Tunnels in loose rock and soft soils are liable to disintegrate and, therefore, a lining is provided to strengthen their sides and roofs so as to prevent them from collapsing. The objectives of a lining are as follows.
(a) Strengthening the sides and roofs to withstand pressure and prevent the tunnel from collapsing.
(b) Providing the correct shape and cross section to the tunnel.
(c) Checking the leakage of water from the sides and the top.
(d) Binding loose rock and providing stability to the tunnel.
(e) Reducing the maintenance cost of the tunnel.
Sequence of Lining
The lining of a tunnel is done in the following steps.
1. In the first stage guniting is done to seal the water in rock tunnels.
2. Concrete lining is done either in one attempt as in the case of circular tunnels or by separately tackling the vest, the sidewall, and the arch. For small tunnels that measure 1.2 to 3.0 m in diameter, the concrete lining can be provided by the hand placing method. In the case of bigger tunnels, concrete pumps or pneumatic placers are used for placing the concrete.
3. The concrete is cured to its maximum strength. If the humidity inside the tunnel is not sufficient, curing can be done by spraying water through perforated pipes.
4. The different types of lining practices adopted by Indian Railways depending upon ground conditions are depicted in Fig. 30.12.
Types and Thickness of Lining
Theoretically, the lining provided inside tunnels may be of timber, iron, steel, brick, or any other construction material but in practical terms the lining provided most
commonly is that of reinforced concrete or concrete surface. Concrete lining is provided in tunnels because of (a) its superiority in structural strength, (b) ease of placement, (c) its durability, and (d) lower maintenance cost.
The thickness of concrete lining depends upon various factors such as conditions of the ground, size and shape of the tunnel, soil pressure, and the method of concreting. The thickness of concrete is calculated by the following empirical formula:
T = 0.083D (30.1)
where T is the thickness of the lining in centimetres and D is the diameter of the tunnel in metres.
On the basis of field experience, railway engineers have devised a thumb rule of providing 2.5 cm of lining for every 30 cm of the diameter of the tunnel. As per this thumb rule, the thickness of the lining of a tunnel with a 1 m diameter would be (100/30) x 2.5 cm = 8.3 cm.