# Key Technology of TBM Excavation in Soft and Broken Surrounding Rock
## Appendix C Verification of matching relationship between blade spacing and vertical force
### Key Technology of TBM Extraction in Soft and Broken Stratum  
#### 4.5.1 Small Pilot Tunnel Excavation  
Start Conditions  
During TBM Construction in soft and broken strata, when the TBM cutter head is stuck, it is necessary to start a small pilot tunnel excavation to get rid of difficulties. On the one hand, the collapsed rock in front of the cutter head is removed through the small pilot tunnel to get rid of the cutter head. On the other hand, the pilot tunnel provides working space for pipe shed construction. This is embodied in the following two conditions:  
Figure 24: Site picture of grouting reinforcement effect of pipe shed.  
Figure 23: Construction photos of pipe shed construction site.  
1. The unfavorable geological section is long, and the TBM has been jammed. It is necessary to use the small pilot tunnel to clear the broken rock accumulated in front of the cutter head;
2. It has been confirmed by advance drilling that the surrounding rock in front is broken (RQD < 20), and the section is long, and the groundwater is relatively developed, so it is necessary to use small pilot tunnel pipe shed grouting to pre-strengthen the front stratum.  
Excavation and Support of Small Pilot Tunnel  
The small pilot tunnel is arranged at the main arch crown and enters from the opening between the two arches behind the shield tail. The clearance height in the small pilot tunnel is 1.3 m, the width of the arch is 1.2 m, and the length is 6.225 m. The small pilot tunnel is supported by chemical grouting peripheral consolidation + temporary support of square timber + HW150 type steel support frame + 140 mm channel steel longitudinal connection + foot locking anchor pipe + super front small conduit + shotcrete (depending on the surrounding rock). The structure of the small pilot tunnel is shown in Figure 25.  
Make use of the pilot tunnel to expand the excavation space \(\oslash\)76 and advance the pipe shed to reinforce the surrounding rock in front. The construction length of the pipe shed is determined according to the length of the unfavorable geological section in front, which is generally 25-30 m. For example, at the location of the main tunnel jammer, according to the revealed length of the horizontal guide crushing section and the advanced geological prediction, it is preliminarily judged that the length of the unfavorable geological section in front is 12-18 m, the construction length of the advance pipe shed is 25 m, the construction range is 67\({}^{\circ}\) of the arch, and the spacing is 40 cm. There are 16 pipes in total. The pipe shed shall be built in the expanded tunnel. According to the construction experience of the pipe shed in the geological section with a poor horizontal guide, the pipe shed can be built 30 cm below the outer contour surface of the TBM excavation (mainly considering the operating height of the drilling rig and personnel). Considering the influence of subsidence in the front of the pipe shed during the construction, the pipe shed shall be set at an angle of 1-3\({}^{\circ}\), and the pipe shed shall be divided into sections with a length of 1.5 m. The pipe sections shall be connected by sleeves, and the joints of adjacent pipe sheds shall be staggered. The staggered length shall not be less than 1.0 m, a 3 M slurry stop section shall be set at the end of the pipe shed, and plum blossom holes shall be arranged at other parts. For pipe  
Figure 25: Structure and physical drawing of small pilot tunnel.  
shed grouting, double-liquid slurry and cement slurry shall be injected first. If the slurry leakage is serious, chemical slurry shall be used instead. The layout of the pipe shed is shown in Figure 26.