Failure Characteristics of Shallowly Buried Twin Fractures in Sandstone: A Single-Face Unloading Triaxial Test

During the excavation of underground engineering, the presence of fractured rock mass exerts a non-negligible influence on the stability of the project. In this study, a series of single-face unloading tests under true triaxial compression is conducted to investigate the fracture mechanics, crack distribution, and failure modes of mirrored twin fractures sandstone under three filling conditions (two unfilled fractures, two filled fractures, and coexisting unfilled and filled fractures), exploring the failure characteristics of fractured rock mass during shallowly buried tunnel excavation. The main ideas are as follows: (1) the gypsum filling material has a certain impact on the strength and failure mode of twin fractures sandstone samples. (2) The fitted line of acoustic emission b-value indicates that the proportion of low-amplitude events during the rock loading and unloading processes of the two unfilled fractures sandstone sample is larger than the filled sandstone samples. Moreover, according to the normalized average frequency and ratio of rise time to amplitude values, the result shows that the different filling conditions of the preset fractures exhibit a tensile failure mode. (3) Cluster analysis of the relationship among peak frequency, amplitude, and time of twin fractures sandstone samples under varying filling conditions indicate that the amplitude and peak frequency increase with time but decrease correspondingly in the postpeak stage. (4) The results of the Particle Flow Code in two dimensions (PFC2D) 6.0 simulation reveal that a small number of microcracks first emerge at the tip of mirrored twin fractures under the three filling conditions and then these microcracks will continuously expand toward the upper and lower ends of the sample. The results will offer a thorough scientific basis for the excavation of underground engineering.