## 4 Characteristics of Loose Zone in Yuntunpu Tunnel
### Analysis of Loose Zone Test Results  
Four test sections were selected for the typical weak surrounding large rock deformation section of the Yuntunpu Tunnel test section (Section 1: D6K234 + 309, Section 2: D5K231 + 948, Section 3: D5K226 + 824.4, Section 4: D5K226 + 824.4). Each section is arranged with a measuring point on the left and right side walls, with a drilling depth of 20 m and an aperture of 100 mm. Due to construction site conditions, only one measuring point is arranged on the left side wall of the tunnel for Section 3. The test results are shown in Figures 7-13. In the figure, L represents the drilling depth, and V\({}_{\rm p}\) represents the elastic longitudinal wave velocity of the surrounding rock. (1) Analysis of Test Results for Section 1  
The test results of Section 1 are shown in Figures 7 and 8. The construction method of this experimental section adopts two-step excavation. According to the variation of surrounding rock wave velocity with drilling depth, it can be found that the 7\(\mu\) small mileage direction face has severe water intrusion due to the right drilling hole. Therefore, the left side wall is mainly used as the main reference value for analyzing data. The wave velocity of the surrounding rock on the left side wall undergoes a drastic change of around 12 m. Based on the proposed criteria for evaluating the loose zone of the surrounding rock, it is comprehensively determined that the range of the loose zone is around 12 m.  
Figure 8: Variation of wave velocity of surrounding rock of right wall-drilling depth (D6K234 + 309). Figure 7: Variation of wave velocity of surrounding rock of left wall-drilling depth (D6K234 + 309). (4) Analysis of Test Results for Section 4  
The test results of Section 4 are shown in Figures 12 and 13. According to the variation of surrounding rock wave velocity with drilling depth, it can be found that the testing process of the drilling hole set in the direction of 5\(\mu\) small mileage is relatively smooth. Based on the comprehensive comparison of the wave velocity variation characteristics of the surrounding rock on both sides of the side walls and the proposed evaluation criteria for the broken rock zone, the range of the loose rock zone caused by tunnel excavation is approximately 14 m.  
Figure 11: Variation of wave velocity of surrounding rock of right wall-drilling depth (D5K226 + 824.4). Figure 12: Variation of wave velocity of surrounding rock of left wall-drilling depth (D5K225 + 620). Figure 13: Variation of wave velocity of surrounding rock of right wall-drilling depth (D5K225 + 620). (5) Summary and analysis of loose zone test results  
The summary of the loose zone test results is shown in Figure 14. From Figure 14, it can be seen that the loose zone of the Yuntunpu Tunnel ranges from 12 to 14 m. Affected by the grade and structure of the surrounding rock, the wave velocity variation characteristics exhibited by different grades of surrounding rock vary greatly. The wave velocity increases and decreases with increasing drilling depth for weak rock masses. For harder rock masses, the wave velocity shows a trend of first improving and then stabilizing with increasing drilling depth. The difference in the trend of changes between soft rock and hard rock is mainly due to the significant impact of stress concentration caused by soft rock tunnel excavation on the wave velocity of the rock mass. The tunnel has a wide range of loose zones, and severe large deformation problems occurred during construction. Therefore, to prevent the development of loose zones, it is necessary to implement a collaborative approach of long and short anchor rods to actively control the development of the loose zones and the deformation of the surrounding rock. After excavating the upper bench, short anchor bolts should be promptly applied and prestressed to form a \"bearing ring\" in the shallow surrounding rock.