Tunnels are underground constructions used for transportations. The features, advantages, disadvantage and methods of tunneling in construction is discussed.
The tunnel engineering is one of the most interesting disciplines in engineering. The work is complex and difficult throughout its course, even though it is interesting.
The tunnels are defined as the underground passages that are used for the transportation purposes. These permit the transmission of passengers and freights, or it may be for the transportation of utilities like water, sewage or gas etc.
The operations and the constructions are carried out underground without disturbing the ground surface. This operation is called as the tunneling.
It is found that the first tunnel was made by the Egyptians and the Babylonians, 4000 years ago. This tunnel served the purpose of connecting two buildings in Babylon. The length of this tunnel was found to be 910m, which was brick lined. The connection was from the royal palace to the temple.
The largest tunnel in ancient times was constructed between Naples and Pozzuoli, in 36.B.C. This tunnel was 4800-foot-long, 25-foot-wide and 30 foot in height. It was a road tunnel.
During the second half of the twentieth century, the development and progress of explosives and sophisticated equipment were flourished, that made the tunneling process more feasible.
It is hence dependent on the experience and the knowledge of the engineer. The tunneling, work better for different approaches under different circumstances. Mainly the tunneling works are carried out by the civil engineers.
This discipline will let us know, that the knowledge on structural and concrete technology is not sufficient. An equal and thorough skill and knowledge on geology, mechanical engineering, geomechanics and the new construction technologies are also key.
The two main factors that help in the efficient route of the tunnel are the alignment restraints and the environmental considerations. A proper inspection on the nature of soil, rock, the water table level, and all the alignment restraints had to be made before fixing the route. The underground, as we know is heterogeneous in nature.
The site chosen for tunneling is such a way that the inconvenience and difficulty that is caused to the environment in that area including living is minimum.
The tunneling method chosen depends on the ground conditions, the water table level, the tunnel drive length and the diameter, the tunnel depth, final utility requirements, the shape of the tunnel and the risk of building.
The tunneling approach gain certain advantages compared with other approaches, which are mentioned below:
The tunneling method gains certain disadvantages, which is due to its complexity and difficulty. Some of them are:
There are always debate arising on the cost that is consumed between the open cuts and the tunneling approaches, to conclude which is more economical. Consider the following factors:
a) Open cuts are found very costly at deep cutting for soil with varying nature and slopes. At this situation, tunneling approach will bring more economy than the open cuts. This approach will account for the large volume of excavation, which is costly.
b) When the substance of drilling is rock, open cut performs well with less amount of excavation and finds cheaper. While tunneling approach is found difficult to show its activity.
c) Based on the requirement of material for nearby filling, an open cut method can be suggested, but the tunneling is found comparatively economical in working. For depth of cutting greater than 60 feet, the method of tunneling is always recommended.
They are short approach and long approach. There are two approaches based on the open cuts on the either ends of a slope. The approach is said to be short, when the hill slope is very steep in nature, as shown in figure.1.
The cost of this mainly depends upon the topography of the considered area. The approach is said to be very long, when the slope of the hill is very flat, as shown in figure.2. 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
Certain factors that must kept in mind in the tunneling processes are:
Fig.3: Surface Alignment and the provision of grade for the tunnel
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