A ride on TULA
Two tunnel boring machines are currently digging the tunnel system of the European XFEL. Our newsletter team paid one of them a visit.
To be honest, its speed is not what you would call breathtaking. At about 15 millimetres per minute, you must try hard to notice it at all. But if you focus on the walls of concrete that were not there a few hours ago, you see: it is moving and you with it.
Welcome to TULA! That’s not only an acronym for “TUnnel for LAser”, but also the name of one of the two tunnel boring machines that are currently digging through the underground of Schenefeld and Hamburg. Their mission: to create the 5.8 kilometre long tunnel system for the European XFEL—6 to 38 metres below ground.
TULA is a small marvel of engineering, though a lot less comfortable than even a London underground subway train. TULA was made in Germany by Herrenknecht AG, the world market leader in the construction of tunnel borers. If somewhere in the world a tunnel is to be bored through ground comprising sand, rocks, marlstone, and water, chances are high that the tunnel boring machine is from this manufacturer located in the Black Forest in the south-west of Germany.
TULA has a price tag of 18 million euro, it is 71 metres long, weighs 550 tons, and has a diameter of a bit more than six metres. With that size, TULA is big, but not the biggest of its kind. There are tunnel boring machines with over three times its diameter. Inside the complex network of tubes, pressing cylinders, rock cutters, tongs to crush stones of up to half a metre, and steel, your guts feel and your ears hear the sheer power of this colossus–1903 kW or 2587 PS.
During the current shift, Martin Kühnel is in charge of TULA. The 29 year old engineer is a tunnel builder with heart and soul. There is joy, almost pride in his eyes and voice when he guides the visitor through TULA. It is his fourth tunnel construction site. Just before his assignment to the European XFEL underworld, he participated in the tunnel boring of the Gotthard tunnel—which is admittedly a bit longer than the European XFEL: “I came to tunnel boring by chance. But now I cannot imagine doing anything else. It’s challenging, you have to swiftly react to new situations. If something stops working, you cannot just say: ‘Well, let’s get another tunnel boring machine’, as you would do with an above-ground excavator. You have to fix the problem—quickly.”
Tunnel boring is no office job indeed. In fact, tunnel boring used to be a quite dangerous business. But thanks to modern technology this has changed. However, there is still the habit to christen the tunnels before the first dig is made. In addition, Saint Barbara has a close look over the works—her statue watches the entry of each tunnel. Of course, such christening celebrations are also welcome opportunities to see the sun once in a while and have a beer.
Twelve pressing cylinders are used to steer the tunnel boring machine in the right direction. What’s right and what’s wrong is measured by means of laser beams. The comparison of actual and nominal values then results in the adjustment of the pressing cylinders. That’s the job of the shield driver. He has to monitor and adjust all parameters in a small room in the front part of TULA, fully computerized with buttons instead of a steering wheel.
One of the shield drivers is Dieter Preuß. Right now, his job is tougher than usual: “There are lot of rocks in the ground.” That means lots of flushing and the use of the stone crusher. But this situation is not new to him. He has stopped to count the tunnels he has bored so far—in Switzerland, the Netherlands, France, and Germany. He has been in the business for 22 years and seems relaxed. And that’s not only because he trusts the electricians of the team: “The statue of Saint Barbara must be properly illuminated all the time. That prevents harm”, he says with a twinkle in his eyes.
12 people work for TULA on one shift. That number comprises the workers in the tunnel borer itself, but also people who are in charge of the supply of new wall elements and tracks for the tunnel train. There are three such shifts, one shift lasting 12 hours, for eight days. After that, there is a break of 96 hours.
On average, TULA makes 12 metres per day. With that speed, it needs about half a year to build the two kilometre long accelerator tunnel. In a few years, electrons will fly through beamlines here—a bit faster than TULA: They will cover the distance in only a few millionths of a second.
A hydro-shield tunnel boring machine
TULA is a mixshield tunnel boring machine, which can operate in soft ground. Its rotating shield cuts through the ground and is followed by an impressive amount of supporting machinery.
To avoid uncontrolled collapsing of the non-excavated ground in front of the shield and to flush away the excavated sand and rocks, a supporting liquid is used—a suspension of the mineral bentonite and water. The mix of suspension liquid and excavated material is separated on the construction site and the bentonite reused.
By means of pressing cylinders, TULA props itself on the already built tunnel walls and pushes itself forward, while the cutter head rotates up to three times a minute. After one and a half metres, TULA stops and it is time for the built-in erector, a crane that brings pre-manufactured parts of the 30 centimetre thick concrete wall into position. After 30 to 40 minutes, all these tubbings are in place and TULA’s ride starts again. 24 hours a day.
Author: Dirk Rathje