The World Channel Tunnel Engineering Projects Construction Essay
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Published: Mon, 5 Dec 2016
The Channel Tunnel (French: le tunnel sous la Manche), widely recognized as one of the world’s greatest civil engineering projects, is a 50.5km underwater rail tunnel connecting Folkestone, Kent in the UK with Coquelles, Pas-de-Calais in France under the English Channel. Even though it began construction in 1988 and was opened in 1994, the idea to have a cross-channel tunnel was first mooted more than 200 years ago but did not materialize due to political, national security and cost considerations. However, with the tremendous increase in traffic growth, better and alternative means of communication, convenience and speed was necessary and hence the need for an alternative transport route was clearly evident. The need for such tunnel was further compounded with Britain joining the European Community and the cross-channel traffic doubling in the last 20 years (leading to the project), reflecting improved trading between the Britain and rest of Europe. The Channel Tunnel would also be able to provide an alternative competitive link between the transportation systems of the UK and France, providing both speed and reliability to freight deliveries. With the strong endorsement from the governments of both sovereigns, the decision to build the Channel Tunnel was thus made. In April 1985, the British and French governments issued a formal invitation to potential tenderers for the fixed Channel link and eventually the contract was awarded to the consortium Channel Tunnel Group Limited- France Manche S.A. (CTG/FM) (later renamed Eurotunnel).
Figure 1: Project Organization
The Channel Tunnel, with the governments’ intention that it be privately funded and there would not be any government assistance or undertaking, was a build-own-operate-transfer (B-O-O-T) project with a concession. The project organization is shown in Figure 1. In this contract arrangement, Eurotunnel would be the owner cum operator, which was being funded by the banks and shareholders. The governments of UK and France were represented by the Inter-Governmental Commission (IGC), to which the Safety Authority and the Maitre d’Oeuvre (an independent technical auditor) would report to. The IGC would then make final engineering and safety decisions. TML (essentially split from CTG/FM so as to separate the roles of owner/operator and contractor) consisted mainly of five British contractors (Translink Joint Venture) and five French contractors (G.I.E Transmanche Construction) and would carry out the construction works for the Channel Tunnel in a design and build contract. Upon completion of the project, the British and French governments would award Eurotunnel a 55 (which was later revised to 65) year operating concession to repay the banks and shareholders. The Contract was officially signed on 13 August 1986 and the fixed rail was to be fully commissioned in 1993. The services offered by the Channel Tunnel include the Eurotunnel Shuttle (a shuttle service for vehicles), Eurostar passenger trains and freight delivery trains.
TML’s contract was to design, build, and test and commission the fixed rail tunnel. The Channel Tunnel (Figure 2) was designed to have three concrete-lined bores approximately 50km long, with 37.9km undersea and the rest under land at either ends of the English (Cheriton near Folkstone) and French (Pas-de-Calais village of Frethun) terminals (Figure 3). Two of the running tunnels were designed to have an internal diameter of 7.6m while the third was a 4.8m service tunnel running midway between the two and connected to them via 3.3m diameter cross passages at 375m intervals. 2m diameter piston relief ducts connecting the main tunnels at 250m spacing were built to prevent the accumulation of differential air pressures and aerodynamic resistance. To facilitate operations and maintenance, four crossover caverns were built between the two terminals to allow trains to cross between the running tunnels. Two crossovers were laid close to the terminals while the other two were under the seabed, effectively dividing the tunnel into three approximately equal lengths. Figure 4 below shows the main phases of the project.
Two separate rail tunnels were chosen instead of a single large twin-track rail tunnel because this could minimize construction risk while at the same time enhance operations, maintenance and safety. The diameters were finalized after design analysis, development and optimization studies, taking into consideration the operation and support, speed and cost of construction. The service tunnel provided access between the running tunnels during normal and emergency situations and was equipped with a guided transport system. It was also where the water and pumping mains run and functioned as a fresh air supply duct to the tunnels in normal working condition. In addition, the service tunnel would function as a lead tunnel during construction which allowed the workers and engineers to assess and ascertain the uncharted ground conditions before advancing the main tunnels.
Basing on the existing geotechnical investigations, past tunneling expeditions and two additional geotechnical and geophysical surveys carried out by TML on the English Channel along the proposed tunnel line, it was ascertained that there was a distinct sub-unit of the Lower Chalk layer known as the Chalk Marl running continuously between the two terminals. Chalk Marl, made up of alternating bands of marly chalk and limestone, was found to be the best tunneling medium as it was essentially impermeable (due to its high clay content) and provided good short term stability under excavation, thus minimizing the number of supports required (Figure 5). It was designed to be bored in the bottom 15m of the Chalk Marl layer to minimize the ingress of water from the fractures and joints, but above the Gault clay which is susceptible to swelling when wet, imposing high stresses on the tunnel lining. The chalk marl strata dipped gently at less than 5o with smaller displacements of less than 2m due to faulting towards the UK side; whereas the strata dipped severely towards the French side (up to 20o) with much larger displacements of up to 15m (Gueterbock, 1992). Chalk at the French side was also harder, more brittle and fractured. This thus led to the use of different tunneling methods on the English and French sides.
The seaward and landward bores for all three tunnels on the UK side began at Shakespeare Cliff. Construction traffic would enter the tunnel via a new inclined access (Adit A2) at the Lower Shakespeare site, while worker access was built via a shaft driven to the tunnel level from the Upper Shakespeare site (Gueterbock, 1992). Due to the fast construction time required and the relatively dry chalk marl at the UK side, it was assessed that the New Austrian Tunneling Method (NATM) was most suitable for the UK tunnels. One feature of the NATM was the interlinking of design, construction method, sequence and plant and the success of this method depended on the continuous integration of these elements by the tunneling engineers. Six TBMs were used to drive the UK tunnels spanning a total distance of 84km. The TBMs were operated on an open-face mode with a front excavating section and a rear gripper unit which acted as a temporary anchor point when the cutting head drove forward at 1.5m increments (Anderson & Roskrow, 1994). Excavation of the tunnel and erection of the tunnel linings were carried out concurrently. Depending on ground conditions, the thickness of the linings ranged between 380mm and 500mm. Expanded concrete lining was used for the UK tunnels where the unbolted lining was expanded against the excavated ground. Pads on the back of the lining allowed the formation of an annulus to be filled with grout to prevent water ingress (Byrd, 1996). Each 1.5m lining ring was made up of eight precast concrete segments with a key segment. Cast iron lining segments were only used in poor ground conditions.
Over at the other side, the tunnel drives started at the shaft in Sangatte in France. Due to the highly fissured ground resulting in very wet conditions on the French side, a different type of TBM known as the Earth Pressure Balance Machine was used. The TBMs were designed to operate both in open and closed modes. Close mode is characterized by the sealing off of the machine from the spoil around it and the cutting head, thus keeping pressure on the dirt in front as it excavated and allowing the machine to work in the dry as the pressure in the machine was higher than the outside. The arrangement of seals on the TBM allowed it to withstand up to 10 atmospheric pressures. When the TBMs reached dryer and more favourable grounds, they could then switch to open mode. While precast sections were also used on the French side, the materials used were different owing to the different soil conditions: neoprene and grout sealed bolted linings made of cast-iron and high strength concrete (Anderson & Roskrow, 1994). The French tunnels were made of six 1.4 to 1.6m wide segments plus a key segment. A total of 5 TBMs were employed on the French side, and the bores from the UK and France were to finally meet in the middle of the English Channel in the tunnel breakthrough phase.
The Channel Tunnel project was huge by any standard, with a number of key factors that could potentially impact the parties involved: bi-nationality, private funding (thereby effectively transferring most of the financial risks to the contractors), schedule and cost. To stay attractive to investors and banks alike, the project had to meet the following priorities: minimum risk of cost overrun, minimum operating cost and maximum traffic revenue. It was recognized, from the outset, that the main challenge of the project was to resolve the logistical support associated with large scale tunneling and the fast-track nature of this project. The management, finance and technical challenges related to this project would be explored in the subsequent paragraphs.
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