# Estimating RQD for Rock Masses Based on a Comprehensive Approach
## 1 Introduction  
Rock mass stability is of paramount importance for various aspects of human life, including production, safety and development. The discontinuities in the rock mass separate it into numerous irregularly shaped blocks, consequently leading to the disruption of its continuity and integrity [1; 2; 3; 4]. This, in turn, significantly impairs the mechanical properties of the rock mass, making these discontinuities the primary determinant of rock mass failure [5; 6; 7; 8]. As such, the accurate acquisition of information concerning the development of rock mass joints assumes critical significance in assessing and ensuring rock mass stability. Rock Quality Designation (RQD), defined as the percentage of intact drill core pieces longer than 10 cm recovered during a single core run [9], becomes a critical index for quantifying the degree of rock pointing. The determination of the RQD is influenced by an array of factors such as the nature of rock mass fracturing, the methodology and quality of sampling, the orientation of the borehole relative to the joint structure, and others. Specifically, an increase in fracture density typically corresponds with a proliferation incrack size, quantity and extension, concomitantly decreasing the RQD [10; 11]. The fidelity of the RQD is contingent upon the sampling process, which encompasses the caliber of drilling equipment, the proficiency of technical operations and the integrity of the core preservation. RQD estimations predominantly depend on core samples retrieved through drilling activities. The accuracy of these estimations is sensitive to the angular relationship between the borehole trajectory and the prevailing fractures. Parallel drilling orientations in relation to the fracture plane tend to result in an underestimation of RQD values, whereas perpendicular orientations may lead to their overestimation [12]. Due to its easy accessibility and operability [13; 14; 15], the RQD is widely used in geotechnical engineering practice. RQD acquisition has attracted many studies, ranging from (i) field observations and (ii) theoretical calculations to (iii) simulation modeling. Field observations of RQDs are implemented using drilling cores and other equipment-based measurement methods to evaluate rock quality. Common examples include those utilizing core drilling, digital borehole cameras and acoustic waves [16]. For instance, [PERSON] [17] predicted the RQD using drill performance parameters. [PERSON] et al. [18] presented an RQD prediction method based on the correlation between the standard deviation of the rotary drilling energy and the RQD. [PERSON] et al. [19] proposed a rock drillability index to classify rock quality using drill cores. [PERSON] et al. [20] used digital borehole camera technology to evaluate the integrity of the rock mass. [PERSON] et al. [21] utilized the borehole wall integrity core length obtained from the borehole camera to verify the RQD methods. Recently, [PERSON] et al. [22] used ultrasonic technology to analyze the property of the excavation disturbed zone around a tunnel. [PERSON] et al. [23] developed relationships between P-wave velocity and RQD based on engineering and geophysical borehole data. These observation approaches offer crucial actual data for geological studies and evaluations of the exploration region. However, these methods require specialized, costly equipment and highly qualified technicians. Theoretical calculations of the RQD use mathematical models or methods to derive it. Successful examples include [PERSON] [24], [PERSON] et al. [25], [PERSON] [26] and [PERSON] et al. [27]. [PERSON] [24] and [PERSON] et al. [25] linked linear density to the RQD based on assumptions of discontinuity spacing obeying negative exponents and log-normal, respectively. [PERSON] [26] concluded that the RQD is affected by the size of the rock mass and deduced the relationship between the RQD and the volumetric joint count. [PERSON] et al.