Abstract
Rock quality designation (RQD) is a critical index for quantifying the degree of rock mass jointing; it is widely used for evaluating the qualities and stabilities of engineering rock masses. However, the use of traditional RQD may yield inaccurate assessments because only core pieces longer than 100 mm are counted. To enhance the utility of RQD, generalised RQD was introduced. Based on the modified blockiness index (MBi), the determination of the optimal threshold of generalised RQD was performed. In this work, 35 types of hypothetical three‐dimensional joint network models were constructed, and their generalised RQD values (with different thresholds) and MBi values were measured. The correlation between the standard ratings of MBi and RQD was assessed; based on this correlation, the theoretical RQD values of the 35 models were derived. The reasonable thresholds of the generalised RQD were determined according to the theoretical RQD values, and the optimal threshold of generalised RQD was obtained using 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 with the optimal threshold determined in this study can properly quantify the degree of jointing of a given rock mass.
Highlights
A rock mass is a natural substance that is composed of different compositions and complex structures produced by geologic processes
Rock quality designation (RQD) [2] is a critical index to quantify the degree of jointing and is typically applied in various rock engineering worldwide, including hydraulic engineering, underground and surface mining, and rock tunnel; RQD has been used for over 50 years
The RQD is routinely used in practices, this concept has several inherent limitations; for example, the RQD index only considers core pieces longer than 100 mm and may result in a final RQD value that does not agree with the practice [3]
Summary
A rock mass is a natural substance that is composed of different compositions and complex structures produced by geologic processes. From the perspective of engineering practicability, the RQD values accompanied by an optimal threshold should exactly quantify the degree of jointing to lay the groundwork for the subsequent tasks, e.g., rock mass quality classification and rock mass stability assessment. The typical practice in rock engineering applications is to use RQD to quantify the degree of jointing; knowing the RQD value is crucial in many rock mass quality classifications and stability assessment systems, such as the rock mass rating system [11], Q-system [12], and Qslope-system [13, 14]; the associated empirical relation with the determination of support schemes has been well developed. The block percentages and MBi values were determined using the GeneralBlock program [9], in the following steps: (1) construct a three-dimensional joint network model, (2) calculate the volumes of the rock blocks that are separated by joints, and (3) determine the block percentages and MBi values based on Eqs. (1) and (2)
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