HOCHTIEF is one of the leading tunnel builders. Currently we are expanding underground transport network systems in metropolises worldwide. © HOCHTIEF

Underground

We go diving in the Black Forest, awaken sleeping concrete in the Alps and fill in holes under the River Thames. Our tunnel construction teams are real hits. And not just because they cause a hell of a row with drilling and explosives.

HOCHTIEF is one of the leading tunnel builders. This sounds fairly straightforward but there is a lot to it. Our experts blast, drill or dig. And they use the longest tools in the world – our tunnel drilling machines are several hundred meters long. They constantly develop new methods to reach the end of the tunnel even faster and more safely. And when we have finished with our job, this changes the route of millions of people. Through the Alps – much faster with the Gotthard Base Tunnel, a mega project. Through London – no problem with the new train line, for which we drilled under the River Thames. Currently we are expanding underground transport network systems in metropolises like New York, Copenhagen, Sydney or Hong Kong. Exciting jobs, where every project is a technical masterpiece.

Diving in the Black Forest  
This “swimming pool” is really something. 800 meters long, 25 meters wide and up to 20 meters deep. Chock full of groundwater. And this has to be removed, because in the future trains are to run here. The Rastatt Tunnel, which we are currently working on, belongs to the Intercity Express (ICE) railway route Karlsruhe-Basel. If you want to get rid of water, you have to use divers. The underwater experts go to the base of the “pool” and smooth the surfaces of the construction pit, which are still undulating after the excavation work. And they screw steel plates onto hundreds of anchors, which serve as the basis for the concrete which is then pumped onto this, two meters thick, and gradually dispels the water. We are working our way forwards along a length of eight soccer fields. Each hour 80 cubic meters of concrete flows through the pump, a volume equivalent to 550 completely filled bath tubs. It must be precisely distributed. Until the base is hard, compactly sealed and so secure that the water can flow out of the basin. These are all preparations for being able to use the tunnel drilling machine. With this machine the work then really gets going.

Sleeping Concrete
Concrete hardens. Sometimes faster and sometimes slower. But what can be done if the journey to its operating location in a tunnel is several kilometers long and hence becomes very time-consuming? Generally a concrete mixer truck cannot drive in a tunnel construction site. The concrete mixture therefore has to contain a formula which keeps it usable in accordance with the transportation time. The concrete is allowed to “sleep” and then only “awaken” when it is being used for construction. Our tunnel construction experts for instance used such “waking agents” for the Gotthard Base Tunnel in Switzerland. This is to ensure that the quality of the tunnel tube is still excellent after 30 kilometers of underground transportation route and to make sure that no lumps form.

Filling in Holes
We keep cool, even if millions of cubic meters of water flow over our construction site. For instance, during the construction of a tunnel under the River Thames for a rail connection in London. Extreme water pressure bears down on the River Thames tube. The nearer we worked to the river bed during tunnel driving, the higher was the risk of water intrusion due to geological fault. The soft ground was particularly permeable at the interfaces to the connecting tunnels. Determined, coordinated and careful action is required if large quantities of water suddenly leak and enter the tunnel. With our know-how, there is no danger that the River Thames dries up like an emptied bath tub when the plug is pulled out. We fill holes with shotcrete and continue working until we see the light at the end of the tunnel.

The Language
Tunnels are “driven through” if a mountain, a city center, a river or the sea is in the way. The people who build them, also called miners, move enormous volumes of material, work with the highest pressures and expect new challenges every day. Their guiding principle “it is dark in front of the pickaxe” still applies today despite sophisticated geological surveys. Because, after all: despite precise planning and geological surveys, you never know what awaits you deep under the surface of the ground when the tunnel driving has already started.

The Refrigerator Trick
In sensitive areas – for instance in the middle of a major city, underneath a railway line or in a nature conservation area – the surface cannot be allowed to move much when a tunnel is being built underneath. But what can be done if the groundwater cannot be lowered or can only be marginally lowered and the ground is very soft? This is where we use the “refrigerator trick“. We drill holes around the tunnel cross-section, pump cooling agents into the ground – and the refrigerator is ready. And we can drill or dig in ice just as easily as in solid stone. For instance, this is how we built the underground railway station Brandenburger Tor in Berlin. And of course when we had finished drilling, we thawed the ground again.

Sudden Big Bang
Sensitivity is required when we blast. Everything must be precisely coordinated with another if we are underground and enter ground in a conventional way where nobody has ever stood before. If the drilling machines have done their job the explosives are filled into the boreholes, wired up and then it’s “take cover”. And the hellish noise can occur – ignition successful. In this way we break off one to two meters of the hard stone – depending on geology and tunnel diameter. Around one kilo of explosives is sufficient for a cubic meter of tunnel excavation. Depending on the diameter and length of the tunnel, a few hundred kilos of explosive material can be used. This is followed by shotcrete, steel arches and anchors to secure profile of the tunnel. Finally the excavators do their job and clear up, before the drilling and blasting starts again. In this way we move forwards, meter for meter. And like a big bang a new transport route is suddenly created.

Bridge in the Tunnel
Sometimes after the big bang suddenly more rock falls down than planned. Then we have to be careful. Caves the size of detached houses can appear. Which was what happened, for instance, during the construction of the Irlahüll Tunnel on the railway route between Nuremberg and Ingolstadt, where we stumbled upon huge caves which also impressed experienced speleologists. You then don’t just pour concrete into them, but instead consult seismologists and geologists and make test bores. This results in special solutions for tunnel construction, because ultimately an intercity express (ICE) must be able to shoot through here at a speed of 300 kilometers per hour. We therefore built a bridge in the tunnel. It runs across critical geological areas, to ensure that the high-speed trains stay on track. And so that watercourses can continue flowing through the stone, like they have been doing for millions of years.

07.06.2017
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