Tunnel Design and Cunstruction

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Table of Contents

Introduction
Underwater Tunnels
Tunnel Construction Methods
The Mersey Tunnel
Conclusion
Reference List

Introduction

Tunnels refer to any enclosed passageway. Usually, tunnels pass underground to bypass natural barriers and to reduce the cost and impact of aboveground construction. This paper looks at various aspects concerning tunnel construction. The main issues of interest include the construction of underwater tunnels, construction of tunnels in soft soils, and the examination of tunnel construction methods.

Underwater Tunnels

The need to construct tunnels underwater arises from several situations. First, it may make more sense to construct a tunnel under a large river, or the sea instead of a bridge to avoid interfering with marine traffic. For instance, instead of constructing a bridge to link the mainland to an island, it may be better to construct a tunnel to avoid interrupting marine traffic around the island.

Underwater tunnels may be constructed to provide a means of crossing a river. In this case, a bridge may be ruled out because of costs, especially of the river serves as a waterway. In addition, undersea tunnels are ideal where ferries are impractical or impossible to use. Sea canals mainly provide access to large islands or join two main bodies of land separated by a sea.

Underwater tunnels are also ideal because they do not require closure during bad weather. Bridges or ferries, on the other hand, cannot operate full time because they are affected by bad weather. Underwater tunnels are challenging to construct depending in the depth of the water and the length of the required tunnel.

Usually underwater tunnels do not run on the bed of the waterway, but below the bed of the water body. Construction of a tunnel on the bed of the water body adds more challenges to the tunnel construction process.

Tunnel Construction Methods

One of the important decisions that engineers must make in the tunnel design process is the construction methods that they will use. Construction of structures on the surface of the land does not compare to the complexities of tunnel construction. Tunnel construction usually takes place underground. A long tunnel usually has more complexities if the geological landscape changes very much.

However, even in areas with uniform geological make up, the local rock and soil characteristics can affect the construction of tunnels. Engineers usually use four tunnel construction methods. Factors affecting the choice of the tunnel construction methods are as follows:

First, the depth of the tunnel is one of the main issues of concern. Shallow tunnels built within cities to serve as aqueducts or as underground highways allow engineers to use trenches. If the tunnel needs to pass under a water body or through a hill, the engineers will favor methods that allow them to bore a hole in the rock mass.
Secondly, the construction methods will depend on the purpose of the tunnel. Tunnels used for transporting water and sewerage, or for laying communication cables may only be a few feet in diameter. However, if a tunnel is needed to provide a transport path for a freeway, the construction methods will vary from the smaller tunnels.
The third issues engineers bear in mind when deciding in the best tunnel construction methods to use is the stability of the surrounding material. If the tunnel is passing through solid rock, then the methods for use will be different compared to tunnel passing through loose sand or clay formation. The method will also vary if the tunnel is supposed to pass under a water body or below the water table.

Engineers also choose tunnel design and construction methods based on the seismic activity of a given region. If the tunnel passes through an area with high geological activity, the design needs to factor the impacts of these activities on the operation of the tunnel. Research shows that tunnels bored in depths of up to 500 meters have better resistance to seismic disturbances compared to tunnels bored in shallower depths.

The four main techniques used in the construction of tunnels are as follows:

The first technique is the cut and cover tunneling. This method involved digging a trench, and then covering it to create a tunnel. The actual methods employed depends on the space available for construction and the final use that the tunnel is intended.

It is popular in areas where tunnels are needed to provide more motorways under existing roads or in other squeezed spaces. In the top down variation, engineers cut out channels on the extreme ends of the tunnel, and then construct walls. They then construct a roof creating a chamber.

Excavation works go on under the roof of the tunnel before the final construction of the floor of the tunnel. In the bottom up variation, after the walls are built, the engineers excavate the area in between the walls and then build the tunnel from the ground up, finishing with the roof of the tunnel.

The second method of tunnel construction is the blast and drill method. This method applies in areas where it is necessary to bore through a rock to create a tunnel. Engineers use times explosives to loosen rock, and then remove the debris. This method is ideal for mountainous areas and in places where the bedrock is solid.

However, due to variation in rock formations, it is necessary to carry out drilling operations in areas with loose rock formation where blasting may result in total collapse of the tunnel. In these cases, some construction work is also required to provide support for the tunnel.

The third main tunneling technique is bored tunneling. This method employs the use of a Tunnel Boring Machine (TBM). TBMs make tunneling predictable, and can often be more cost effective. The main application of TBMs is that they make it possible to dig long tunnels.

They work best in areas where the surrounding material is strong enough to support long tunnel sections without the need for support. These machines work best within a given range of rock hardness. If the rocks are very hard, the wear and tear of the machines can be very high. In this case, it is better to use blast and drill methods.

Another advantage of using TBMs is that they can be equipped with sensors to determine when the rock formations change. This helps in making decisions regarding how to proceed. The fourth method of tunneling is the Sequential Excavation Method (SEM). This method is build around the understanding that tunneling reorients the internal stresses in rock formations.

As such, SEM aims at creating an environment that is quite similar to the preexisting stresses in the rocks. This is achieved by the use of careful calculation of stresses and using structures in such a way that the overall forces remain as they were before the construction of the tunnel. This leads to a reduction of the quantity of construction materials usually needed to stabilize tunnels dug in soft soils.

The Mersey Tunnel

The Mersey tunnel system is composed of three tunnels. The three tunnels are the Kingsway tunnel, the Queensway tunnel, and the Mersey Railway tunnel. The first two tunnels carry vehicular traffic while the last one is meant for trains. The tunnels were all constructed to ease traffic congestion at the river crossings.

At the time of their construction, the tunnels held records for the longest tunnels ever to be constructed. The fact that they were located beneath the riverbed meant that water was an ever present challenge. The tunnels still require pumps to remove water. The water is not only the result of seepage from the river, but ground water as well.

Conclusion

In conclusion, the construction of tunnels is going to improve with time, because more tunnels will be needed to increase connectivity in various parts of the world. Tunnels are also very attractive because of the limited environmental impacts they produce.

In the case of the Mersey tunnels, three important links were made without affecting the flow of beauty of the river over above. In this sense, tunnels provide an ideal way of ensuring that human transport and communication needs do not destroy the environmental aesthetics of the planet.

Reference List

BAGCT 2013, Methods for Tunneling Construction. Web.

Case Maker 2000, What is Rapid Application Development. Web.

Engrailhistory 2012, The Mersey Tunnel. Web.

FHWA 2011, Technical Manual for Design and Construction of Road Tunnels Civil Elements. Web.

Infoplease 2013, Tunnel Design and Construction techniques. Web.

Lean, G. 2006, Disappearing World: Global Warming Claims Tropical Island. Web.

Macdonald, M. 2013, Tunnel Construction Methods. Web.

MTUA 2013, Mersey Tunnels Users Association. Web.

Rivers, M. 2012, Doing business Down Under. Web.

Schuetze, C. F. 2012, Travelling with a Smaller Carbon Footprint. Web.

Siterman, E. 2009, Value engineering in Dubai. Web.

Wang, J.-N. 1993, Seismic Design of Tunnels: A Simple State-of-the-Art Design Approach. Web.

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