Case Study of Program-Assisted Q-System Rating Based on Point Cloud Analysis
Authors: Chiu C. C. , Liu C. Y. 
Published: August 2024

This study develops a computer program to assist in rating the Q-system of jointed rock masses. The analysis is based on photogrammetric analysis of point clouds. Three key rating terms, rock quality designation (RQD), joint set number (Jn), and joint roughness number (Jr), can be calculated by the program without an expert's manual rating. To identify major outcrop joint surfaces, the random sample consensus algorithm was employed. Subsequently, we calculate the weighted joint density (wJd) and its corresponding RQD based on the major joint surface. The spherical K-means clustering was involved to determine the number of dominant joint sets, which assist the acquiring of the Jn value. In addition, the a/L method was adopted to calculate the joint roughness coefficient (JRC) and further convert it into Jr. The results of our analysis align with manual Q-system ratings, demonstrating the feasibility of the proposed computer program.

1. INTRODUCTION

The Q-system (Norwegian Geotechnical Institute, 2022), a semi-quantitative rock mass classification method proposed by Barton in 1974 (Barton et al., 1974) has been widely used by geotechnical engineers (Akram et al., 2019; Barton, 2002). It is used for evaluating the rock mass characteristics of underground openings, providing engineers with the corresponding excavation methods and suggesting appropriate supports (Akram et al., 2019). Moreover, it can be applied in field mapping to estimate the quality of rock masses, which has been validated worldwide.

However, its applicability to the quality evaluation of jointed rock may be limited. First, because of the rock mass heterogeneity, engineers must perform several Q ratings around the outcrop to obtain the Q value distribution. However, outcrops may sometimes be covered by plants or soil or exposed at hard-to-reach locations. Therefore, the lack of a survey method limits the quality of the rating. On the other hand, multiple landslides occurring simultaneously in a region during typhoon season are common, necessitating a rapid survey for emergency situations. Geologists are required to conduct all investigation steps rapidly, including photogrammetry, survey, sampling, and especially, Q-system rating. Therefore, reducing the survey time is critical.

The Q-system is calculated using the following equation:

(equation)

where RQD is the sum of the length of all core pieces that are more than 10 cm long as a percentage of the total core length; Jn is the rating for the number of joint sets; Jr is the rating for the roughness of the least favorable of these joint sets in the same domain; Ja is the rating for the degree of alteration or clay filling of the least favorable of these joint sets or filled discontinuities in the same domain; and Jw is the rating for the water inflow and pressure effects, which may outwash discontinuity infillings in the same domain; SRF is the rating for faulting, for strength/stress ratios in hard massive rocks, for squeezing, or for swelling in soft rock in the same domain. A detailed rating of each parameter can be obtained from the handbook(Norwegian Geotechnical Institute, 2022). For the Q-system, three of these six parameters, RQD, Jn, and Jr, are highly related to the distribution and the roughness of the joints. Therefore, these three parameters can be calculated by evaluating the spatial information from the outcrop surface.