What is Tunnel Engineering ?

Introduction :

Tunnel Engineering:

• The branch of Civil Engineering which deals with the  design, construction and maintenance of tunnels is known as Tunnel Engineering.

            So for as the subject of 'Tunnel Engineering' or 'Tunnels' is concerned it also includes the study of  necessity of tunnels, history of development of tunnels, classification of tunneling and lining, ventilation and drainage of tunnels etc.

Tunnels:

• The underground passages which are constructed without removing or disturbing the ground surface are known as Tunnels.

Elizabeth River Tunnels

        These are engineering structures in the form of artificial galleries, passages or communication routes beneath the ground , under the bed of a stream or through a hill or mountain.

Uses of Tunnels:

i) They are used for transportation of men, material and goods.

ii) They are used for conduction of water to generate power in which case they are known as hydro-power  tunnels.

iii) They are also used for carrying oil, sewage, gas etc.

Necessity of tunnels:-

          It is always desirable to achieve economy in the cost of construction and maintenance of road and railway projects. Considering this factor and safety of the traffic and other public utilities, the provision of tunnels becomes necessary under the following circumstances :-

i) When the surface route of railway track or road for reaching the other side of a hill is much longer and, therefore, costlier in construction than a tunnel.

ii) When depth of open cut for reaching the other side of a hill is more than 20 m and it is costlier to construct and maintain it than a tunnel.

iii) When there is necessity of carrying public aminities like water, oil, gas  etc.

iv) When damage to transportation system of strategic importance is to be avoided and safty to traffic is desired during aerial warfare.

v) When the two terminal station, separated by a mountain, are to be connected by the shortest route.

vi) When acquisition of valuable land and property for railway or road project is to be avoided.

vii) When there is necessity of conduction of water for the generation of power.

Advantages and disadvantages of tunnels:-

          The advantages and disadvantages of providing tunnels are discussed below:-

Advantages:-

i) They carry railway lines, roads and public utiltils like water, oil and gas etc., across streams or mountains economically.

ii) They eliminate excessive cost of maintenance of an open cut, subjected to land slides.

iii) They connect the two terminal stations by the shortest route.

iv) They provide protection to communication routes against damage during aerial warfare.

v) They facilitate conduction of water to generate power.

Disadvantages:-

i) They may be costlier in construction as compared to open cut.

ii) They require more time in their construction as compared to open cut.

iii) They require special equipment and methods for their construction.

iv) They require skilled labour and supervision in their construction.

v) They may cause suffocation if not properly ventilated and thus, result inconvenience to the passengers.

Approaches in Tunneling Method:-

There are two approaches based on the open cuts on the either ends of a slope. They are short approach and long approach. The approach is said to be short, when the hill slope is very steep in nature, as shown in figure.1.

The approach is said to be very long, when the slope of the hill is very flat, as shown in figure.2. The cost of this mainly depends upon the topography of the considered area. In high altitudes, these approaches will be bounded with snow or may be blocked by the heavy landslides. These are the factors that would cause the decision of open cut or tunnel method.

Fig.1: Short Approach in Tunneling

Fig.2: Long Approach in Tunneling

Alignment and Grade in Tunneling Process:-

Certain factors that must kept in mind in the tunneling procedures are:

• The best and economical alignment was chosen must be straight in nature
• Tunnel should have a grade, which is less than the outside. It is observed that in the railway tunnels, constant slipping of the wheels takes place due to the wetness of the rails. This reduces the hauling capacity of the locomotives.
• 0.2% gradient must be provided to ensure proper drainage.
• When it comes to long tunnels, two grades at the either ends must be provided (That rise from each end then towards the center as shown in figure-3).

Fig.3: Surface Alignment and the provision of grade for the tunnel

• If the grade is provided on one side, instead of either side, the effectiveness of ventilation can be increased.

Dust Prevention in Tunnel Construction:-

The dust accumulation within the tunnel is the after effect of conduction the blasting, drilling mucking operations. If the dust accumulated amount is beyond the permissible limit, it creates harmful effects for the people working and to the surrounding environment.

The blasting of rocks as a part of tunneling operation will emit a high amount of silica in the environment. The intake of same will cause a disease called the “silicosis” which is fatal.

Dust Control Methods in Tunnels

1. Wet drilling

1. Use of vacuum hood

1. Use of respirators

The methods used to control dust accumulation are:

Now modern drilling machines are available, that make use of water to drill. Here the area that is to be bored is made wet, resulting in a reduction of dust emission.

The figure below shows a vacuum hood that is used in tunneling work. A hood is fitted to a drill face. This is at the rock face. This is connected to an exhaust pipe. The exhaust pipe takes in the dust formed by the drilling process and that is taken safely out of the tunnel. This reduces no flying of dust to outside.

Another common and efficient method to prevent dust inhalation is using respirators. These are well designed units that are worn by the miners and workers, who are highly exposed to the dusty environment.