What are the secondary lining standards for tunnels?

The secondary lining is a moulded concrete or reinforced concrete lining applied on the inner side of the initial support during the tunnel construction, and it forms a composite lining with the initial support.

Relatively speaking, the secondary lining and the initial support refer to the inner lining constructed with concrete and other materials under the condition of the initial support of the tunnel, so as to achieve reinforcement support, optimize the route waterproof and drainage system, beautify the appearance, and facilitate installation The functions of communication, lighting, monitoring and other facilities can meet the requirements of modern highway tunnel construction. So what are the standards for the secondary lining of the tunnel?

The tunnel concrete maintenance formwork

1. In order to ensure the quality of the lining project, the second lining construction of the general section of the tunnel (including the tunnel body, the open tunnel, and the widened section) should adopt the full-section formwork trolley and pumping operation.

2. The secondary lining of the tunnel entrance section should be constructed in time. If the excavation exceeds 50m, the excavation should be stopped and the secondary lining construction should be stopped; the secondary lining of the tunnel entrance and the soft rock section in the tunnel should be constructed as soon as possible. Under circumstances, the distance between the second lining and the tunnel face is not more than 200m; the distance between the second lining work surface and the bottom paving work surface is generally 30m, and the distance from the low side wall work surface is generally 50m to ensure the normal construction progress of the second lining.

The tunnel concrete maintenance formwork

3. Before the construction of the second lining, laser measurement should be carried out on the initial support section, and the non-compliant ones should be dealt with.

4. When the underground water in the cave is confirmed to be corrosive to the lining structure through laboratory tests, different types of corrosion-resistant concrete should be used for different types of corrosion according to the requirements of the drawings. When there is no design requirement, the change shall be reported in time.

5. When the grade of surrounding rock changes, the grade of the lining section should also be changed accordingly, but the supervisor’s approval is required. The lining of the section with poor surrounding rock should be extended to the section with good surrounding rock, and the general extension is 5m.

Tunnel waterproof board laying formwork

6. The installation quality of tunnel waterproof and drainage facilities, embedded parts and reserved cavern templates, etc. must meet the design and specification requirements.

7. The construction unit shall entrust a qualified professional inspection unit to inspect the secondary lining steel bar, the thickness of the protective layer, and the void condition. The contractor shall carry out rectification and treatment of items that fail to pass the inspection.

8. In order to ensure that the lining does not invade the boundary of the tunnel construction, the designed contour line can be appropriately expanded when setting out.

9. For the completed lining section, continue to observe the stability of the second lining, pay attention to phenomena such as deformation, cracking, and intrusion into the clear space, and record in time.