Tunnel construction in the Alps

MAN AND MACHINE ARE STRUGGLING TO BUILD A 53-KILOMETER LONG TUNNEL THROUGH THE FRENCH ALPS.

A sound of creaking and cracking comes from the rock. Then a dull thud. It’s enough to send a jolt of fear through a visitor to this dusty tunnel deep in the mountain. Dominik Stewart, though, is quite relaxed. “That was a steel anchor,”says the engineer,“torn off in the rock.“ The workers are drilling holes eightmeters deep into the walls in which inch-thick metal bars are then cemented into place. The anchors stabilize the walls, their tips driven deep into the surrounding rock. But the mountain is protesting at these pinpricks, snapping off anchors here and there as if they were toothpicks.

Stewart, the site manager, and his crew are also securing the newly dug tunnel sections with steel arches. There is a continual creaking as parts of the arches shift with a jolt under the pressure of the rock. Between the steel arches the workers are installing concrete blocks the size of beer crates. “Crumple elements,” they’re called. Slowly but surely the mountain rock settles on the blocks, crushing and fragmenting them.

The forces at work in the Saint-Martin-de-la-Porte shaft are brutal. The shaft is the first stage of the high-speed rail link between Lyon and Turin. The key to the route is a 53-kilometer long base tunnel between the French town of Saint-Jean-de-Maurienne and Venaus in Italy. The tunnel will be excavated from both ends as well as from four intermediate points along its path—access shafts from which the tunnel can then be extended right and left. Bilfinger Berger, together with its French subsidiary Razel, is involved in two of these shafts, at Modane and Saint-Martin-de-la-Porte.

Work in Saint-Martin-de-la-Porte has been underway since March 2003. The men have fought their way a kilometer and a half or more through the mountain. They still have another 700 meters to go. Under normal conditions, tunnelers with special machinery can manage 20 to 25 meters per day. The workers in Saint-Martin-de-la-Porte are advancing at a meter a day, if that.

COMPLEX GEOLOGY
The black rock is so soft that the excavator can just scrape it away. Once a few cubic meters have been broken out, the digger speedily withdraws and the shotcrete pump takes over. In a wild dance, the operator on a platform lift jigs the spray back and forth.“The shotcrete is intended to stop the wall from falling in on us,”Stewart explains. Every few days the face of the tunnel is also pierced with 120 glass fiber anchors to prevent the rock from breaking away uncontrollably.“Of course there are technical reasons for securing the face, but the main aspect is the safety of the crew,” says Stewart.

Stewart picks up a few chunks of rock and crumbles them in his bare hands:“Coal and oil shale.”Not far from the site is the Col du Télégraphe, a massif familiar from the mountain stages of the Tour de France: but inside it looks very different. “The geology is incredibly complicated,” says Stewart.“The rocks are pushed together with no structure, just one beside another in crushed and distorted layers. It’s chaos!”

SQUEEZING THE TUBES
The tunnel builders have plate tectonics to thank for this confusion. The African plate is pushed northwards and butts against the European continental plate. As the two come together, the Alps are forced upwards like folds in a carpet. Layers of rock that once lay above one another are tilted diagonally and even swirled about.

As the experts have discovered, the inside of the mountain is far less peaceful than the peaks above.“The pressure of the mountain from the right-hand side is such that in ten weeks it squeezed the tunnel by up to two and a half meters,” Dominik Stewart relates.The previously symmetrical cross section of the shaft was squashed right in. As a result, the tunnelers changed tactics in their battle with the hill and adopted a new method. Phase one involves driving the shaft forward with caution. Every few hours the excavation stops and the men secure the roof and walls of the tunnel with steel arches and mesh—lumps of rock are constantly crashing down somewhere or other into the intercepting nets. “The safety frames are so flexible that as the mountain moves we can accommodate up to 60 centimeters of distortion,” Stewart explains.

In phase two the wall claddings are then reinforced.The crumple elements are positioned and more robust steel arches installed. Within days, cracks are visible in the concrete crumple elements. In this phase the mountain can still move by up to 50 centimeters.

Finally, in phase three, the floor and walls are concreted. The concrete walls are a meter thick. The floor, which was temporarily back-filled, is dug out and finished so that together with the walls and roof it forms an oval. Stewart smiles: “At this point there is no further convergence!”

In phase one, the cross section of the shaft measures 112 square meters. Once the mountain has settled and the walls are made safe with a concrete shell, the final area is half the size at 54 square meters. For every meter they advance, the miners use 500 meters of steel anchoring, 160 meters of glass fiber anchors and up to 90 cubic meters of concrete.

A few kilometers from Saint-Martin-de-la-Porte, up the valley in the direction of Italy, lies the Modane site where the second exploratory shaft and subsequent base tunnel access is being dug. Here the bore winds like a corkscrew down into the mountain. The conditions are far different from those at Saint-Martin: twice a day a ton and a half of explosives are detonated. The quartzite is so stubborn that after five blast holes the drill bit is worn out. Despite this, workers in Modane are advancing far faster than their colleagues in Saint-Martin-de-la-Porte. Daily progress is around seven meters.With 1000 meters left, it should take them until late summer to reach their destination. In the geological chaos of Saint-Martin, there is about half as far to go—how long that takes will depend on what other surprises the mountain holds.

(Text: Mathias Rittgerott, Photos: Frank Schultze)