## 4 Application of Middle Rock Pillar Classification Method
### Application of Middle Rock Pillar Classification Standard  
The scores of each index were evaluated using the existing survey data according to the site conditions of the project in the research section. From the borehole data, it can be seen that the integrity coefficient \(K_{\
u}\) was 0.74, and it can be seen from Table 6 that it was a relative integrity middle rock pillar. Using the integrity coefficient linearly corresponding to the score, \(T_{1}=(34-26)/(0.8-0.6)\times(0.74-0.6)+26=31.6\). The middle rock pillar consisted of granite strata. According to the survey report, the uniaxial compressive strength \(R_{c}\) was 93.61 MPa, and it can be seen from Table 7 that it is relatively hard rock. Through linear correspondence of the hardness degree to the score, \(T_{2}=(42-34)/(120-70)\times(93.61-70)+34=37.78\). The normal water inflow in the research area was 4.375 L/min \(\times\) 10 m, and the maximum water inflow was 13.19 L/min \(\times\) 10 m. It can be seen from Table 8 that the permeability score \(T_{3}\) was in the range of \(-4\) to 2, and \(T_{3}\) was taken as \(-4\). The minimum width of the middle rock pillar in the research section was 1.6 m, and the span of the mainline and ramp A was 14 m. Therefore, the width-span ratio at the most dangerous location was W/B = 0.11. As shown in Table 10, \(T_{4}\) was \(-16\). According to a survey report, the main joint direction of the research section was N10-39 \({}^{\circ}\)E/65-85 \({}^{\circ}\)S. The included angle between the structural plane and the left tunnel axis was 0\({}^{\circ}\)-30\({}^{\circ}\), and the included angle between the structural plane and the right tunnel axis was 60\({}^{\circ}\)-90\({}^{\circ}\). The dip angle of the structural plane was 60\({}^{\circ}\)-90\({}^{\circ}\). Therefore, the comprehensive evaluation score was 0.60. As shown in Table 11, \(T_{5}\) ranged from \(-5\) to \(-3\), and \(T_{5}\) was taken as \(-5\). To calculate the initial stress score of the middle rock pillar, the initial strength-stress ratio should first be determined according to Equation (3). According to the survey report, \(R_{c}=93.61\) MPa, the gravity of granite formation \(\gamma\) was 26 kN/m\({}^{3}\), and the burial depth of the middle rock pillar was H = 100 m, namely, \(\sigma_{\max}=\gamma\cdot\)H = 26 \(\times\) 100 = 2600 kN/m\({}^{3}\); thus, the initial strength-stress ratio was \(S=R_{c}/\sigma_{\max}=93.610/2600=36.00\). According to Table 12, it belongs to the low-stress state, and the corresponding score \(T_{6}=-2\). Based on the above analysis, the total score \(T\) and the corresponding grade of the middle rock pillar can be determined, as listed in Table 15.  
According to Table 13, the middle rock pillar is classified as Grade III. However, moderately weathered granite was present in this area, and its uniaxial compressive strength \(R_{c}\) was 60-70 MPa. After calculation, it belonged to Grade IV, that is, the total middle rock pillar in this section belonged to Grade III-IV surrounding rock, which was lower than the Grade III surrounding rock in the original design scheme. The reason for the decrease in the grading was that the low strength of the moderately weathered granite area in this section and the minimum width-span ratio (0-0.3) evaluated belonged to the range of great mutual influence, so the middle rock pillar in this section belonged to Grade IV. With the excavation of the two tunnels, the clear distance gradually increased, and the width-span ratio also increased. When the width-span ratio was greater than 0.3, the interaction between the two tunnels decreased, and the deducted score decreased. The grade of the middle rock pillar increased to Grade III. Therefore, relevant measures such as reducing footage, reducing the staggered distance between two tunnels, and increasing temporary support measures should be taken in the Grade IV section to ensure the construction safety of medium rock inclusion and play an early warning role in the construction to avoid potential safety hazards.