Tunnel lining structure and design requirements

The tunnel lining structure is an important part of the underground building structure, and its structural form can be selected according to the type of the stratum, the function of use and the level of construction technology. The structural forms mainly include a semi-lining structure, a thick arched thin wall lining structure, a straight wall arched lining structure, a curved wall structure, a composite lining structure and a multi-arch tunnel structure.

First, tunnel lining structure form, stress characteristics and applicable conditions

1, half lining structure

In the hard rock formation, if there is no danger of collapse of the side wall, only the top rock may have partial slippage, but only the top lining, no side wall, only a layer of cement mortar covering not less than 20mm thick, that is, semi-lining structure .

2, thick arch thin wall lining structure

In the medium hard rock formation, the force exerted by the dome can be transmitted to the rock mass through most of the arch, making full use of the strength of the rock. This structure is suitable for use in surrounding rock with low horizontal pressure and poor stability. For large-span, high-span wall caverns in stable or basically stable surrounding rock, if it is difficult to use the construction conditions of the spray anchor structure, or if the spray anchor structure is not waterproof enough, it can also be considered.

3, straight wall arch lining structure

Tunnel structures in general or poor rock formations, usually with vaults and side walls, form a unitary structure, namely a straight wall arched lining structure, a widely used tunnel structure.

4, curved wall lining structure

In poor rock formations, the rock mass is loosely broken and easy to collapse. The lining structure is generally composed of arch rings, curved side walls and inverted arch floor to form a curved wall lining structure. The lining structure has relatively good mechanical properties, but it has high requirements on construction technology, which is also a widely used tunnel structure.

5, composite lining structure

The composite retaining structure generally considers that the surrounding rock has self-supporting ability. The supporting function is firstly to reinforce and stabilize the surrounding rock, so that the self-supporting ability of the surrounding rock can be fully exerted, so that the surrounding rock can be allowed to undergo certain deformation and thus thinning. The thickness of the support structure. During the construction of the project, a flexible thin-layer shotcrete is generally applied to the wall of the cave. If necessary, a bolt is provided at the same time, and the spray layer is thickened by repeated spraying, and a mesh reinforcement is added to the spray layer to stabilize the surrounding rock. After the deformation of the surrounding rock tends to be stable, it is applied as a permanent support for the inner lining. The composite lining structure is often composed of initial support and secondary support. When the waterproof requirement is high, a waterproof layer must be added between the initial support and the secondary support.

6, multi-arch tunnel structure

In addition to examining engineering geology, hydrogeology and other related conditions, and also subject to line requirements and other conditions, tunnel design also needs to consider comprehensive comparisons of safety, economy and technology. Therefore, for small and medium-sized tunnels (100~500m), which are not particularly long in length, especially in small and medium-sized tunnels with complex geological conditions and complicated land acquisition restrictions, arch tunnels are often used.

Second, the technical requirements for tunnel lining structure design

1, determination of lining section type and geometric size

The type of tunnel lining structure shall be determined according to the geological conditions, construction conditions and use requirements of the surrounding rock of the tunnel. The tunnels of high-speed, first-class and second-class highways shall be composite lining; the road tunnels of the horizontal, third and third grades shall be under the conditions of I, II and III surrounding rock, except for the type and size of the lining structure outside the tunnel section. According to the requirements of use, surrounding rock grade, surrounding rock geological conditions and hydrogeological conditions, tunnel embedding position, structural stress characteristics, combined with engineering construction conditions and environmental conditions, comprehensive analysis and determination by engineering analogy and structural calculation.

During the construction phase, the support parameters should also be adjusted according to the on-site surrounding rock monitoring and measurement and on-site geological tracking investigation. If necessary, it can be determined through trial analysis. In order to facilitate the use of standard arch stencils and equipment, the type of lining should be determined to be as small as possible, and the contours of the arches of the same span should be the same. Generally, measures such as thickness adjustment and local reinforcement are adopted to adapt to different geological conditions.

2, the choice of lining materials

The lining structural material should have sufficient strength, durability and water resistance. Under special conditions, it is also required to have corrosion resistance and frost resistance. From the economic point of view, the lining structural materials must also meet the requirements of low cost and easy mechanized construction.

3, general construction requirements for lining structures

(1) Protective layer of concrete

The reinforced concrete lining structure, the minimum thickness of the concrete protective layer of the stressed steel bar is 20mm for the general assembled lining, the inner layer of the cast-in-place lining is 25mm, and the outer layer is 30mm. If it has an aggressive medium, it can be increased to 50mm, and the steel mesh sprayed concrete is generally 20mm. As the thickness of the section increases, the thickness of the protective layer should also be appropriately increased.

(2) Over-excavation or under-excavation of lining

In the construction of the tunnel structure, the excavation size of the cavern cannot be completely consistent with the size of the shroud designed by the lining, which leads to the problem of over-excavation or under-excavation of the lining. Excavation usually increases the workload of backfilling, while undercutting does not guarantee the cross-sectional dimensions of the lining. Therefore, there are certain restrictions on super and undercut. The lining allows for over-undercutting to be calculated according to the design of the hole.

Cast-in-place concrete linings are generally not allowed to undercut. If there is an individual point undercut, the depth of the undercut part entering the lining section shall not exceed 1/4 of the thickness of the lining section, and shall not exceed 15cm and the area shall not exceed 1m2. Generally, the tunnel lining structure, the average over-excavation allowable value shall not exceed 10~15cm. For some key parts of the cavern, such as the ring-shaped beam rock platform of the dome, the arched rock platform with thick arched wall lining (and semi-lining), The periphery of the cavern, etc., the allowable value of over-excavation should be strictly controlled, generally not more than 15cm.

(3) Setting of deformation joints

Deformation joints generally refer to settlement joints and expansion joints. The settlement joint is provided to prevent the structure from being deformed and fractured due to local uneven subsidence, and the expansion joint is provided to prevent the structure from being cracked due to thermal expansion and contraction or swelling and shrinkage. Therefore, the settlement joint is provided to satisfy the deformation requirements of the structure in the vertical and horizontal directions, and the expansion joint is provided to satisfy the deformation requirement of the structure in the axial direction. The width of the settlement joint and expansion joint is greater than 20mm and should be set vertically perpendicular to the tunnel axis.

Tunnel lining structure classification and design requirements