# Rock Fragmentation Characteristics by TBM Cutting and Efficiency under Bi-Lateral Confinement
## 3 Results
### Crack Development between Disc Cutters  
2.1 Variations of Crack Development with Increase of Major Confining Pressure while Minor Confining Pressure is Constant  
As shown in Figure 8a, there is no obvious crack development in the cross section between two plastic deformation zones when the confining pressure is 5 MPa. The fractured surface contains an approximate triangular shape, and the breaking surface is straight and flat. A crack could develop due to the combined action of shear failure and surface tension. Simultaneously, a crack with a certain deflection angle appears below each plastic deformation zone, and the broken area is relatively small. As \(\sigma_{2}\) increases to 10 MPa, the shape of the fractured surface varies and becomes nearly semilunar-shaped with an uneven depth, as illustrated in Figure 8b. At the same time, when \(\sigma_{2}\) is 5 MPa, the pit is slightly deeper. A crack starts to grow in a plastic defamation zone, and then crack propagation forms, leading to an interconnected crack between the two plastic deformation zones. Additionally, when \(\sigma_{2}\) increases to 15 MPa, the depth of the fractured surface increases slightly, the crack initiation angle decreases somewhat, and the deflection angle increases sharply. Accordingly, breaking is simultaneously made by the internal tensile cracks. When \(\sigma_{2}\) increases to 20 MPa, the shape of the fractured surface is similar to the surface when the confining pressure reaches 5-10 MPa, as depicted in Figure 8d. The fractured surface is also likely to be formed by internal tensile cracks. However, the level of crack development is low, and there are only two internal cracks besides the cracks formed between the disc cutters. When \(\sigma_{2}\) increases to 25 MPa, the shape of the fracture's surface is similar to the surface created by those two types of confining pressures, as shown in Figure 8e. However, it can be seen that when the depth is large, the crack development in the plastic deformation zone is not clear enough. Moreover, a new type of crack is observed that does not originate with the plastic deformation zone below the disc cutters. The cracks are randomly distributed within the rock with varying lengths. This type of crack has been reported in a previous study performed by [PERSON] et al. [29] as well, and leads to breaks. Figure 8: Crack development in the cross section with increase of \(\sigma_{2}\), while \(\sigma_{1}\) is 5 MPa. 2.2 Variation of Crack Development with Increase of Minor Confining Pressure while Major Confining Pressure is Constant  
When \(\sigma_{2}\) is equal to 20 MPa, the variation of the crack growth with the increase of \(\sigma_{1}\) is displayed in Figure 9. When \(\sigma_{1}\) is equal to 5 MPa, the profile of the fractured surface is low in the middle and very close to the edges. When \(\sigma_{1}\) increases to 10 MPa, the shape is roughly similar to when \(\sigma_{1}\) is only 5 MPa, as shown in Figure 9b. Although the lowest point is deeper than the former situation, the contour is deeper when the confining pressure is 5-20 MPa. Additionally, there is an obvious crack connecting two plastic deformation zones below the fractured surface. Therefore, when the confining pressure is 10-20 MPa, the increase in \(\sigma_{1}\) leads to a decrease in crushing depth, preventing the propagation of the cracks from the top to the bottom, and resulting in shallow and small fractured surfaces. Figure 9c illustrates that when \(\sigma_{1}\) increases to 15 MPa, the range of crack development is larger than that of the confining pressure of 10 and 20 MPa. In addition, the fractured surfaces between disc cutters are clearly varied. These showed similar shapes--low in the middle and high towards the edges--and there are obvious internal cracks, directly propagating. Therefore, the increase in \(\sigma_{1}\) prevents crack development in the depth direction, resulting in a decrease in depth of the fractured surface as well.