What is tunnel lining?  Tunnel lining refers to the permanent support structures constructed along the periphery of a tunnel or underground cavity to prevent deformation or collapse of the surrounding rock or soil (known as the "enclosure"). This technique is commonly applied in tunnel engineering and hydraulic channels.

   Types of tunnel lining:

   Depending on geological conditions and design requirements, tunnel linings might be:

   ① Temporary linings: Installed immediately after excavation to stabilize the tunnel until a more permanent solution can be applied.

   ② Primary linings: Provide initial support to limit deformations.

   ③ Permanent linings: Serve as the final, enduring support and finish for the tunnel.

   Why is tunnel lining necessary? The primary purpose of tunnel lining is to provide structural stability and safety. It serves several critical functions:

   1. Structural Stability:

   After excavation, the surrounding rock or soil can be unstable. The lining reinforces the tunnel, distributing and bearing the loads from both the ground and the tunnel itself, which is crucial for preventing deformations or collapse.

   2. Waterproofing and Protection:

   Linings prevent water infiltration that could weaken the structure or lead to deterioration. They also shield the tunnel from aggressive soils, corrosion, and environmental wear, thereby extending its service life.

   3. Functional and Aesthetic Requirements:

   Besides structural functions, linings help maintain the desired cross-sectional shape of the tunnel, ensure a smooth interior for vehicular or rail traffic, and contribute to overall safety and operational efficiency.

   Materials used for tunnel lining can vary depending on the specific requirements and conditions of the project. Common materials include:

   ① Concrete and Reinforced Concrete: These are widely used due to their strength and durability.

   ② Masonry: Traditional methods involve using bricks or stones.

   ③ Shotcrete: A mixture of concrete sprayed onto the tunnel surface, sometimes reinforced with a steel mesh.

   ④ Cast Iron or Steel: Used in some specialized applications.

   ⑤ Other Metals: Occasionally employed based on specific needs.

   The design and construction of tunnel lining must consider factors such as the type of enclosure, water conditions, and geological characteristics. Properly executed tunnel lining is essential for ensuring the long-term safety and functionality of tunnel structures.