Tunnel lining is a critical component of tunnel construction, providing structural stability, waterproofing, and durability. The choice of materials depends on factors such as geological conditions, tunnel purpose, design lifespan, and cost-effectiveness. Below are the primary materials used in modern tunnel lining:

   1. Concrete

   Concrete is the most widely used material for tunnel linings due to its strength, durability, and adaptability. Two main types are employed:

   ① Cast-in-place concrete: Fresh concrete is poured into formwork installed behind the tunnel excavation face. It creates a continuous lining that conforms to the tunnel’s shape and resists ground pressure.

   ② Precast concrete segments: These are factory-made segments transported to the site and assembled using tunnel boring machines (TBMs). They provide rapid installation and consistent quality, commonly used in soft-ground tunnels (e.g., urban metro systems).

   Reinforced concrete, incorporating steel rebars or fibers, enhances tensile strength and crack resistance.

  2. Steel

   Steel components are often integrated into tunnel linings for added structural support:

   ① Steel fiber-reinforced concrete (SFRC): Steel fibers mixed into concrete improve toughness and reduce cracking under dynamic loads (e.g., seismic activity or heavy traffic).

   ② Steel arches or ribs: Used in conjunction with shotcrete or lagging in drill-and-blast tunnels, these provide immediate support in unstable rock or soil conditions.

   ③ Steel plates or liners: Applied in pressurized tunnels (e.g., hydropower waterways) to withstand internal water pressure.

   3. Reinforcement and Fiber Additives

   ① Steel Reinforcement & Steel Fibers: Traditional rebar or steel fibers improve the tensile strength and ductility of the concrete, helping to control cracking and increasing resistance to impacts and fire-induced spalling.

   ② Polypropylene & Other Synthetic Fibers: These are added to shotcrete mixes to enhance durability, reduce plastic shrinkage, and improve resistance to explosive spalling during fires.

   4. Polymer-Based Materials

   Polymer-Based Liners and Membranes: Products like AGRUFlex use advanced polyolefins and high-grade resins to create flexible, waterproof membranes that can be applied either as sprayed layers or prefabricated sheets. Such systems are particularly useful in preventing water ingress and corrosion.

   5. Alternative and Fabricated Materials

   In some cases, materials such as stone concrete, rubble concrete, or even prefabricated cast iron blocks are used for lining specific tunnel sections. These alternatives are typically selected based on local material availability and project-specific engineering requirements.

   6. Specialized Materials

   ① Fire-resistant coatings: Intumescent paints or boards protect linings in transit tunnels during fire events.

   ② Corrosion-resistant alloys: Stainless steel or coated reinforcements are used in aggressive environments (e.g., saline groundwater).

   Each material plays a role in addressing the unique challenges of tunnel construction—whether it’s exploiting the strength of the surrounding rock with a thin shotcrete layer in NATM or ensuring rapid installation and long-term durability with precast segmental linings in TBM-driven projects.