Tunnel Linings

Definition:

Tunnel linings are the internal walls or structures constructed within a tunnel to provide support, reinforcement, and protection against various external factors, including ground pressure, water infiltration, and potential deformations.

Uses/benefits:

The uses and benefits of tunnel linings include:

Structural Support: Tunnel linings offer structural support to the tunnel, distributing the loads from the surrounding ground and preventing excessive deformation or collapse.
Ground Stabilization: Linings help stabilize the surrounding ground by resisting lateral pressures and controlling potential ground movement, reducing the risk of instability.
Water Management: Linings act as a waterproof barrier, preventing water ingress into the tunnel and controlling groundwater seepage, which can otherwise compromise the stability and functionality of the tunnel.
Protection against Corrosion: Linings provide a protective layer against corrosive elements, such as groundwater with high salt content, chemicals, or gases, thereby increasing the lifespan of the tunnel structure.
Enhanced Safety: Tunnel linings improve the safety of the tunnel environment by reducing the risk of rockfalls, erosion, or other hazards, ensuring the protection of both users and infrastructure.
Aesthetic Appeal: In some cases, tunnel linings are designed with aesthetic considerations, enhancing the visual appearance of the tunnel and blending it with the surrounding environment.

Specifications:

The specifications for tunnel linings depend on several factors, including geotechnical conditions, tunnel diameter, anticipated loads, expected service life, and specific project requirements. Some common specifications include:

Material Selection: The choice of materials for tunnel linings can vary, including cast-in-place concrete, precast concrete segments, shotcrete, reinforced concrete, or tunnel boring machine (TBM) rings.
Thickness and Reinforcement: The thickness of the lining and the reinforcement requirements depend on the anticipated loads and ground conditions. Reinforcement elements such as steel bars or fibers may be incorporated to enhance strength and control cracking.
Waterproofing: Specifications may include provisions for waterproofing systems, such as membranes, coatings, or grouting, to prevent water infiltration and ensure a watertight tunnel.
Fire Resistance: For tunnels that may be exposed to fire hazards, specifications may include requirements for fire-resistant materials or additional fire protection measures to enhance the safety of the tunnel.
Surface Finish: Depending on the project requirements, specifications may include provisions for the surface finish of the tunnel lining, such as smooth or textured surfaces, or the application of protective coatings or linings.

Types:

There are several types of tunnel linings used in various projects, including:

Cast-in-Place Concrete: This involves pouring concrete directly into formwork within the tunnel to create a continuous lining.
Precast Concrete Segments: These are precast concrete segments that are assembled to form a ring-shaped lining, commonly used in tunnel boring machine (TBM) excavation.
Shotcrete: Shotcrete involves spraying a mixture of cement, aggregates, and water onto the tunnel surfaces, forming a layer of concrete for immediate support.
Steel or Composite Linings: In certain cases, steel plates or composite materials may be used as tunnel linings, providing high strength and durability.
Brick or Masonry: Traditional brick or masonry linings can be used for smaller tunnels or in historical contexts, providing both structural support and aesthetic appeal.

General requirements (Concrete Lining)

1. Sequence of Lining:

The size and shape of the tunnel, the characteristics of the rock mass, the type of formwork, etc. are some of thefactors considered when choosing the sequence in which the tunnel lining should be installed (IS: 5878 (part V)-1976).

For Horse -Shoe, D - shaped and other wide & Flat-Bottomed tunnels:Placing concrete to form the kerbs first, followed by side walls & arch and finally the invert.
Circular section / when bottom of tunnel is narrow: Placing concrete to form the invert first followed by sides and arch
For small and circular tunnels: placing concrete for the invert, side walls and arch all at one time.

2. During the process of driving the tunnel, it is desirable to install a primary lining above and between the steelsupports in the event of a soft rock mass or collapsing rock strata. IS:5878 (part III) -1972 describing the differenttypes and methods of primary and main lining.