQD (RQDr), and its corresponding direction is called as the representative direction of RQD. In this study, to obtain the RQDr and its representative direction, theoretical formulas are derived to calculate the RQD in any direction based on some known RQDs in different directions. The proposed formulas consider the variance of the discontinuity orientations and sizes for a discontinuity set, use the traditional concept of RQD, and possess the associated simplicity; they only require sampling a few additional discontinuity orientations. Therefore, the proposed approach is easy to understand and apply in engineering practices. An anisotropy index of jointing degree (AIjd) is defined to quantitatively describe the anisotropy of jointing degree for rock masses. The range of AIjd is from 0 to 100, and the larger value of AIjd reflects stronger anisotropy of jointing degree of a rock mass. In this study, a hypothetical three-dimensional discontinuity network is constructed by performing Monte Carlo simulations, based on which the generated data are analyzed to validate the proposed method. The comparison between the calculated and actual values of the RQD shows that (a) the difference between the calculated and actual values of the RQD is <4 for >86.0% of the 1368 scanlines examined with respect to 12 scanline groups, except SG4–1 group, for which the proportion is 66.2%; (b) the difference between the calculated and actual values of the RQD is <6 for >96.2% of the 1368 scanlines examined with respect to 12 scanline groups, except SG4–1, for which the proportion is 86.7%; and (c) the proposed method of calculating the RQD in any direction based on some known RQDs in different directions is satisfactory. In addition, the comparisons show that the calculated RQDr and representative direction exhibit a good agreement with the actual results, and hence, the proposed method for this purpose is satisfactory. Moreover, an actual rock mass of a slope around the Qingshan Lake National Forest Park in China is investigated, and the results show that the proposed method is valid for that rock mass.