# Rock Fragmentation Characteristics by TBM Cutting and Efficiency under Bi-Lateral Confinement
## 3 Results  
### Fractured Surface Morphology Analysis  
Taking the fractured surface generated by the cutting of disc cutters of TBM at the confining pressure of 10-20 MPa as an example, a typical fractured surface is shown in Figure 5, in which the direction of the disc cutters' axis is indicated in the \(X\)-direction, the \(Y\)-direction is parallel to the grooving direction, and the \(Z\)-direction shows the penetration direction. Figure 5 shows the surface depth measured using an uncrushed original surface, which is marked with white and zero depth. In addition, dark blue represents the deep surface, and the maximum depth is 30 mm. It can be seen that the fractured surface is mainly located within a range enclosed by the adjacent grooving and surface of an intact rock. The deepest point of the typical fractured surface is located roughly in the center, while its depth decreases from the middle to the edges. However, the variations in depth lead to the formation of small islands and partial deep pits. 1.1 Variations of Fractured Surface with Increase of Major Confining Pressure while Minor Confining Pressure is Constant  
Figure 6 illustrates the typical variation of a fractured surface, which \(\sigma_{2}\) increases after conducting two penetrations, while \(\sigma_{1}\) is constant (5 MPa). Figure 6a shows a fractured surface when \(\sigma_{2}\) is 5 MPa. The deepest point is located in the middle; once the TBM disc cutters are sequentially penetrated, the maximum depth is measured 11.9 mm. Additionally, an island is formed, and the deepest point of the fractured surface is on both sides of the island. The volume of the fractured surface is roughly 25.634 mm\({}^{3}\), which is calculated by the built-in software. The fractured surface after two penetrations is small, indicating that the breaking effect under the confining pressure is not remarkable. Furthermore, \(\sigma_{2}\) increases to 10 MPa. As shown in Figure 6b, the yellow part has a depth in the range of 12 to 16 mm, which has been increased in comparison with Figure 6a. Furthermore, it is revealed that the bottom of the fractured surface contains a high peak and a low pit in the \(X\)-direction, in which the maximum depth and volume are about 16.2 mm and 33.682 mm\({}^{3}\), respectively. Increases in the maximum depth and volume indicate that the rock breaking tends to grow along the depth direction when \(\sigma_{2}\) increases from 5 to 10 MPa, and the effect of rock breaking under 10 MPa confining pressure is improved as well. As \(\sigma_{2}\) increases to 15 MPa, the size of the fractured surface in the \(Y\)-direction increases to 80 mm, as displayed in Figure 6c. An island is formed in the middle of the fractured surface, in which the top of the island has a depth of 15 mm. The maximum depth of the fractured surface reaches 22.1 mm. The bottom of the fractured surface is relatively flat, with a depth of 15 mm. Meanwhile, the volume of the fractured surface increases sharply to 61.370 mm\({}^{3}\). This indicates that when \(\sigma_{1}\) is 5 MPa, the crushing amount of rock mass between disc cutters is increased 1.8 times once \(\sigma_{2}\) increases from 10 to 15 MPa; when \(\sigma_{2}\) increases to 20 MPa, the shape of the fractured surface is similar to the counterpart. When \(\sigma_{2}\) is 15 MPa, the maximum depth is 21.8 mm, and the volume decreases slightly to 58.768 mm\({}^{3}\). When \(\sigma_{2}\) increases to 25 MPa, as depicted in Figure 6e, the fractured surface represents a typical spherical morphology: the middle is low and the depth gradually decreases towards the edges. The maximum depth and volume reach 27.1 mm and 87.031 mm\({}^{3}\), respectively. It can be seen that the breaking efficiency of disc cutters of TBM is at the maximum when \(\sigma_{1}\) and \(\sigma_{2}\) are 5 and 25 MPa, respectively. Based on the achieved results, with the increase of \(\sigma_{2}\), the rock is broken more effectively after performing two penetrations. Figure 5: A typical fractured surface.