Shear instability identification method and its damage characteristics based on automatic recognition of three-dimensional curvature of limestone joint surfaces

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.

Keywords

shear instability discrimination / shear damage / joint surface curvature / Otsu threshold segmentation / critical threshold

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