## 5 Tunnel Excavation
### Tunnel Boring Machine Excavation  
In the area with a high overburden, the conventional excavation method for the headrace tunnel was replaced, in order to expedite the excavation and construction of the project, for approximately 20 km in Lot C2. However, the lack of foresight regarding geological problems was disastrous for TBM-based tunneling at this stage of the project owing to the high overburden. Two rock behaviors were scrutinized under the high-stress environment along the longitudinal section of the headrace tunnel, i.e., (1) competent brittle rock (rock burst) and (2) weak rock (squeezing). In the planning stage of tunneling using TBMs for excavation, the primary issues were the squeezing probability of up to 500 mm, as was indicated for the weak rock mass, and rock bursting in brittle rock under a high-stress environment. Similar concerns were also encountered during the construction of the Gotthard Base Tunnel, where the project was successfully finalized using open (gripper) TBMs [42]. Similar machines were chosen for this project, which was suited for the aforementioned possible behaviors. The two TBMs, named TBM 697 and TBM 696, excavated a considerable portion of the twin headrace tunnels, and they ascended from the southwest to northeast directions at a slope of 0.8% upstream. The two tunnels were designed in parallel with a modified separation distance of 55 m, which was originally 33 m [17; 25]. The purpose of this modification was to reduce the induced stress in the inter-tunnel pillar. For both of the TBMs, the support installation equipment facilities were present at location L1. This section was 6 m behind the cutter head shield, as shown in Figure 6. Section L2 was 60 m after section L1 and it featured an additional facility for shotcrete spray robots and rock bolting. Figure 6: Open gripper tunnel boring machine. Initially, both machines were facilitated with two separate and independent systems for performing advance exploration, which are listed below [42]:  
1. Tunnel Seismic Tomography System
2. Probe drilling  
After the unexpected high horizontal stress prediction that substantially contributed to the severity of the rock bursts, two more investigation systems were added in the areas with frequent occurrences of rock bursts in order to measure the following: (1) the uniaxial compressive strength of the rock for rock burst prediction and (2) microseismic monitoring. Overcoring tests were conducted for measuring the in situ stress in this area to understand the in situ stress environment and avoid rock bursts [37]. When considering the stability after excavation, the main difference between the conventional and mechanical/TBM excavation approaches are the disturbances that they create in the rock mass at the periphery of the excavation site. In mechanical excavation, the disturbance in the rock mass is minimal, as compared to that in conventional excavation [9; 43]. As the tunnel depth increases, the number of problems that are associated with high stresses also increases in TBM excavation [44]. At shallow depths, conventional excavation can cause undesirable effects, such as rock mass quality degradation, over excavation, and blast vibration [45]. However, the negative effects (rock mass degradation) are actually helpful when tunneling through hard rocks at an overstressed depth. In this situation, the fractures that formed by blasting create a protected area through the migration of possible stress concentration away from the surrounding areas of the excavation, thus reducing the risk of rock bursts [46]. This is one of the reasons why rock bursts were rarely observed during the construction of the NJHEP. However, this conventional excavation approach requires the optimization of each activity in the tunnel construction cycle in order to accelerate the rate of advancement [47]. Table 4 lists the progress achieved through TBM-based tunneling and the conventional method of tunneling in Lot C2.