Characteristics of stress thresholds of granite after triaxial dynamic impact treatment

Abstract

The geostress and rock blasting in underground engineering may greatly affect the stress thresholds of surrounding rock. In this study, pre-damage impact tests were first conducted on granite under varying confining pressures (5, 10 and 15 MPa) and numbers of impacts (1, 5, 10 and 15 impacts). Then, uniaxial compression tests were undertaken on the pre-damaged granite to study the evolution of stress thresholds using the crack volume strain method and acoustic emission method. The crack damage stresses determined by the two methods were compared. Additionally, based on the rise time amplitude and average frequency, the evolution law of microcracks inside rock specimens was revealed, and an improved acoustic emission method was proposed. The results indicated that as the number of impacts increased, the crack closure stress, crack damage stress, and peak stress of granite specimens initially rose and then declined, while they continuously increased with the confining pressure. The proportion of shear cracks first declined and then rose with greater number of impacts and decreased with higher confining pressure, and that of tensile cracks showed the opposite trend. The improved acoustic emission method was more accurate in identifying the crack damage stress.

1. Introduction

As metal deposits are currently mined at increasing depths, the impact loads generated by roadway excavation, blasting, and other activities in high geostress environments can cause irreversible damage and instability of surrounding rock, posing a significant challenge to deeper mining [1]. To ensure the safety and efficiency of underground mining structures, identifying the deformation and strength features of the surrounding rock has high practical significance.

However, characterizing rock mechanical behavior by the peak strength alone does not meet the requirements of practical engineering applications [2]. Under loading conditions, hard brittle rock generally experiences five stages, including crack compaction, linear elastic deformation, crack stable extension, crack unstable extension, and post-peak failure. The stresses demarcating the five failure stages are known as the stress thresholds, which generally comprise the crack closure stress σcc, crack initiation stress σci, crack damage stress σcd, and peak stress σf [3 −5]. Therefore, identifying the stress thresholds and analyzing the crack closure, crack initiation, and expansion process of impact damaged rock are of significance for stability assessment and control of underground engineering rock.

Several methods are currently used to determine stress thresholds considering the strain parameters of rock. LAJTAI et al [6, 7] and BRACE et al [8] analyzed the lateral strain, axial strain, and volumetric strain curves, and the points in the curves obtained from the test data where the deviation from a straight line started to occur were taken as σcc and σci points. EBERHARDT et al [9, 10] and CAI et al [11] estimated σcc using the onset point of the axial stiffness plateau, and the sudden drop point of the axial stiffness to determine σcd in rock. LI et al [12] proposed a new volumetric strain response method to determine σci in rock. GONG et al [13] identified the damage stress threshold of rock...