Tunnel Engineering

Deals with planning, design, construction, safety and maintenance of tunnels.

A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.

A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. The central portions of a rapid transit network are usually in tunnel. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.

Secret tunnels are built for military purposes, or by civilians for smuggling of weapons, contraband, or people. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely.

History Of Tunnel Engineering

Ancient Era

 The best description of ancient tunneling practices was provided in Henry S. Drinker’s seminal treatise on this topic titled Tunneling, Explosive Compounds, and Rock Drills that was published by John Wiley & Sons in 1878. Drinker claimed that his book discussed all tunneling activities from the time of Ramses II until 1878. Drinker was a professor at Lehigh University and his research including nearly 2,000 tunnel case histories with particular emphasis on tunneling practices in Germany, Italy, Great Britain and the United States; although tunneling practices in more than 15 other countries were also documented.

As one might expect, the majority of early tunnels were constructed for use as tombs and/or religious practices where the builders were free to choose their sites so as to avoid the need for ground support. With time, however, tunneling began to be used for drainage, water supply, sewers and transportation projects (including canals) where tunnel location became paramount and various, sometimes difficult, ground conditions needed to be managed. In his discussion of tunnel projects, Drinker is liberal in his use of the word “astonishing” when describing the size, shape, depth and length of these ancient openings such as two-lane “highways” extending for thousands of feet. For one project, Drinker discussed the use of 30,000 workers toiling for 11 years.

In his treatise, Mr. Drinker refers to his “Modern Era” of tunneling beginning in the 1700s with the regular use of black powder and various methods of timbering. Final linings were provided using bricks, dressing stone and cement, but only when absolutely required by bad ground. As recorded, the vast majority of ancient tunnels were constructed in rock until the late 1700s when consideration was given to tunneling under the Thames River in London. Apparently, the French had built a soft ground tunnel for a canal in 1803 that required continuous arching and this set the stage for numerous similar projects in the following 50 years. In Great Britain alone, it was estimated that a total of more than 200,000 feet of canal tunnels had been built by 1850. It was also during this period of time that tunnels began to be built for railroads that vastly increased both tunnel size and the need for tunneling through difficult ground. Foremost among these railroad projects was the Mont Cenis Tunnel in Europe and the Hoosac Tunnel in the United States.

Whenever bad ground was encountered (whether faulted material in rock or any type of soft ground), ancient tunnelers invariably resorted to some form of timbering to support the ground both at the face and along the length of the tunnel. Sometimes, timber was used for the final lining, but, more often than not a final lining of brick, dressing stone or cement was installed inside the timber support. One of the great costs associated with using timber for tunnel support was the large size of opening required in order to allow for construction of the inner, final lining. During this process, literally scores of carpenters were employed and entire forests were denuded in order to provide the raw materials.

Tunnel Engineering Features, Advantages and methods of Tunneling in Construction

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 tunnelsProvision 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

Tunneling Disadvantages

• 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

 Senior Tunnel Engineer Salary

The average salary for a Senior Tunnel Engineer is $92,648 per year.