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
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.