Experiment on Rockburst Process of Borehole and Its Acoustic Emission Characteristics

Abstract

In the present study, structural model test using rectangular prismatic granite specimen 200 mm × 200 mm × 200 mm with a horizontal central circular hole of 78 mm diameter was conducted to investigate a rockburst process of borehole. Strain measurement system and high-speed camera were used to capture the rock responses during rockburst. Acoustic emission (AE) system was adopted to monitor the associated AE signals during the rockburst process, and to locate the positions of micro-cracks and subsequently quantitatively investigate the cracking mechanisms. In addition, scanning electron microscope (SEM) was also used to identify the micro-cracks of fragments. The experimental results indicate that rockburst process is characterized by significant spatial distribution and structural responses. As the circumferential stress of surrounding rocks increase, there are some local rockbursts interlacing at different regions of compressive stress concentration along the opening axis direction before overall rockburst. These local rockbursts continued to develop and coalesce, eventually forming overall rockburst. A local rockburst in the present test can be composed of several bursts and persist for a longer period of time than that in true-triaxial tests using rectangular prismatic specimen. Hoop effect, stress gradient around the opening, and last V-shaped bands were accurately simulated. According to AE analysis results, quiet period characterized by few AE hits with high amplitude and a sharp increase in AE energy can be used as an early warning signal for overall rockburst. The time and position of rockburst are related to the spatiotemporal distribution of AE event density, which can be used as a potential indicator for rockburst prediction. During the rockburst process, tensile cracks occupied most of the total micro-cracks, and tensile splitting dominated the failure process. Shear cracks due to tensile cracks interaction initiated at 67% of the spalling strength (tangential stress for spalling failure at the opening boundary) of the borehole.