## 4 Results  
The results for all cases show that a microcracking or breakout zone, consisting of micro-tensile and micro-shear cracks, was formed between the side of the borehole and its adjacent free space due to rock splitting process. In Figure 12, the micro-tensile and micro-shear cracks generated due to rock splitting can be seen in black and red colors respectively. The grey color in the breakout zone represents the displacement vectors in the model. When the split-wedge system started splitting the sides of the borehole, micro-shear cracks were formed around the periphery of the borehole. These micro-shear cracks did not propagate further than a few centimeters from the wall of borehole. Afterwards, the micro-tensile cracks started propagating horizontally from the tip of the counter-wedge towards the adjacent free space. This horizontal propagation of micro-tensile cracks can be viewed as a result of the thrust from the counter-wedge. In most of the cases, the propagation of the micro-tensile cracks created a primary fracture surface that separated the breakout zone from the rock material. In some cases, however, one or more secondary fracture surfaces were also observed to either emanate from the primary fracture or being developed individually due to the rock splitting action. The depth of breakout zone did not seem to vary with the borehole spacing and ranged between 1 m and 1.4 m. The maximum values of borehole spacing for which the fracture surface reached the adjacent free surface in all cases of modulus ratio for soft rock, normal rock, and hard rock are presented in Figure 13. Since, the modulus ratio concept is not considered in typically used vales of maximum borehole spacing, a comparison between the numerical analysis results and the typically used values of borehole spacing can be useful. The numerical analysis results show that the maximum borehole spacing values for all three types of intact rock using low modulus ratio were slightly less than the typically used values of borehole spacing. However, in the cases of average modulus ratio and high modulus ratio, much greater values  
Figure 12: Illustration of rock splitting process through crack propagation in soft rock (in yellow); at average modulus ratio, (**a**) initial rock splitting model (0.5 m borehole spacing); (**b**) final rock splitting model (1.1 m borehole spacing). of maximum borehole spacing are proposed to be used in all types of intact rock as compared to the typically used values. The maximum borehole spacing tends to decrease with the increase in UCS value of intact rock because the crack formation due to the counter-wedge pressure on the borehole walls becomes easier as the strength of intact rock decreases, since the in-situ stresses are not involved in the modeling process. Whereas, the maximum borehole spacing increases with the increase in modulus ratio due to the brittleness of rock because the increase in Young's modulus of intact rock increases the strain energy stored in it which results in the brittle nature of rock. Furthermore, the breakout zones formed in all of the three intact rock types were also investigated by decreasing the depth of drilled boreholes. As shown in Figure 14a-c, respectively, the borehole depth values of 1.4, 1.3 and 1.2 m were simulated in order to investigate the maximum borehole spacing for all cases of modulus ratio in soft, normal, and hard rock using the splitting width of 20 mm. The maximum values of borehole spacing in all three types of intact rock obtained by using the borehole depth values of 1.4, 1.3 and 1.2 m were recorded and the results obtained by using low modulus ratio, average modulus ratio, and high modulus ratio are presented separately in Figures 15-17, respectively. The effect of increase in modulus ratio was observed to be overall greater than that of decrease in UCS on the increase in maximum borehole spacing. Therefore, it is proposed to consider the effect of modulus ratio of intact rock during the selection of borehole spacing in any rock strength.