Basic knowledge of tunnel lining

The tunnel concrete maintenance formwork

Tunnel lining and initial support refer to the inner lining constructed with concrete and other materials under the condition that the tunnel has been initially supported to achieve reinforcement support, optimize the route drainage system, beautify the appearance, and facilitate the setting of communication. The role of lighting, monitoring and other facilities to meet the requirements of modern highway tunnel construction. Due to the construction conditions of the tunnel and the limitation of the construction environment, the construction of the two-line waterproof concrete in the tunnel is more difficult than the construction of the open-air concrete.

First, the main role

The function of the tunnel lining varies with the surrounding rock. For the stable surrounding rock of I, the deformation of the surrounding rock and the initial support is small and it tends to be stable soon, so the tunnel lining is basically not subjected to the surrounding rock pressure. The main function is waterproof and storage, and the flat lining surface is also conducive to ventilation and beauty. For the hard surrounding rock which is basically stable in II, although the deformation of the surrounding rock and the initial support is small, the surrounding rock pressure of the tunnel lining is not Large, but considering the long-term operation of the tunnel during the long-term operation of the anchor steel corrosion, the surrounding rock relaxation zone is gradually compacted, the initial support quality is unstable and other factors, so the tunnel lining is applied to increase the support lining safety; for III- V surrounding rock, due to the soft and broken rock mass, as well as rheological, expansion pressure, groundwater, etc., or due to shallow burial, bias and construction, etc., the self-stability of surrounding rock is very poor, only relying on initial support is not enough To suppress the development of surrounding rock deformation, the tunnel lining applied at this time will bear a large part of the late surrounding rock deformation pressure.

Second, the basic requirements

1. The degree of adhesion between the initial support and the tunnel lining will have an impact on the stress state of the composite lining. When there is a gap between the support and the lining, especially the concrete filled in the vault is not dense in real time, the surrounding rock pressure of the arch is saddle-shaped, that is, the vault is small and the arch is large, and even the outer side of the lining is pulled.

2. When the support and the lining are tightly bonded at the beginning, the radial force and the tangential force can be transmitted between the two layers, which can be checked according to the overall structure. When a waterproof layer is provided between the two layers, the combined structure is used to check and only the radial load is transmitted.

3. In order to prevent water leakage in the cave, the deformation of the tunnel lining should be controlled during design.

4. If there is a gap between the support and the lining, it should be backfilled and compacted.

Third, the calculation of tunnel lining

Due to the layered application of the composite lining, the time should be considered, and the calculation can be carried out according to the viscoelastic-plastic finite element method; the composite lining can also be approximated by the elastoplastic theory or the eigen curve method; On-site measurement data and model test results, as well as composite lining design adopted by internal and external data: For the external rock of tunnel lining with 30%-50% surrounding rock pressure for II-V surrounding rock, calculate by load-structure model . It should be pointed out that due to the insufficiency of the research on the mechanism of the tunnel lining, and the geological conditions of the surrounding rock are ever-changing, it is difficult to calculate the tunnel lining accurately. It is recommended to use simple and practical calculation methods. For weak surrounding rock with complex geological conditions, the tunnel lining calculation results of the above several methods should be compared to determine a reasonable and simple calculation method.

Fourth, the determination of the time for tunnel lining

For surrounding rock with better self-stability, tunnel lining should be applied after the surrounding rock and initial support deformation are basically stable, and the following conditions should be met:

1. The displacement rate around the tunnel has a significant slowdown trend.

2. The displacement speed of 1 m above the arch and the middle of the side wall is less than 0.1-0.2 mm/d, or the sinking speed of the dome is less than 0.07-0.15 mm/d.

3. The displacement value before applying the tunnel lining should reach 80%-90% of the total displacement value.

4. The surface cracks of the initial support no longer continue to develop.

5. When it is still difficult to meet the above conditions when taking certain measures, the tunnel lining may be applied in advance and should be strengthened.

For shorter tunnels and better self-stability of surrounding rock, tunnel lining can be applied after the entire tunnel is penetrated in order to reduce the interference between the various processes.

Fifth, measures to strengthen tunnel lining

For the complex geological conditions, the self-stability of the surrounding rock is poor, or in the case of shallow buried, biased, expansive surrounding rock, when the deformation of the surrounding rock and the initial support is not stable, it needs to be applied as a tunnel lining in advance. At this time, the tunnel lining is subjected to a large surrounding rock pressure, so the following measures are needed to strengthen the tunnel lining:

(1) Change the shape of the lining to adapt to the surrounding rock pressure, reduce the bending moment of the lining, and make the lining section basically under pressure.

(2) Improve the concrete marking, or use reinforced concrete, steel fiber concrete and other materials that can improve the bending strength.

(3) Applying the invert arch to form a closed structure for the lining to improve the overall rigidity of the structure and reduce the deformation of the surrounding rock.

(4) Measures such as advanced support and grouting to strengthen surrounding rock to increase the strength of the rock mass and improve the overall stability of the surrounding rock.