Numerical Experimental Investigations of the Effect of Confining Pressure Unloading Rate on Sandstone Mechanical Properties

Abstract

Purposes This work is conducted to exclude the superimposed effect of natural rock specimen discreteness and multiple stress path conversion in test process on results, and to further investigate the influence mechanism of the confining pressure unloading rate on rock failure and instability. Methods The discrete element method was used to carry out homogeneous numerical specimens under constant axial pressure and two fixed confining pressure unloading rates. The macroscopic and mesoscopic mechanical properties and laws of the specimens under two confining pressure unloading rates are compared and discussed. Findings The results show that the macroscopic and mesoscopic fracture distributions have certain similarity and regularity under the two conditions, the load-bearing strength of the specimen in slow confining pressure unloading rate is higher than that in fast condition, and the local fracture distribution density at the fast unloading condition is relatively larger. The axial and confining pressure “falls” and “rises” during the two unloading rate tests, which reflects that the specimens have undergone a multi-stage sudden and progressive failure process. The continuous accumulation of local damage leads to the overall instability of the specimens. The number of tensile fractures under the two unloading rates is significantly larger than that of shear fractures, and the growth rate of tensile fractures is greater than that of shear fractures. The slow unloading condition relatively fully mobilized the local bearing capacity of the specimen and improved the ultimate bearing capacity of the specimen. Conclusions Under the two conditions, the distribution of fractures composed of more than 20 AE events is the most uneven. With the increase of AE event numbers creating each fracture, the overall seismic magnitude increases gradually, and the overall energy release during the fast unloading process to form fractures becomes relatively higher, while slow unloading transfers stress at a relatively slow rate, resulting in a relatively lower, smooths, and uniform energy release during fracture formation.

Keywords

• confining pressure unloading rate
• numerical experiment
• mechanical properties
• discrete element method
• sandstone