THE CHANNEL TUNNEL

Have you ever considered the possibility of existence of an underground tunnel between two countries divided by sea !? It seems impractical but this feat has already been achieved back in the 1990s . England and France are connected through an underground tunnel system below the English channel since 1994 called the English channel.

Given the controversial history between the two countries , the formation of this megastructure proved to be very significant in improving the bilateral relationship of England and France. It also proved to be a boon for the businesses in both the nations. 

A project ,one of it's own kind was taken up by the engineers who were unaware of all the technical difficulties they will have to face and overcome. 

So let's dig into this without any further ado !!

Where is the Channel tunnel?

England and France are separated by a 34 kilometers stretch  of English Channel. Beneath the sea bed of this English Channel exists the Channel tunnel connecting the town of Folkestone ( in England) and the commune of Sangatte ( in France) . It is 100 meters below the sea level.

MAP OF THE CHANNEL TUNNEL

Funding :-

Channel tunnel, also called the Euro tunnel was a risky project because something like it was never attempted before. Both the governments were concerned about it's failures, which by the way are way too many to count . Funding such a high risk project can lead to irreparable losses . This is why the Channel tunnel pitched by Albert Mathieu could not receive  government  funds .

This is when the private companies like Eurotunnel(now known as Getlink(https://www.getlinkgroup.com/en/)and Bechtel came forward with funding for this megastructure. 1.5 billion Euros was set as the initial budget for constructing the tunnel excluding the interior construction. 

In 1986 both countries agreed on working on this project which eventually began in December 1987. 

THE BASIC IDEA :-

The basic idea of the construction was to dig the tunnel from both the ends ( English shore and French shore) and meet up somewhere in the middle ( 18 kilometers from either ends). They were to dig two railway tunnels and one maintenance tunnel in the middle of the two railway tunnels. In addition to this they also had to dig several vents interconnecting these main tunnels.

This complex system of pathways meant that a gap of 34 kilometers would be covered by tunneling a length of 143 kilometers .

England and France have always been competitors . This trend continued in this project as well. Engineers and workers at the both the ends were determined to reach to the middle of the channel first. It was like a digging race for them. Both the countries were very different from each other with respect to the work force and technology . English workers were more experienced than the French workers . Their tunneling machines too were more advanced as compared to that of the French.                                              England certainly was the favorite to win this tunneling race . 

TUNNELING BEGINS 

Workers started tunneling from both the ends in December 1987 . They were using tunnel boring machines (TBM) to dig into the sea bed . The machines were 200 meters long and weighed 1100 tonnes . It was a challenge to use TBM for digging into the sea bed because of the hard quality of rocks and soils found there.  Also the  heavy weight of the machine meant that handling it below the sea level would be a challenge. It also posed the danger of exposing fault lines that could lead to a potential earthquake. 

TBM

 So in order to improve the performance of the TBM it was designed specifically to work in such conditions . The structure was strong enough to drill through the strata of blue chalk . The spoil ( unwanted rock powders) accumulated within the machine was taken off regularly to keep the machine light. The powerful motors and the efficient centrifugal system meant that these TBM could remove 10 tons of rock per revolution. 

English TBMs were superior to the TBMs used by France. They were more efficient in terms of speed and power delivery. But the French machines had a vital advantage of having a waterproof structural skeleton that allowed them to bore through wet soil lands as well. This would prove to be a huge boon for the French workers ahead in the project.

As the tunneling continued , the roof of the tunnel was being covered by arch shaped concrete slabs . They are shaped like a downward facing concave surface which bears the compressive loads . In this case the load would be of the entire sea bed and the volume of water in the English channel. To bear such heavy loadings the concrete slabs were reinforced and designed to resist the vibrational disturbances due to resonance of railway tracks.

THE BIG PROBLEM :-

As the tunnel was being dug from both the sides(England and France), they had to coordinate properly in order to meet perfectly coaxially in the middle. But it was not an easy task considering that they were tunneling towards each other starting from a far fetched distance of 36 kilometers within the sea bed.  

But, why was it important for the tunnels to meet coaxially ? Why could not they afford a bent at the point of junction? 

Well, the answer lies in the physics involved with the circular motion.

So basically any curved path is a small portion of a circle with a certain radius. If there was to be a bent in the junction of the tunnels , it would behave as a curved path for the rapidly travelling trains. As we know, any body that travels through a curved path gets associated with a centrifugal force that tries to sway away the body from it's path towards a direction radially outwards . 

Centrifugal force and radius of curvature of the path are related as :-

F = (MV^2)/R

Basically , the centrifugal force is inversely proportional to the radius of curvature of the path. Hence, greater the deviation between the axes of the 2 tunnels greater the bent. This would mean that a high curvature path would be observed that would have a small radius of curvature. Small radius of curvature would mean that the fast moving trains would have to experience centrifugal force of very large magnitude that can cause serious accidents. 

So, the engineers calculated the maximum allowable deviation between the axes of the two tunnels which came out to be 2.4 meters  . 

The margin of error was extremely small. Bringing two tunnels towards each other from a separation of 36 kilometers with an accuracy of 2.4 meters (along all possible radial directions) would require some magic beyond human capabilities . 

Coaxial deviation 

THE SOLUTION :-

Engineers had realized that they can't rely on human skills to pull off this big challenge. So they decided to convey surveys for tracking an underground path of tunneling which ensures that the tunnels from  both ends meet coaxially in the center . The surveying was done through air(via helicopters) to give infinite directions for navigation. Yet it proved to be futile as it was impossible to predict the accuracy of the survey. 

In this situation ,Geology turned out to save the day. The idea was to detect various layers of rocks under the sea bed and bore along that layer of rock which would be best suited for boring and construction purposes. This would make sure that the two tunnels would meet coaxially as both would follow a common trail of layers of rock.

Geologists came into action. They  started searching for the best kind of rock for this purpose. The rock would have to be durable, water resistant and workable. The geologists found their answer in Blue Chalk. It was both durable and water proof. The strata of this rock was continuous from English shore to the French shore at an average depth of 100 meters below the sea bed . This was the path along which the two tunnels would meet at the center.

LASER TECHNOLOGIES OF TBM:-

Steering a TBM is not an easy job, specially when it needs to be steered through a sea bed . Prior information about the upcoming path is necessary to avoid the situations of collision. For this ,laser technology assists the TBM by predicting it's exact coordinates .

It involves the laser stations (sources of laser light) projecting laser beam onto the laser target boards that are installed in the TBM. Complex computer algorithms then interpret and analyze the collision of these laser beams and come up with the coordinates and possible pathways for the TBM .

  

Coordinate interpretation

TOUGH WORKING CONDITIONS :-

Two big challenges regarding the working conditions were :-

• Scarcity of fresh air
• Danger of flood and fire 

In March 1988 , near the English end of the channel ,workers noticed water seepage into the tunnel. The incoming water tasted salty which meant that it was The English Channel that had found it's way into the tunnel. The seepage rate reached to a mammoth value of 300 liters / minute . This started creating  cracks in the tunnel roofs which could lead to structural collapse.

Reason behind this mishap could have been a possible exposure to the fault lines within the sea bed because of tunneling. In situations like these, French TBMs would have thrived, given that they were waterproof.

Eventually around 1989 Jan things became normal for the English workers. Nevertheless  this event certainly slowed down their progress.

Supply of concrete slabs :-

The concave concrete slab was a very important structural element of the tunnel as it was the only compression bearing member. Production and delivery of such an element was an important aspect in this construction work. These slabs were produced and stored at the shores of the respective countries. French shore being a planar land had an advantage of easy production and storage. This was not the case with the English shore as it comprised of stretches of hills that made production and storage work quite tedious for them. To go with this problem ,they also had space woes.

The solution to this problem was solved by spoils. Spoil is the waste material produced in the process of digging . It contains soils ,rocks , organic material etc. This spoil was recycled as a foundation material for concrete factories and storage houses at the shores of England.

The meeting moment :-

Finally after 3.5 years  of boring ,the 2 tunnels met 18km from their respective shores.

It was the British tunneler Gream Fogg  who crossed the intersection for the first time and stepped into the French tunnel. For the first time in 13000 years Britain connected with the main European land.

The project was accomplished ,that too with an insane accuracy of 0.35 m deviation( margin of error =2.4 m) . 

Now the TBMs were stuck in the middle of the tunnel. To clear the path , the English TBM was buried into the seabed making a clear way for the French TBM to proceed towards the English shore. 

Post tunneling :-

2 more years were invested in making the interior of the tunnels. It included tracks ,cooling systems, interconnecting vents, lights etc.

So, finally in May 1994 the tunnel was officially inaugurated for it's commercial use.