The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks. In this study, fine-, medium-, and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) measurements. The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes. The inflection points of the b-value occurred earlier with an increase in grain sizes, indicating that the larger grains promote the generation and propagation of microcracks. The distribution characteristics of the average frequency (AF) and the ratio of rise time to amplitude (RA) indicate that the proportion of shear microcracks increases with increasing grain size. The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size, which may intensify the microcrack evolution. Moreover, analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation. The study found that granites with larger grains had a higher proportion of high-displacement regions, which can lead to larger-scale cracking or even spalling. These findings are not only beneficial to understand the pattern of micro- crack evolution with different grain sizes, but also provide guidance for rock monitoring and instability assessment. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.

For expedited transportation, vehicular tunnels are often designed as two adjacent tunnels, which frequently experience dynamic stress waves from various orientations during blasting excavation. To analyze the impact of dynamic loading orientation on the stability of the twin -tunnel, a split Hopkinson pressure bar (SHPB) apparatus was used to conduct a dynamic test on the twin -tunnel specimens. The two tunnels were rotated around the specimen's center to consider the effect of dynamic loading orientation. LS-DYNA software was used for numerical simulation to reveal the failure properties and stress wave propagation law of the twin -tunnel specimens. The findings indicate that, for a twin -tunnel exposed to a dynamic load from different orientations, the crack initiation position appears most often at the tunnel corner, tunnel spandrel, and tunnel floor. As the impact direction is created by a certain angle (30 degrees, 45 degrees, 60 degrees, 120 degrees, 135 degrees, and 150 degrees), the fractures are produced in the middle of the line between the left tunnel corner and the right tunnel spandrel. As the impact loading angle (a) is 90 degrees, the tunnel sustains minimal damage, and only tensile fractures form in the surrounding rocks. The orientation of the impact load could change the stress distribution in the twin -tunnel, and major fractures are more likely to form in areas where the tensile stress is concentrated. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY -NC -ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).