Building the World's Largest Rail Tunnel
Simon Löw and Herbert Einstein

Switzerland will soon boast the world’s longest rail tunnel, one that will stretch 57 kilometers. In an effort to improve its rail system, the country is in the midst of a huge construction project to build two new rail tunnels through the Alps. The tunnels will cut through the mountains deeper than any other tunnel, as the country is pressured to increase its transportation system’s capacity for rail freight.

The tunnels will cross important parts of the deeper core of the Swiss Alps, which have a complex tectonic and metamorphic history. Both base tunnels run essentially perpendicular to the main tectonic units and sit beneath 2,500 meters of rock. Their depth, combined with the complexity of the geology, make the project particularly challenging.

From below: Pictured is a view up the access shaft for the Sedrun-Gotthard base tunnel. The Gotthard base tunnel will stretch 57 kilometers through the Alps as part of a railway improvement program in Switzerland. Photo by Herbert Einstein.

The longer of the two tunnels, the new Gotthard tunnel, will run 57 kilometers, making it the world’s longest. The other tunnel, Lötschberg, will stretch 34.6 kilometers. Both will sit lower in the mountains than any of the many rail tunnels or highway tunnels previously built in Switzerland. At greater depths, slopes leading into the tunnels aren’t as steep, and heavier trains can pass through.

The need for the new tunnels started as early as the 1960s when Europe experienced rapid economic growth. One consequence of this growth was a highway building boom that produced several long highway tunnels through the mountains between 1964 and 1980, some of them as long as their predecessor railway tunnels. By 1998, the freight being transported through and across the Alps by rail and truck within the arc between Mount Cenis (France) and Brenner (Austria) equaled about 98 million tons, more than triple the amount 30 years earlier.

Over time, the amount of freight being carried by truck in Europe has eclipsed the amount being carried by rail. But, in Switzerland, rail traffic had remained dominant because the country maintained a 28-ton limit for truck freight transport. Now, as part of several agreements with the European Union, Switzerland raised that limit to 40 tons to match other European countries. As a result, truck traffic through Switzerland and on transalpine roads is increasing and will continue to increase, a trend that the Alpine countries find unacceptable. Thus, in the hopes of alleviating truck traffic, they are building high-capacity rail links through the Alps, links that require long, deep base tunnels: the Maurienne-Ambin tunnel in France, the Brenner tunnel in Austria, and the Gotthard and Lötschberg tunnels in Switzerland. The construction will also connect Switzerland into Europe’s high-speed rail network.

Fueling the displeasure over truck traffic was the Oct. 24, 2001, accident in the existing Gotthard Highway Tunnel. Two trucks collided, causing a fire that smoldered for several days and closed the tunnel long enough to seriously disrupt traffic patterns through the Alps. Eleven people died in the accident. In March of 1999, a tunnel fire also claimed lives when a cigarette butt caused a truck carrying flour and margarine to explode in the Mont Blanc tunnel that connects France and Italy. Thirty-nine people died. And in May of 1999, a collision in Austria's Tauern tunnel caused a fire that killed 12 people.

The new Gotthard and Lötschberg tunnels are part of Switzerland’s NEAT project: Neue Eisenbahn Alpentransversalen or New Transalpine Railroad Link. The major components of NEAT are the Gotthard and Lötschberg-Simplon axes but with additional improvements along the access lines.The main axes parallel the present roads and railways, but their base tunnels, Gotthard and Lötschberg, will be deeper and longer than any of the existing tunnels and than the existing tunnels of the same names.

The Gotthard base tunnel will be 600 meters deeper than the existing Gotthard tunnel. The Lötschberg base tunnel will be 550 meters below the current Lötschberg tunnel. The existing Simplon tunnel, 20 kilometers long and at an elevation of 700 meters, is a part of the new system. Several more tunnels no more than 10 kilometers long will be built to create an access system that will join the existing tunnels with their corresponding base tunnels, creating a four-track system. The existing tunnels can allow 250 trains per day, but the new base tunnels will increase that capacity by 300 trains per day.

Working with geologic uncertainties

Some sections of the two tunnels may cause considerable difficulties during construction, difficulties that translate into added cost and time. Also, even in well explored cases like these two tunnels, it is not possible to precisely predict what will be encountered, an uncertainty inherent to most geologic and geotechnical projects. In addition, even if the geology were precisely known, construction processes always include variability. Although risk analysis is a well-known tool in business administration and has been so for a number of decades, it is not frequently used in major infrastructure projects. For the two transalpine tunnels, it was applied extensively not only regarding cost and time but also regarding operational safety.

The feasibility and design stages of the project paid close attention to selecting the most suitable tunnel alignments with respect to the geology. Important geological criteria in this process were: avoiding tunnel sections with very high overburden and stresses; finding the most stable ground conditions at locations of multifunction stations; limiting the length of the tunnel sections that are in weaker rocks (Triassic evaporates, phyllites, shists and cataclasites); optimal tunnel orientation with respect to critical tectonic structures (shear zones, faults, foliation); and protecting groundwater resources while minimizing the flow of groundwater into the tunnel. In addition, other criteria, such as short total tunnel length and short access ramps or shafts, accessibility, environmental protection and large tunnel radii had to be considered. These factors led to final tunnel alignments that follow good ground conditions over long tunneling intervals.
Nevertheless, geologically problematic structures or conditions could not be entirely avoided.

Preliminary site investigations, conducted primarily between 1990 and 1999, focused efforts on reducing, quantifying and mitigating geological uncertainties and risks. Simon Löw and colleagues reported the results of these investigations at the GeoEng 2000 International Conference on Geotechnical and Geological Engineering in Melbourne. One example follows.

Water from fault zones

It is known through numerous tunnels and hydropower drifts built in the eastern Aar and Gotthard massifs during the past 130 years that some of the steeply inclined fault zones in these units are highly permeable and could lead to initial tunnel inflows of 100 to 1,000 liters per second. As tectonic forces push the rocks against and past each other, the rock is ground into pieces, much the way a powder forms when two pieces of concrete scrape past each other. These cataclastic faults create broken-down rock that is much more permeable than the original rock, easily allowing water to pass through.

Tunnel fire: Trucks wait in line as smoke billows from an exhaust exit of the Gotthard tunnel near Airolo, Switzerland. On Oct. 24, 2001, two trucks collided in the tunnel, igniting a fire that killed 11 people. The accident and others in transalpine highway tunnels have fueled sentiment that too many trucks use the roads and tunnels. Austria, France and Switzerland are working on large construction projects in an effort to increase the freight capacity of their transalpine railway systems. AP Photo/KEYSTONE, Giuliano Giulini

The mean total spacing between faults in the area of the eastern Aar and Gotthard massifs is on the order of only 35 meters; they are very close together. But the mean spacing between faults that would produce inflow is only every 200 meters. Therefore, only a small percentage of all faults mapped at the surface and described in inventories of large faults are expected to be permeable at a critical level. At the same time, predicting the location and properties of fault zones 1,000 to 2,500 meters below ground surface — the elevation of the tunnel — is an extremely difficult or even impossible task, but many people are working on understanding relationships between fault mechanics and rock permeability.

Fault zones do not only pose problems to miners during excavation, but also to the environment. It is known from many tunnels already built in the Alps that large tunnel inflows in crystalline rocks can draw down the water table. Such water table drawdowns can dry out springs located many kilometers away from the tunnel alignment. Even more spectacularly, geodetic measurements performed in the Aar and Gotthard massifs above the existing Gotthard Highway Tunnel show that localized tunnel drainage from just one permeable fault zone has created surface settlements on the order of 12 centimeters over distances of many kilometers.

These settlements are unexpected because they occur in stiff crystalline rocks above a tunnel that has an overburden of 1,000 meters. As reported in a Ph.D. thesis by Christian Zangerl of ETH (Swiss Federal Institute of Technology) in Zurich, model calculations show that these settlements cannot be explained purely by closure of fractures, but that properties of the intact rock also contribute to the observed phenomena. Strong local variations of surface settlements could possibly cause serious damage to arch dams, which sit in nearby valleys. For such reasons, detailed localization and characterization of critical faults at tunnel elevation is performed with new exploration techniques carried out from the advancing tunnel face.

Modernizing transalpine crossings

Engineers have tackled the Alps many times before. Transalpine traffic in Europe existed in prehistoric times and it was important for the Celts and particularly the Romans. One can state that the founding of the Swiss Confederation in the 13th century was closely related to the transportation corridor over the Gotthard Pass. As a matter of fact, only an engineering feat — the construction of a suspended roadway along the steep walls of the Schöllenen Gorge, and a bridge across this gorge — made the Gotthard an acceptable route. Once this was achieved, it benefited from being one of the shortest European North-South routes.

This route and others across mountain passes were, in essence, mule paths. The next major development, in the first half of the 19th century, was when roads were built over several mountain passes, making stagecoach and wagon transportation possible. In the second half of the 19th century and in the early 20th century, railroad connections were built across the Alps, first without tunnels or with only short tunnels and then, starting with the Mont Cenis tunnel, including major tunnels.

Since then, engineers have constructed many rail and highway tunnels through the Alps. The current project is one of the most ambitious. Changing policies, populations and economics demand two more, and engineers and geologists are working together to meet the challenge.

Löw is chair and professor for engineering geology at the Swiss National Institute of Technology in Zurich and has been involved in the planning and realization of Switzerland's railway improvement project for the last 10 years.

Einstein is a professor of civil and environmental engineering at the Massachusetts Institute of Technology. He and colleagues at MIT and in Switzerland developed and applied a decision analysis tool to estimate construction cost and time of the Gotthard- and Lötschberg base tunnels. E-mail him at:

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