Secondary lining plays a crucial role in composite lining systems and is an important component of tunnel structures. It is mainly used to enhance the overall stability of tunnels and provide long-term support and protection. Composite lining usually includes initial support (such as shotcrete, anchor rods, and steel arches) and secondary lining, and the combination of the two can more effectively cope with various geological conditions and stress changes encountered during tunnel excavation and later operation.

　　Secondary lining plays the following important roles in composite lining:

　　I、 Bearing function

　　1. Provide long-term stable support

　　-The initial support mainly bears the immediate pressure of the surrounding rock after excavation during the construction process, but over time, the deformation of the surrounding rock may continue to develop. As a structure constructed in the later stage, the secondary lining can provide long-term stable support for the tunnel and jointly bear the surrounding rock pressure with the initial support.

　　-For example, in soft rock tunnels, the creep characteristics of the surrounding rock are obvious, and secondary lining can effectively control the long-term deformation of the surrounding rock, ensuring the safety and stability of the tunnel structure.

　　2. Take on some of the water pressure

　　-When there is groundwater in the tunnel, the secondary lining can bear some of the water pressure, especially in the case of local defects in the waterproof layer. The secondary lining can play a certain role in waterproofing and anti-seepage, preventing groundwater from damaging the tunnel structure.

　　-In areas like Shenzhen with abundant groundwater, the anti water pressure effect of secondary lining is particularly important.

　　II、 The role of safety reserve

　　1. Improve the safety factor of the tunnel

　　-As a safety reserve, secondary lining can play a timely role in preventing serious accidents such as tunnel structure collapse when local damage or insufficient bearing capacity occurs in the initial support.

　　-For example, when a tunnel encounters unexpected disasters such as earthquakes or explosions, secondary lining can provide additional protection for personnel and equipment inside the tunnel.

　　2. Ensure construction safety

　　-During tunnel construction, the application of secondary lining can provide a relatively safe working environment for construction personnel. Especially in areas with complex geological conditions and poor rock stability, timely construction of secondary lining can reduce construction risks and ensure smooth progress of construction.

　　3、 Waterproof function

　　III. Form a waterproof barrier

　　-The secondary lining is usually poured with concrete with good impermeability performance, and a waterproof layer is set between it and the initial support to jointly form the waterproof system of the tunnel. The secondary lining can effectively prevent groundwater from seeping into the interior of the tunnel, ensuring the normal functioning of the tunnel.

　　-Taking tunnels in the Shenzhen area as an example, due to their proximity to the ocean and high groundwater levels, the waterproof effect of the secondary lining is crucial for the durability of the tunnel.

　　2. Prevent leakage from damaging the structure

　　-The leakage of groundwater not only affects the functionality of tunnels, but also causes corrosion and damage to tunnel structures. The good waterproof performance of the secondary lining can avoid the erosion of groundwater on the initial support and internal facilities of the tunnel, and extend the service life of the tunnel.

　　IV、 Aesthetic and decorative effects

　　1. Improve the internal environment of the tunnel

　　-The surface of the secondary lining can be leveled and decorated to make the interior of the tunnel more beautiful and tidy, improving the comfort and safety of driving.

　　-For example, fireproof and anti-corrosion coatings can be applied to the surface of the secondary lining, and lighting facilities and signage can be installed to provide good visual effects and directional information for tunnel users.

　　2. Enhance the overall image of the tunnel

　　-For some important traffic tunnels or urban underground passages, the appearance quality of the secondary lining directly affects the overall image of the tunnel. Through careful design and construction, the secondary lining can become a beautiful scenery line in the city.

　　The role of secondary lining in composite lining systems is multifaceted. It not only enhances the safety and stability of tunnel structures, but also provides various functions such as waterproofing, fireproofing, and anti-corrosion. It is an important component to ensure the long-term safe operation of tunnels.