Abstract
The evaluation accuracies of rock mass structures based on the ratings of the Rock Quality Designation
(RQD) and discontinuity spacing (S) in the Rock Mass Rating (RMR) system are very limited
due to the inherent restrictions of RQD and S. This study presents an improvement that replaces these
two parameters with the modified blockiness index (Bz) in the RMR system. Before proceeding with this
replacement, it is necessary for theoretical model building to make an assumption that the discontinuity
network contains three sets of mutually orthogonal disc-shaped discontinuities with the same diameter
and spacing of discontinuities. Then, a total of 35 types of theoretical DFN (Discrete Fracture Network)
models possessing the different structures were built based on the International Society for Rock Mechanics
(ISRM) discontinuity classification (ISRM, 1978). In addition, the RQD values of each model
were measured by setting the scanlines in the models, and the Bz values were computed following the
modified blockiness evaluation method. Correlations between the three indices (i.e., Bz, RQD and S) were
explored, and the reliability of the substitution was subsequently verified. Finally, RMR systems based
on the proposed method and the standard approach were applied to real cases, and comparisons between
the two methods were performed. This study reveals that RQD is well correlated with S but is difficult
to relate to the discontinuity diameter (D), and Bz has a good correlation with RQD/S. Additionally, the
ratings of RQD and S are always far from the actual rock mass structure, and the Bz ratings are found to
give better characterizations of rock mass structures. This substitution in the RMR system was found to
be acceptable and practical.
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