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.
Construction of First Tunnel
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 connection was from the royal palace to the temple. The length of this tunnel was found to be 910m, which was brick lined.
Construction of Largest Ancient Tunnel
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.
The tunneling, work better for different approaches under different circumstances. It is hence dependent on the experience and the knowledge of the engineer. 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 essential.
Selection of Tunneling Route
The two main factors that help in the efficient route of the tunnel are the alignment restraints and the environmental considerations. The underground, as we know is heterogeneous in nature. 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 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 construction.
Advantages of Tunneling
The tunneling method gain certain advantages compared with other methods, which are mentioned below:
- The tunneling procedure is more economical in nature, compared to open cut trench method when the depth is beyond a limit
- The surface life or ground activities like transportation are not disturbed when tunneling is undergone.
- The method ensures high-speed construction with low power consumption
- Reduces Noise Pollution
- These methods have freedom from snow and iceberg hazards, in areas of high altitudes
- Surface and air interference is restricted for tunnels
- Provision of tunnels with easy gradients, help in reducing the cost of hauling
- For the transportation of public utilities, tunneling method has a remarkable advantage compared to the bridge.
- The dangerous open cut to a nearby structure, when it is needed, is solved by the tunneling method
- The tunneling grant greater protection in aerial warfare and bombing conditions
The tunneling method gains certain disadvantages, which is due to its complexity and difficulty. Some of them are:
- The initial investment cost for commencing the tunnel is high compared to the open cut method.
- Highly skilled and experienced designers and engineer team only will work best for this operation.
- Higher and constant supervision from the start to the end of the tunneling project is necessary without any compromise
- Highly sophisticated and specialized equipment are necessary to perform the tunneling operations.
Comparison between Open Cuts and Tunneling
There are always debate arising on the cost that is consumed between the open cuts and the tunneling techniques, 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. This method will account for the large volume of excavation, which is costly. At this situation, tunneling method will bring more economy than the open cuts.
b) When the material of drilling is rock, open cut performs well with less amount of excavation and finds cheaper. While tunneling method 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.
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.