Key Takeaways & Executive Findings
- •• Shear damage area proportion on limestone joint surfaces increases with effective normal stress, while high-curvature proportion decreases. • The critical threshold range for inducing shear fractures in limestone is identified as effective normal stress-to-shear stress ratio between 1.4 and 1.6. • A novel method for automatic recognition of three-dimensional curvature of rock joint surfaces is proposed, enabling quantitative analysis of joint surface morphology. • Pre-peak acoustic emission cumulative energy and damage increase sharply near the ratio of 1.6, validating the critical threshold for shear instability.
Abstract
In deep underground engineering construction, the dominant rock failure mode, whether by tension or shear, influences the engineering instability. Therefore, the critical triggering conditions that induce shear or tensile fractures in rocks urgently need further investigation. This paper designs direct shear tests on intact limestone under different normal stress conditions, using binarization methods supplemented by scanning electron microscopy to explore the two-dimensional fracture damage characteristics of limestone joint surfaces. Based on the three-dimensional morphological characteristics of limestone joint surfaces, a method for automatically identifying the three-dimensional curvature of rock joint surfaces is proposed, quantifying the changes in curvature distribution under different normal stresses. Further analysis focused on the proportion of shear damage and high-curvature areas on the upper and lower joint surfaces of limestone. The study examined changes in the cumulative energy of pre-peak acoustic emission and damage under varying effective normal stress-to-shear stress ratios. These results were used to identify and validate the critical threshold range for inducing shear fractures in limestone. The conclusions indicate that the proportion of shear damage area of limestone joint surfaces is positively correlated with effective normal stress. The proportion of high curvature of limestone joint surfaces decreases with increasing normal stress. Both the rapid growth stage of shear damage area and the rapid descent stage of high curvature proportion occur in the effective normal stress to shear stress ratio range of [1.4, 1.6]. The cumulative energy of pre-peak acoustic emission and damage under different effective normal stress to shear stress ratios increase sharply around the ratio of 1.6, further verifying that the effective normal stress to shear stress ratio range of [1.4, 1.6] is the critical threshold range for inducing shear fractures in limestone.
1. Introduction
As the development and utilization of deep space resources have become a future trend of human activities, the stability of underground rock mass engineering is becoming increasingly important. With the increase of triaxial stress, the rock failure mode transitions from tensile-shear composite failure to shear failure, making shear fracture the main failure characteristic of rock under high stress [1]. Shear (Mode II) fracture of rock, characterized by easy initiation, long propagation distance, and instantaneous propagation, has become a common failure form in rock mass engineering [2, 3], such as rock slope sliding [4], fault or structural plane sliding-type rock burst [5−7], and fault sliding [8].
BARTON et al [9, 10] advanced this research by conducting extensive shear experiments, introducing the roughness coefficient, and establishing the JRC-JCS model, which correlates the shear strength of rock with its roughness. Numerous scholars have built upon this model in their research endeavors. INDRARATNA et al [11] performed direct shear tests on rock masses with regular structural planes under varying normal stresses, validating the relationship between normal stress and shear strength. GRASSELLI et al [12, 13] developed a three-dimensional morphological function, which incorporates various morphological parameters into the shear strength formula. Building on the Grasselli model, CHEN et al [14] introduced a roughness index capable of reflecting anisotropy. CAO et al [15] explored the relationship between the fractal dimension of joint surfaces, the fractal dimension of profile lines, and the joint roughness coefficient (JRC). Through extensive indoor shear loading experiments (e.g., four-point bending, punch shear, compact double shear tests), RAO et al [16, 17] discovered that rock undergoes tensile (Mode I) fracture rather than shear (Mode II) fracture under pure shear loading. For brittle rocks with significantly lower tensile strength compared to compressive strength, later
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ZHANG Shi-chuan, SONG Shi-long, SHEN Bao-tang, LI Yang-yang (2025). Shear instability identification method and its damage characteristics based on automatic recognition of three-dimensional curvature of limestone joint surfaces. Journal of Central South University. https://doi.org/10.1007/s11771-025-6103-3
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Frequently Asked Questions
What is the critical threshold range for inducing shear fractures in limestone?
The critical threshold range is an effective normal stress-to-shear stress ratio between 1.4 and 1.6, as identified by the study.
How does the proportion of shear damage area on limestone joint surfaces vary with effective normal stress?
The proportion of shear damage area is positively correlated with effective normal stress, meaning it increases as the effective normal stress increases.
What method was proposed for identifying three-dimensional curvature of rock joint surfaces?
The study proposed an automatic recognition method based on three-dimensional morphological characteristics of joint surfaces, using binarization and scanning electron microscopy to quantify curvature distribution.
What is the significance of the pre-peak acoustic emission cumulative energy in this study?
The cumulative energy of pre-peak acoustic emission increases sharply near the ratio of 1.6, which validates the critical threshold range for inducing shear fractures in limestone.
What are the main applications of this research?
The findings are relevant for deep underground engineering, helping to predict and prevent shear-induced instability in rock masses, such as rock slopes, faults, and structural planes.
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