Abstract
In the coal mine industry, the instability failure of an extraction borehole is one of the main factors leading to the decrease in gas extraction rate. The study on borehole strength and failure mode with different sizes can provide a theoretical basis for the optimization and selection of a gas extraction borehole. In this paper, biaxial loading tests are carried out on similar coal specimens, and the prediction model of borehole strength is proposed and verified. At the same time, numerical simulation of similar specimens is carried out to analyze the failure process of boreholes of different sizes. The results show that hole size has a great influence on hole strength, and the hole strength decreases with an increase of hole size. Size change has a great influence on the strength of a small-size borehole, and the strength of the borehole gradually stabilizes with an increase of size. The process of borehole failure can be divided into three stages, borehole top failure, borehole deformation, and crack extension. Stress concentration and a large number of cracks occur on the two wings of the borehole, and these cracks reduce the strength of the borehole. The stress concentration area of the borehole is X-shaped distribution, and the through crack mainly extends along this area and leads to the failure of the specimen.