Size and Shape of a Tunnel

The size and shape of a tunnel depend upon the nature and type of ground it passes through and also on whether it is designed to carry a single or a double railway line. The shape of a tunnel should be such that the lining is able to resist the pressures exerted by the unsupported walls of the tunnel excavation.

If the ground is made up of solid rock, then the tunnel can be given any shape. Tunnels in rocky terrains are generally designed with a semicircular arch with vertical sidewalls. In the case of soft ground such as that consisting of soft clay or sand, the pressure from the sides and the top must be resisted. A circular tunnel is generally best suited for resisting both internal and external forces regardless of the purpose for which the tunnel is used. Theoretically, a circular section provides the largest cross-sectional area for the smallest diameter, which provides greater resistance to external pressure. But this type of cross section is more useful for drains carrying sewage and fluids and for aquaducts built for irrigation purposes. For railway track, the circular portion at the bottom of the tunnel has to be levelled in order to lay the track and facilitate the easy removal of muck and placing of concrete. The typical cross section of a tunnel is shown in Fig. 30.1. The sections

commonly adopted for tunnelling and the purposes these tunnels serve are enumerated in Table 30.1.

Table 30.1 Shape and purpose of tunnels

Shape

Purpose

Circular

Water and sewage

Elliptical

Water and sewage mains

Horseshoe

Roads and railways

Arched roof with vertical walls

Roads and railways

Polycentric cross section

Roads and railways

The size of a railway tunnel depends upon the gauge of the railway track and the number of lines. The typical dimensions for railway tunnels are specified in Table 30.2.

Table 30.2 Size of the tunnel

Gauge (mm)

Single line

Double line

Breadth (mm)

Height (mm)

Breadth (mm)

Height (mm)

BG (1676) MG (1000)

4880 - 5490 4270 - 4880

6700 - 7320 6100 - 6700

8530 - 9140 8530 - 9140

6700 - 7320 6100 - 6700

Tunnel Alignment and Gradient | RAILWAY ENGINEERING | Methods of Tunnelling