What Are Segmental Concrete Tunnel Liners? - Trenchlesspedia

Trenchlesspedia Explains Segmental Concrete Tunnel Liners

Tunnel lining is used in both trenchless construction and rehabilitation to maintain the shape of the tunnel and to shield whatever might pass through it. Concrete, especially with steel fiber reinforcement, has proven to be an effective material in the formation of structures, from buildings to bridges to tunnels. The ability to shape concrete for specific uses makes it a versatile option for the trenchless construction industry.

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The manufacture of a concrete tunnel liner segment begins with the specifications for the concrete mix. Not only are the type of concrete and the selection of reinforcement fiber important, it is also necessary to plan for and manage the amount of water to be used in production. The concrete should be wet enough to be inserted into molds, but dry enough to harden with the right consistency.

Each segment is created using molds that are heated to specific temperatures and for a preordained amount of time. Once dry, quality control testing confirms whether a segment is fit for use. Finished segments are labelled and numbered for database control. Large tunnel projects might require many thousands of these segments.

The installation of concrete tunnel liner segments might be performed by special machines, or they could be done by tunnel boring machines during the boring process. Properly placed concrete tunnel liner segments may be part of a tunnel that could last for centuries.

With the rapid development of transportation, tunnels, as important buildings on highway lines, have undergone tremendous changes in terms of tunnel length and construction technology. Tunnel lining is a permanent support structure built with reinforced concrete and other materials along the periphery of the tunnel body to prevent deformation or collapse of the surrounding rock. After the tunnel is excavated, in order to maintain the stability of the surrounding rock, support (lining) is generally required. The structure of the tunnel lining is closely related to the level and construction method of the surrounding rock. The choice of support type should depend on the surrounding rock geological conditions, construction conditions and use requirements.

1. Tunnel lining type

(1) Spray anchor support (lining)

Shotcrete support refers to the use of shotcrete and anchor as the main support means, and through the monitoring and measurement of the surrounding rock to guide the design and construction, so that the surrounding rock becomes a part of the support system, so as to rationally use the ground Carrying capacity to ensure the stability of the surrounding rock tunnel construction method. The bearing structure formed by bolts, shotcrete and surrounding rock can effectively limit the free development of the deformation of surrounding rock and adjust the stress distribution of surrounding rock to prevent the rock mass from falling loosely. It can be used as a temporary support during the construction process, and in some cases it may not be necessary to do long-time support or lining.

Depending on the geological conditions of the surrounding rock, a variety of support forms can be used:

A. Anchor rods used alone are generally only used locally;

B. Shotcrete is used alone, sometimes only for local use;

C. Anchor rods combined with shotcrete are mostly used for the top arch and side walls of underground caves;

D. Anchor rods and shotcrete, the addition of single-layer or double-layer reinforced mesh can improve the tensile strength and crack resistance of the sprayed layer, thereby improving the support ability;

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E. Anchor spraying is added with metal mesh, and rib-shaped support made of I-beam and other shaped steel is added in the spray layer.

The above-mentioned various forms of anchor spraying support, the number of anchor rods used, the depth, the thickness of the spacing spray layer, and the size of the metal mesh and rib support, etc., should be determined according to the actual situation. In order to do a good job of support, it is also necessary to carry out on-site measurement work such as wall rock displacement and deformation. The anchor and shotcrete support often follows the parallel operation of excavation and excavation, especially when tunneling or underground powerhouse construction adopts partial excavation. As the excavation section expands, it can be digged and sprayed until the entire section is completed.

For modern highway tunnels, the excavation width is generally 10m ~ 20m, and the excavation section is 80m2 ~ 150m2. For such a large cross-section construction, the construction support mainly with shotcrete support plays a decisive role in preventing the surrounding rock deformation, bearing the surrounding rock pressure and protecting the construction safety in the initial stage of the tunnel.

(2) Integral lining

Integral lining refers to cast-in-place concrete or reinforced concrete lining, also known as formwork concrete lining. The technological process is as follows: standing mould, pouring, curing, and removing mould. The advantages of integral lining are: strong adaptability to geological conditions, easy to form as required, good integrity and strong impermeability, and can be suitable for a variety of construction conditions (such as available wooden formwork, steel formwork or formwork car, etc.)

According to different geological conditions or surrounding rock levels, the integral lining has two styles of straight wall and curved wall. Straight wall lining is suitable for the situation where the geological conditions are better and the vertical surrounding rock pressure is the main, while the horizontal surrounding rock pressure is smaller. Mainly suitable for the second and third grade surrounding rocks. When in a tunnel, most of the surrounding rock is below grade 4, and only a few sections are grade IV wall rock, straight wall lining can also be used. Straight wall lining is composed of three parts: upper arch ring, vertical side walls on both sides and bottom bottom.

Curved wall lining is suitable for surrounding rock of grade IV and above with good geological conditions and large horizontal surrounding rock pressure. It consists of a top arch ring, side curved side walls and a bottom arch. The function of the invert arch is to resist the surrounding rock pressure at the bottom and prevent the lining from settling, so that the lining forms a ring-shaped closed overall structure to improve the bearing capacity of the lining.

(3) Compound lining

The compound lining divides the lining structure into two or more layers, and the lining of each layer may be of the same form and material, or may be of different forms and materials. At present, the outer layer and the inner layer are mostly used. The most commonly used outer lining is spray anchor support, and the inner lining is integral concrete lining.

The compound lining is applied in combination with spray anchor support and new Austrian construction. A layer of concrete is first sprayed on the surface of the cave wall, and sometimes anchors are also arranged at the same time. After the country is solidified, a thin layer of flexible supporting structure (called initial support) is formed. It can not only allow a certain change in the surrounding rock, but also limit the excessive deformation of the surrounding rock. During construction, regularly measure the deformation of the support, and feed back these deformation information to the construction and structural design, to determine the best time to apply the liner and the appropriate thickness of the liner. After the deformation of the outer lining is terminated or basically stabilized, the inner lining is applied.

The initial support of the composite lining is timely and closely combined with the surrounding rock, so as to protect and strengthen the surrounding rock and give full play to the self-supporting ability of the surrounding rock. After the secondary lining is completed, the inner surface of the lining is smooth and flat, which can decorate the inner wall and enhance the sense of security. It is an ideal structural form and is currently widely used.

2. Choose

Among the three types of spray anchor support (lining), integral lining and complex table lining, the type of boron lining used in highway tunnels should depend on the surrounding rock geological conditions, construction conditions and use requirements. Compound linings should be used for highways, first-grade highways, and second-grade highway tunnels; for third-grade and below highway tunnels, under the conditions of I, Ⅱ, and Ⅲ surrounding rocks, the tunnel opening section should be composite lining or integral lining other sections Spray anchor lining can be used.

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