Lining of Tunnels
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.
1 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.
2 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) × 2.5
cm = 8.3 cm.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.