Refined Modification of the Fractured Rock Mass Classification Method Considering Geometric Parameters of Joints and Block Stability

ABSTRACT Rational determination of surrounding rock grades is critically important for accurately predicting tunnel instability mechanisms and designing support structures scientifically. However, the traditional Q‐system classification method exhibits considerable subjectivity in determining key parameters, especially those pertaining to rock mass integrity, which remains a persistent challenge in engineering geology practice. This study proposes a quantitative modification to the Q‐system by integrating the response relationship between joint geometric parameters and block stability. High‐definition tunnel face images were processed to extract joint spacing and the number of joint sets. Based on numerical simulations of 115 working conditions, a quantitative relationship was established between these joint parameters and block response. Using the entropy weight method, multiple instability indicators—including displacement, number of unstable blocks, volume, and stress—were comprehensively integrated to derive a modified, continuous parameter that simultaneously captures joint set frequency and spacing. Additionally, the two‐dimensional rock block index (RBI 2D ) was introduced to refine the rock quality designation (RQD), enabling a more accurate characterization of rock mass integrity. Engineering applications demonstrated that the modified Q ′ value provides a more reliable assessment of rock mass quality, particularly in joint‐intensive zones or near faulted sections. The proposed approach effectively reduces the subjectivity inherent in conventional assessments and offers a technically robust basis for balancing safety and economy in tunnel construction.