## 2 Test Preparation and Procedures
### Experimental Setup and Testing Method  
The photographic view of the testing system is shown in Figure 2. The uniaxial compressive stress that applied to the sandstone specimens were conducted by a servo-hydraulic rock mechanics testing system (Instron-1346) with a maximum axial loading capacity of 2000 kN. The loading direction is parallel to the height of the specimens. The axial load was imposed incrementally with a constant displacement loading speed of 0.6 mm/min, and the axial displacements were measured by an axial linear variable displacement transducer (LVDT).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Specimen No.** & **W/mm** & **H/mm** & **T/mm** & \(\alpha\)\({}^{p}\) & \(\sigma_{d}\)/MPa** & \(\epsilon_{d}\)/10\({}^{-3}\) & \(E_{b}\)/GPa \\ \hline Intact-1 & 60.54 & 120.77 & 30.75 & – & 76.64 & 6.64 & 12.86 \\ Intact-2 & 60.56 & 120.65 & 30.80 & – & 76.94 & 6.18 & 14.00 \\ Intact-3 & 60.07 & 120.71 & 30.12 & – & 71.50 & 6.46 & 12.37 \\ Sq-0-A-1 & 60.72 & 120.45 & 30.46 & 0 & 53.90 & 5.54 & 10.97 \\ Sq-0-A-2 & 60.27 & 120.22 & 30.02 & 0 & 53.29 & 5.51 & 10.89 \\ Sq-0-B-1 & 60.41 & 120.35 & 30.18 & 0 & 54.55 & 5.75 & 11.45 \\ Sq-0-B-2 & 60.70 & 120.17 & 30.43 & 0 & 57.94 & 5.69 & 11.19 \\ Sq-0-C-1 & 60.66 & 120.38 & 30.12 & 0 & 59.58 & 6.31 & 11.83 \\ Sq-0-C-2 & 60.58 & 120.14 & 30.11 & 0 & 61.16 & 5.83 & 12.09 \\ Sq-45-A-1 & 60.54 & 119.60 & 29.96 & 45 & 46.49 & 5.11 & 9.94 \\ Sq-45-A-2 & 60.53 & 120.13 & 30.40 & 45 & 45.95 & 4.94 & 9.86 \\ Sq-45-B-1 & 60.80 & 120.26 & 29.86 & 45 & 47.71 & 4.90 & 10.33 \\ Sq-45-B-2 & 60.83 & 120.29 & 30.54 & 45 & 48.41 & 4.92 & 10.36 \\ Sq-45-C-1 & 60.64 & 119.56 & 30.19 & 45 & 54.52 & 5.45 & 11.38 \\ Sq-45-C-2 & 60.45 & 120.16 & 29.74 & 45 & 51.40 & 5.26 & 10.90 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Geometrical sizes and mechanical parameters of specimens.  
Figure 1: Geometry of the pre-holed specimens and filling modes.  
DIC technique has been widely used in rock mechanical testing to analyze the rock deformation and fracture on specimen surface. Especially, it provides an effective way to describe the fracture evolution around inclusions in this test. Through the deformation fields of the specimen surface, it is easy to obtain quantitative information that is difficult to be directly observed in high-resolution pixel images. The detailed principle of DIC can be seen in reference [30]. During the test, a charge-coupled device (CCD) camera (Basler PiA2400-17 gm) was applied to capture the images on the specimen surface at a frequency of 15 frames/s under the illumination of a gooseneck lamp. The resolution of the camera is \(2448\times 2050\) pixels, and the length-pixel ratio is 0.0935 mm/pixel. After the test, the digital images collected in the test were post-possessed by VIC-2D image correlation software, and the calculation configurations in DIC post-processing are as follows: subset: 19, step: 4, subset weights: Gaussian weights, interpolation: optimized 8-tap, correlation criterion: normalized squared differences. During the correlation, the confidence margins were under 0.05 pixel controlled by a computing threshold, and the filter size was chosen to be seven in the strain computation.  
Generally, the internal damage of rocks under loading can be reflected by the AE activities [31]. Hence an AE data acquisition system (PCI-2 model) was adopted to detect the damage behavior of sandstone specimens under compression. Using vaseline as a coupling agent, two Nano30 sensors were attached on both sides of the specimen, and gently affixed with cellulose tape, as shown in Figure 2. The AE signals measured in the sensors were amplified by pre-amplifiers with a gain of 40 dB. To avoid the possible noise interference caused by the test equipment and surrounding environment, the recording threshold level was set at 45 dB. During the test, the loading system, DIC system and AE system were started synchronously so as to reveal the relationships between the stress-time curves and surface strain fields and internal AE events in real time.