## 3 Test Results and Discussion
### Axial Stress-Strain Response  
The typical axial stress-strain curves of intact specimen and specimens containing a square hole are shown in Figure 4. It is obvious that the mechanical properties of specimens with a square hole under uniaxial compression are significantly lower than that of the intact specimen, and the reduction extent is affected by the hole inclination angle. The axial stress-strain behavior of sandstone specimens can be approximately divided into four typical stages, including the initial pore compaction stage, elastic deformation stage, crack growth stage and post-peak stage.  
At the stage of crack closure, the stress-strain curves of the specimens show an initial downward concave shape and nonlinear deformation at low stress levels, resulting from the closure of initial micro-defects in rock under compressive squeeze. The initial elastic modulus (the slope of the tangent passing through the origin on the stress-strain curve) of Int, Sq-0-A and Sq-45-A are 3.24 GPa, 2.22 GPa and 1.44 GPa, respectively, and the nonlinear deformation section of the specimen Sq-45-A is more obvious than that of other specimens, which indicates that the pore compaction behavior is not only affected by the existence of prefabricated holes but also by the hole shape. Once the majority of  
Figure 4: Typical stress–strain curves of intact specimen and specimens containing a square hole.  
Figure 3: Effect of heterogeneity on axial stress–strain curves of sandstone specimens under uniaxial compression.  
these micro cracks or pores has been closed, the axial stress grows linearly with the increase in the strain, indicating that the stress-strain curve turns into the elastic deformation stage. Even though the responses of these pre-holed specimens are mainly elastic deformation, some primary tensile cracks around the square hole begin to initiate and stably propagate in this stage. As the increase of uniaxial loading, the stress-strain curves deviate from the elastic behavior and show nonlinear deformation, which is mainly caused by the secondary crack initiation, propagation and coalescence. At the post-peak stage, many macro cracks around the hole grow rapidly, accompanied by stress adjustments inside the pre-holed specimen, resulting in step-like stress fluctuations. For the intact specimen, the axial stress drastically drops to 0 MPa within 0.06 s after the peak stress, and brittle splitting failure occurs during the stress dropping, accompanied with a loud sound.  
The effect of filling mode and hole inclination angle on the stress-strain behavior of sandstone specimen is shown in Figure 5. It can be seen that the stress-strain behavior of the filled specimens is similar to that of the unfilled specimens. They all show plastic-elastic-plastic deformation characteristics before the peak stress. However, for the filled specimen, its mechanical properties are improved compared to the unfilled specimen, and there is still a residual strength after rock failure, which means that the specimen with inclusions remains a certain bearing capacity even after the failure of rock matrix. In addition, under the same filling mode, the mechanical properties of the pre-holed specimens with \(\alpha=0^{\circ}\) are better than those with \(\alpha=45^{\circ}\).