Mechanical Characteristics of Pre-Peak Unloading Damage and Mechanisms of Reloading Failure in Red Sandstone

Abstract

The mining of deep coal resources occurs in a high-stress geological environment as well as an engineering environment of rock excavation and unloading. Research on the re-bearing capacity characteristics and damage mechanism of rock masses damaged by peak front unloading is critical in revealing the destabilization and rupture law of deep rock bodies. The triaxial pre-peak unloading point was controlled to prepare damaged sandstone specimens, and the RMT-150C rock mechanics test loading system and the AEwin USB-type acoustic emission monitor were used to perform uniaxial reloading tests on the pre-peak unloading-damaged sandstone and to monitor the acoustic emission signals during the rupture process. Among them, the peak front unloading point was set to 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, and 90% of the peak strength at 10 MPa of the surrounding pressure for a total of 11 working conditions. The test results show that: (1) The degree of unloading before the peak controls the uniaxial reload deformation characteristics of sandstone. The higher the unloading point, the faster the deformation of the rock sample, even directly into the crack instability extension stage, and the sandstone deformation characteristics transform from plastic—elastic to elastic—viscous. (2) The cumulative energy characteristics of the 40% to 60% sandstone at the unloading point are comparable to those of the complete sandstone and are separated into smooth, steady growth, and secondary smooth phases. The acoustic emission energy characteristics of the 65% and 70% sandstone at the unloading point are mostly focused on during the crack expansion stage. The sandstone’s acoustic emission energy characteristics exhibit a “double peak” occurrence at 75% of the unloading point. The cumulative energy characteristics of the 80% to 90% sandstone at the unloading point reveal a “stepped” rise. (3) Sandstone’s pre-peak unloading rupture morphology influences the reload damage characteristics: 40% to 70% of the specimens at the unloading point exhibit “Y”-type double-slope shear damage features. The predominant cause of specimen damage in 75% of the specimens at the unloading point is secondary primary cracks based on the pre-peak tensile rupture pattern. The damage path of 80% to 90% of the specimens at the point of unloading occurs in shear damage along the pre-peak unloading rupture pattern. (4) A closed crack mechanics analysis model under uniaxial reload was established, and the basic solution of pseudo-force for fine microcracks subjected to far-field stress, the stress intensity factor at the crack tip, and the crack fracture angle were theoretically derived. Furthermore, the relationship between the fracture angle θ of rock compression-shear cracks, the crack angle β, and the friction coefficient f at the crack surface was clarified.

Reloading Mechanical Properties and Particle Flow Simulation of Pre-Peak Confining Pressure Unloading Sandstone

“…The mechanical properties of rock (masses) will be different depending on the stress environment. Scholars have conducted indoor mechanical tests on various types of rocks (masses) under complex stress environments and these can be roughly divided into loading mechanical, unloading mechanical [2], cyclic loading and unloading mechanical [3][4][5][6][7], and damage reloading mechanical tests [8][9][10]. Previous studies [11][12][13] conducted indoor loading tests and studied the stress-strain curves, deformation, and failure characteristics of sandstone and conglomerate specimens under various confining pressures.…”

Mechanism of Stratum Instability and Dynamic Deformation under Discontinuous Boundary Conditions

“…Krzysztof et al [20] found through experiments and numerical simulations that a reasonable support method can effectively improve the stability of the fault area and the rock surrounding the roadway. Zhu et al [21,22] studied the mechanical characteristics of fractured surrounding rock in damaged tunnels and analyzed the mechanical deformation and failure characteristics of fractured surrounding rock through rock mechanics experiments. Yu et al [23,24] carried out three-dimensional geological modeling of a quarry containing close-range double faults based on the site, and through similar simulation experiments and three-dimensional numerical simulation analysis, they derived the form of mutual transformation of the energy and stress of the two faults in the whole mining process of the quarry, the overall joint control mechanism of the faults on the regional peripheral rock structure, and the law of the mutual influence of the faults, and, moreover, they gave a map of the joint action of the two faults in the whole mining process.…”