## 3 Experimental Results and Theoretical Analysis  
### Mechanical Properties  
Before the uniaxial compression experiment, the height and loading area of the specimen are obtained directly by using vernier calipers. Moreover, the axial force and the corresponding height increment of the specimen can be obtained directly by using the Shimadzu I-250 (Japan) testing system. These data were processed to obtain the axial stress-strain curves of selected samples with different flaw inclination angles, as shown in Figure 4b-d. The experimental results showed that the failure process of the specimens under uniaxial compression passed through the stages of original microcrack closure, microcrack nucleation, macrocrack propagation and coalescence. With increasing axial stress, the axial stress-strain curve shows an obvious stress drop phenomenon in Figure 4b-d. According to  
Figure 3: Experimental equipment and procedure. the photographic monitoring results, every stress drop phenomenon corresponded to the initiation or unstable propagation of macroracks and was accompanied by a crackling noise. When the first stress drop phenomenon of selected samples occurred during the compression failure process, the selected samples did not produce a penetrating failure surface. Furthermore, the selected samples continued to be loaded, we found that they could still bear axial stress, which showed that the selected samples were not completely destroyed. Figure 4b-d shows that there are two stress drops before the final failures of the specimens, and we infer that the axial stresses of the specimens may decrease suddenly during the loading process near crack initiation and crack coalescence. From Figure 4b-d, points 1 and 2 on the axial stress-strain curves are stress drop points, and the stress and strain values of stress drop point 1 are the initial crack initiation stress and initial crack initiation strain, respectively. The stress and strain values of stress drop point 2 are the peak strength and peak strain, respectively. The corresponding stress and strain values of stress drop points 1 and 2 are calculated, as shown in Table 1. When the corresponding stress value of a point on the stress-strain curve is 20 MPa, the tangent modulus of this point is defined as the elastic modulus (E) of the specimen in this paper. According to Table 1, the initial crack initiation stress and peak strength gradually decrease with increasing flaw inclination angle. After comparing and analyzing the stress and strain values of stress drop points 1 and 2, we found that the initial crack initiation stress is approximately 90-98% of the peak strength, and the initial crack initiation strain is approximately 93% of the peak strain. Therefore, we infer that the selected samples usually exhibited macrocrack initiation at 90% of peak strength. From Figure 4b-d, point 3 on the axial stress-strain curves is the final failure point of selected samples. The stress value of point 3 on the axial stress-strain curve is not zero, which indicates that the failure specimen still has a certain bearing capacity. The final crack patterns of the specimens are shown in Figure 5. In Figure 5, \"T\" and \"S\" represent tensile cracks and shear cracks, respectively. The blue color represents sandstone grains and the white color represents cracks. From Figure 5a-b, crack coalescence between en echelon flaws is mainly produced by the linkage of two tensile cracks. From Figure 5c, the connection of tensile cracks and shear cracks between en echelon flaws produces a fault. Combining this information with the results of photogrammetric monitoring, the crack initiation between en echelon flaws occurred at the time of stress drop point 1. When the axial stress gradually increased to the peak strength, crack coalescence between en echelon flaws began to occur, so the crack further developed into a larger macrocrack.