Acoustic emission (AE) in coal is anisotropic. In this paper, we investigate the microstructure‐related scale effect on the anisotropic AE feature in coal at unconfined loading process. A series of coal specimens were processed with diameters of 25 mm, 38 mm, 50 mm, and 75 mm (height to diameter ratio of 2) and anisotropic angles of 0°, 15°, 30°, 45°, 60°, and 90°. The cumulative AE counts and energy dissipation increase with the specimen size, while the energy dissipation per AE count behaves in the opposite way. This may result from the increasing amount of both preexisting discontinuities and cracks (volume/number) needed for specimen failure and the lower energy dissipation AE counts generated by them. The effect of microstructures on the anisotropies of AE weakens with the increasing specimen size. The TRFD and its anisotropy reduce as the specimen size increases, and the reduction of fractal dimension is most pronounced at the anisotropic angle of 45°. The correlation between TRFD and cumulative AE energy in the specimens with different sizes are separately consistent with the negative exponential equation proposed by Xie and Pariseau. With the specimen size gain, the reduction of the TRFD weakens with the increasing amount of cumulative absolute AE energy.

To gain insight into the reduction mechanism of water on the dynamic destabilization failure character of coal under mining disturbance conditions, the effect of water on the damage and energy dissipation feature of coal was investigated via cyclic uniaxial compressive loading–unloading test, acoustic emission (AE) monitoring technology, and theoretical analysis. It manifests that the softening effect of water on the mechanical properties of coal is still remarkable, lower uniaxial compressive strength and elastic modulus, while greater axial strain and higher Poisson's ratio are observed under the cyclic loading–unloading condition. Water‐saturated specimens dissipate more energy during the cyclic loading–unloading process, which reduces the proportion of input energy accumulated in the coal mass, thus lowering the burst proneness of coal. A greater amount of damage generates in air‐dried specimens than that in water‐saturated specimens at the first cyclic loading–unloading cycle, while lower damage increments are observed in the following cycles. This may be the reason for the minimal energy dissipation of air‐dried specimens in the following cycles. The elastic modulus behaves logarithmically versus the increasing cyclic loading–unloading cycle counts. The energy dissipated due to the damage evolution and plastic deformation of coal specimens positively correlates with the energy released by AE activities, the AE energy in air‐dried specimens has a greater concentration around the peak axial stress and a stronger correlation with the dissipated energy. The cumulative AE energy and energy dissipation density are linearly correlated during the cyclic loading–unloading process, which indicates a possibility in estimating the damage evolution and the dynamic failure risk of coal via cumulative AE energy, under the cyclic loading–unloading condition.

We explored the dependency of strength anisotropy on loading rate in coal both experimentally and numerically. An improved numerical modeling method was developed for this investigation to reduce the random error caused by the mechanical differences of coal specimens. After that, specimens with different anisotropy angles (the angle between the coring direction and bedding plane orientation) were processed (0°, 15°, 30°, 45°, 60°, and 90°). These specimens were then scanned by the X‐ray Computed Tomography (CT) and tested by unconfined loading frame. Uniaxial compressive simulations were performed on 15 numerical models with different anisotropy angles at various loading rates (1.0 × 10⁻⁶, 5.0 × 10⁻⁶, 10.0 × 10⁻⁶, and 20.0 × 10⁻⁶ mm/step). The results indicate that the uniaxial compressive strength (UCS) of coal generally first decreases and then increases as the anisotropy angle increases from 0° to 90° for all loading rates. It is also found that the UCS increases significantly with the increasing loading rate at anisotropic angles of 30° and 45°, while it is less obvious at anisotropic angles of 0° and 90°. This results in a reduced USC anisotropy at high loading rates. Meanwhile, a negative exponential equation is obtained to describe the variation of UCS with the loading rate at different anisotropic angles. And a unified equation is then developed and verified to define the UCS as a function of the loading rate and anisotropic angle.

Coal mass strength estimation has attracted broad attentions for decades. Investigating the effect of scale and anisotropy on the Uniaxial Compressive Strength (UCS) estimation via P-wave velocity is crucial in upscaling the lab-obtained correlation to coal mass strength evaluation. Microstructures were considered as the major impact factors for the scale difference and anisotropy of mechanical properties in coal. Thus, the microstructure correlated influence of specimen sizes and anisotropy angles on the UCS estimation based on the primary wave (P-wave) velocity were investigated here. Specimens with four diameters (25 mm to 75 mm) and six anisotropy angles (angles between bedding plane orientation and coring direction, 0° to 90°) were processed. Roles microstructures played for the P-wave propagation were explored by the X-ray computed tomography scanning and P-wave velocity tests. The UCS estimation via P-wave velocity in coal were investigated by regression analysis, significances of scale and anisotropy effect on this correlation were quantitatively evaluated by an introduced one-way analysis of variance method. P-wave velocity for specimens with different diameters exhibits a U-shaped variation against anisotropy angles, it reduces at anisotropy angles 0° ~ 45°, while raises within that of 45° ~ 90°, the minimum, maximum, and second largest value were acquired, respectively, at the anisotropy angle 45°, 90°, and 0°. This was considered caused by the complementary variation of angles between directions of P-wave propagation and orientations of bedding plane and major cleats. P-wave velocity wakens in larger specimens, velocity reductions are significantly at the anisotropy angle of 45°, while less obviously at 0° and 90°. While the anisotropy of P-wave velocity strengthens as diameters gain. Specimen sizes and anisotropy angles were proved to affect the UCS estimation via P-wave velocity by the one-way analysis of variance. Exponential correlation was verified more appropriate in the UCS estimation via P-wave velocity in coal, specimens with diameter 38 mm and anisotropy angle 45° were proved optimal for the UCS estimating by P-wave velocity in coal, since the correlation between them is stronger than other diameters and anisotropy angles. This research facilitates us in quickly and accurately strength estimating in anisotropy coal, meanwhile, it also provides references in extending the lab-obtained mechanical properties of coal/rock specimens to coal/rock mass.