I. Tunnel Bore Excavation Methods
1) Main tunnel: Grade V surrounding rock adopts the two-bench ring excavation with reserved core soil method; Grade IV surrounding rock adopts the two-bench method; the widened section of Grade IV surrounding rock adopts the two-bench ring excavation with reserved core soil method; Grade III surrounding rock adopts the full-face method. The last 15 m before the left and right tunnel portals are broken through using mechanical cold excavation.2) Auxiliary pilot tunnel: As the pilot tunnel has a small cross-section but shallow cover, the two-bench method is adopted, and its excavation method may follow that for Grade IV surrounding rock.3) Internal cross passage: As the cross-section is small but the cover is deep, the full-face method is adopted.
II. Excavation Principles1) No excavation without advanced geological prediction.2) Excavate one section and support it immediately; it is strictly prohibited to complete excavation without support.III. Excavation and Support Sequence and Step-Distance Requirements
1) Excavation and support sequence for the reserved core soil method: 1. Ring excavation of the upper section I, II, III → 2. Install primary support of the upper section ① → 3. Excavate the core soil of the upper section IV → 4. Skip-sequence excavation of the lower section V → 5. Install primary support of the lower section ② → 6. Skip-sequence excavation of the lower section VI → 7. Install primary support of the lower section ③ → 8. Construct the invert ④ → 9. Backfill the invert ⑤ → 10. Cast the integral secondary lining ⑥.
2) Excavation step-distance principles: The reserved core soil length of the upper bench should be 3–5 m, and its width should be 1/3–1/2 of the tunnel excavation width. The cyclic advance of the upper-bench arched pilot excavation is determined by the spacing of the primary support steel frames (XS5b steel frame spacing 0.6 m), and shall not exceed 1.8 m.
3) Excavation and support sequence for the bench method: During construction, first excavate the upper section I, then install primary support ①, then skip-sequence excavate the lower section II and install the corresponding primary support ②, construct the invert lining and invert filling ③④, and after the primary support tends to stabilize, construct the arch-wall secondary lining ⑤.
4) Excavation step-distance principles: (1) The upper bench excavation height is 6.7 m, and the lower bench excavation height is 3.0 m; (2) The longitudinal length of the upper bench is 20 m; (3) The invert lining is 30 m from the upper bench; (4) The arch-wall secondary lining is 40 m from the invert lining.
5) Excavation and support sequence for the full-face method: During construction, first excavate the full face I, then install primary support ①, and after the primary support tends to stabilize, cast the integral secondary lining ②.
6) Full-face excavation step-distance principles: (1) The excavation of the working face per cycle shall not exceed 3 m; (2) According to the Technical Specifications for Construction of Highway Tunnels (JTG/T 3660-2020): in sections without invert, the central drainage ditch should preferably be excavated by cutting; however, generally to save one process, it is excavated simultaneously with the working face. Since the specification uses "should preferably," this is not a mandatory requirement here; the method adopted is mainly determined by the subcontractor's construction organization; (3) After muck removal is completed, wire mesh and shotcrete should be applied immediately; it is strictly prohibited to apply wire mesh and shotcrete only after two or more excavation cycles; (4) The distance from the arch-wall secondary lining to the working face shall strictly not exceed 120 m, and shall be controlled at 100 m during construction to ensure construction safety.
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