Effects of loading rates and mine water immersion on the mechanical characteristics of coal under Brazilian test conditions

The mechanical properties of coal pillars are crucial for evaluating the stability of underground water reservoirs in coal mines. This article examines the fracture mechanical behavior of coal in response to mine water immersion, layer direction, and loading rate. Eight types of specimens were studied, featuring inclination angles between the applied force and the bedding plane of 0°, 15°, 30°, 45°, 60°, 75°, 90°, and the Divider type. The loading rates ( V ) tested were 0.005 ​kN/s, 0.02 ​kN/s, 0.05 ​kN/s, and 0.1 ​kN/s. The results indicated that after immersion in mine water for 30 days, the Brazilian splitting strength ( BSS ), splitting modulus ( E m ), and absorbed energy ( U a ) of coal decreased by 51.35%, 52.37%, and 44.60%, respectively, compared to the non-immersion samples. The primary reason for this phenomenon is that the production rate of micropores and small pores resulting from mine water immersion surpasses their conversion rate to mesopores and macropores. This imbalance leads to the fragmentation of the internal structure of coal and the interconnection of pore fracture zones, thereby significantly weakening its bearing capacity. It has been observed that the relative proportions of failure mechanisms along and across the bedding plane directly influence the variations in coal mechanical properties at different θ values. Additionally, BSS , E m , and U a of coal gradually increase with an increase in loading rate, which is due to the reduced duration of coal damage development and evolution, subsequently lowering the probability of activating weak structures.