What are the classification of tunnel lining? Tunnel lining is a critical structural component that ensures stability, safety, and durability in underground constructions. Its classification depends on materials, structural design, construction methods, and functional requirements. Below are the primary classifications:

  1. By Material

   ① Concrete Lining: The most common type, used for its compressive strength and durability. It includes plain concrete and steel-fiber-reinforced concrete (SFRC) for enhanced crack resistance.

   ② Reinforced Concrete (RC) Lining: Incorporates steel rebars to handle tensile stresses, ideal for tunnels in seismically active zones or under heavy loads.

   ③ Shotcrete Lining: Applied pneumatically for rapid support, often used in the New Austrian Tunneling Method (NATM). It may include additives like accelerators for quick setting.

   ④ Segmental Lining: Precast concrete or steel segments assembled by Tunnel Boring Machines (TBMs). These are modular and efficient for mechanized tunneling.

   ⑤ Sprayed Waterproof Membrane Lining: Combines shotcrete with waterproof membranes (e.g., HDPE sheets) to prevent water ingress in challenging hydrogeological conditions.

   2. By Structural Design

   ① Monolithic Lining: Cast in situ as a continuous structure, providing uniform load distribution. Common in shallow tunnels with stable geology.

   ② Composite Lining: A dual-layer system consisting of a primary lining (e.g., shotcrete and rock bolts) for immediate support and a secondary lining (e.g., RC) for long-term stability.

   ③ Flexible Lining: Designed to accommodate ground deformations, often using yielding elements or compressible materials in soft ground or squeezing rock conditions.

  3. By Construction Method

   ① Cast-in-Place Lining: Built on-site using formwork, suitable for irregular tunnel shapes or custom designs.

   ② Precast Lining: Factory-manufactured segments (concrete or steel) installed by TBMs, ensuring high precision and speed in uniform cross-section tunnels.

   ③ Spray-Applied Lining: Utilizes robotic shotcrete application, particularly in drill-and-blast tunnels or rehabilitation projects.

   4. By Functional Purpose

   ① Load-Bearing Lining: Prioritizes structural integrity to withstand ground pressure, water pressure, and external loads.

   ② Waterproofing Lining: Focuses on impermeability, integrating membranes or chemical additives to prevent leakage.

   ③ Abrasion-Resistant Lining: Used in hydraulic tunnels or mining conveyances, often incorporating hard aggregates or steel fibers to resist wear.

   5. Specialized Linings

   ① Fire-Resistant Lining: Includes materials like calcium silicate boards or vermiculite concrete to withstand high temperatures in transit tunnels.

   ② Drainage Lining: Features built-in channels or porous materials to manage groundwater, reducing hydrostatic pressure.

  6. Based on Function and Timing

   Primary Lining: Installed immediately after excavation, this temporary or initial support minimizes deformations and stabilizes the tunnel face. Typical methods include:

   ①  Shotcrete (sprayed concrete)

   ②  Steel ribs or sets

   ③  Rock bolts and mesh reinforcement

   Secondary Lining: Applied once the ground is stabilized, the secondary lining provides the permanent support, water-tightness, and durability required for the tunnel’s service life. This is often achieved through:

   ①  Cast-in-situ concrete

   ②  Precast concrete segments

   ③  Composite systems combining different materials

   Tunnel lining selection depends on geological conditions, project requirements, and economic factors. Modern advancements, such as fiber-reinforced polymers (FRP) and smart sensors embedded in linings, further enhance performance monitoring and longevity. Engineers must balance technical feasibility, sustainability, and cost-effectiveness to optimize tunnel safety and functionality.