In the previous article, we systematically introduced tunnel secondary lining disease detection methods ranging from visual inspection to ground-penetrating radar and ultrasonic testing, which is akin to conducting a comprehensive "physical examination" for the tunnel. After accurately "diagnosing" the location and nature of diseases through precise detection, the next crucial step is to "prescribe the right medicine" and adopt scientific and effective measures for treatment. As the final installment of this series, this article focuses on specific treatment methods for various tunnel secondary lining diseases, aiming to restore structural health and ensure the long-term safety of the tunnel.

1. Treatment Methods for Cracks

(1) Surface Sealing Method: Suitable for cracks with small widths. This method involves applying waterproof coating or pasting waterproof membrane on the crack surface to prevent moisture from entering and further expanding the crack. It features simple construction and low cost but has limited effectiveness for wider cracks.

(2) Grouting Method: Applicable to cracks with larger widths. By injecting materials such as cement slurry or epoxy resin into the cracks, this method fills the gaps and enhances the integrity and strength of the concrete. It has complex construction processes and higher costs but offers excellent treatment results.

(3) Structural Reinforcement Method: Used for cracks that seriously affect tunnel structural safety. Common approaches include pasting carbon fiber sheets and adding reinforced concrete arch sleeves. This method involves high construction difficulty and cost but effectively improves the bearing capacity and safety of the tunnel structure.

2. Treatment Methods for Water Leakage

(1) Drainage Method: Ideal for areas with severe water leakage. It involves installing drainage grooves or pipes on the secondary lining surface to divert leaking water outside the tunnel, reducing erosion on the secondary lining structure. The method is simple to construct and low-cost but fails to address the root cause of water leakage.

(2) Grouting Method: Targets the source of water leakage. By injecting materials like cement slurry or polyurethane into the leakage source, it blocks water seepage channels to achieve water stopping. It has complex processes and higher costs but delivers effective water leakage control.

(3) Waterproof Coating Method: Applied to the surface of the tunnel secondary lining. It forms a waterproof layer by brushing waterproof coating, preventing moisture from penetrating the secondary lining structure. The method is simple and low-cost but requires high quality of the waterproof coating to avoid issues such as peeling.

3. Treatment Methods for Spalling

(1) Cleaning and Repairing Method: Suitable for areas with small spalling areas. First, loose concrete in the spalled area is thoroughly cleaned, then repaired with materials like cement mortar or epoxy resin to restore the surface flatness of the secondary lining structure. It has simple construction and low cost but is ineffective for large spalling areas.

(2) Structural Reinforcement Method: Used for large spalling areas or those seriously endangering tunnel structural safety. Methods include pasting carbon fiber sheets and adding reinforced concrete arch sleeves. Despite high construction difficulty and cost, it effectively enhances the bearing capacity and safety of the tunnel structure.

4. Treatment Methods for Cavities

(1) Grouting Filling Method: Applied to cavity areas. By injecting materials such as cement slurry or cement mortar into the cavities, this method fills the voids and improves the compactness and strength of the secondary lining structure. It involves complex construction and higher costs but achieves good treatment effects.

(2) Local Demolition and Reconstruction Method: Used for cavity areas that severely affect tunnel structural safety. It entails demolishing the secondary lining concrete in the cavity area and recasting concrete to restore the integrity of the secondary lining structure. This method has high construction difficulty and cost but completely resolves cavity issues.

In summary, a mature and targeted technical system for treating various tunnel secondary lining diseases—including cracks, water leakage, spalling, and cavities—has been established. From surface sealing and grouting filling to structural reinforcement, each method has specific application conditions and repair objectives. Successful treatment relies on accurate judgment of disease causes and precise control of construction processes. In practical engineering, one or more methods are often flexibly combined based on specific conditions to achieve optimal repair results. Through the scientific application of these treatment methods, we can effectively eliminate potential safety hazards of tunnel structures, significantly extend their service life, and provide solid guarantee for the safe and smooth operation of transportation infrastructure.