Tunnel lining refers to a permanent structure that supports and maintains the long-term stability and durability of the tunnel. Its functions are: to support and maintain the stability of the tunnel; to maintain the space required for train operation; to prevent the weathering of surrounding rocks; to relieve the influence of groundwater. Therefore, the tunnel lining must have sufficient strength, durability and a certain degree of frost resistance, impermeability and erosion resistance. The tunnel lining is mainly composed of arch ring, side wall, invert and bottom plate. In order to drain the water in the tunnel, a drainage ditch (central drainage ditch or side ditch) is also set in the tunnel.

After the tunnel was excavated, the original balance of the stratum around the tunnel was destroyed, causing deformation or collapse of the tunnel. In order to protect the stability of the surrounding rock and ensure driving safety, the tunnel must have a supporting structure with sufficient strength, that is, the tunnel lining.

The tunnel lining is a permanent support structure built with reinforced concrete and other materials along the periphery of the tunnel body to prevent deformation or collapse of the surrounding rock. Different lining forms can be used in different surrounding rocks. Commonly used lining forms are shotcrete lining, shotcrete lining and composite lining. In most cases, composite lining is used. Composite lining is often divided into primary support (primary lining) and secondary support (secondary lining).

Commonly used tunnel lining materials are: concrete and reinforced concrete, rubble concrete, aggregate stone or concrete precast blocks, and shotcrete. The lining structure of the tunnel is mainly determined according to the geological and topographical conditions of the tunnel, considering the rationality of the structural force, the construction method and the construction technology level and other factors. The structure, shape and size of the tunnel lining differs due to its use, topography, geology, construction and structural performance and other conditions.
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