Composite lining is a commonly used structural form in modern tunnel engineering. It combines the advantages of primary support and secondary lining to achieve tunnel stability and long-term safety. The design concept of composite lining is based on the self-bearing capacity of surrounding rock. Through a reasonable support system, the bearing function of surrounding rock can be fully utilized to reduce the impact on the environment while ensuring the safety and stability of the tunnel.

　　Advantages of composite lining:

　　1. It can give full play to the respective roles of primary support and secondary lining to adapt to different surrounding rock conditions. Primary support can quickly seal the surrounding rock and control the deformation of surrounding rock; secondary lining serves as a safety reserve to improve the long-term stability and durability of the structure.

　　2. It has good waterproof performance. By setting up a waterproof layer, it can effectively prevent the infiltration of groundwater.

　　3. For tunnels with complex and changeable geological conditions, it has good adaptability and flexibility, and can adjust support parameters according to actual conditions.

　　Disadvantages of composite lining:

　　1. The construction process is relatively complex, which increases the construction difficulty and construction period.

　　2. The interaction between primary support and secondary lining is relatively complex, and the design and calculation are difficult.

　　3. The cost is relatively high and requires the use of more materials and equipment.