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.