Under construction for nearly 20 years, the Gotthard Base Tunnel has now been officially inaugurated. Around four million tons of concrete went into the 57-km tunnel – around forty times as much as into the Burj Khalifa skyscraper in Dubai, the tallest building in the world. BASF supplied concrete admixtures, cement injections for sealing, and fire protection mortar for the construction project.
In the 20-km long construction section between Erstfeld and Sedrun, admixtures from BASF were added to the concrete used in the two parallel tunnel tubes and the cross-connections that interlink the tubes every 300 m. “The concrete used for the work underground had to satisfy contradictory requirements: the concrete for the kilometers of section into the mountain had to remain workable for many hours and was not allowed to harden. But during shotcreting, the concrete was required to set lightning-fast. The was one of the biggest challenges in the Gotthard Tunnel,” said René Bolliger, Head of Underground Construction at BASF for Germany, Austria, and Switzerland. The solution consisted of an effective combination of concrete admixtures from BASF – its MasterGlenium superplasticizers make the concrete specially flowable and workable. BASF, in addition, developed a special admixture for the Master-Set product family for use in the Gotthard Tunnel, which considerably retards hydration of the cement. In this way, by combining BASF superplasticizers, the concrete was put to optimal use after long transport routes and under the high temperatures in the interior of the mountain. The system, consisting of superplasticizer and long-time retarder, was used both for initial stabilization following excavation of the tunnel as well as during concreting of the inner shell.
“We work continuously on innovations that make our customers even more successful. Our most recent example for this is MasterEase UG 3904 for use in underground construction. The advantages of this product include the benefit that, in combination with non-alkaline shotcrete accelerators, it can be mixed and pumped even more effectively than before,” says Bolliger.
Concrete solutions for deep shafts
Two vertical shafts, more than 800 m deep, extend from the mountain station of Sedrun down to one of the two multifunction stations in the Gotthard Base Tunnel. There, solutions by BASF were used as well: this area was first stabilized with shotcrete, as for the tunnel itself, and subsequently provided with a wall of in-situ concrete. “Delivery of the concrete down into the shafts via a downpipe represented a special challenge. We had to ensure that the constituents of the concrete would not separate while dropping,” explained Bolliger. By selection of the best MasterGlenium concrete superplasticizer and by optimally adjusting batching to the conditions at the site, BASF experts succeeded in keeping the concrete workable even after falling several hundred meters under gravity. To guard against water ingress, a MasterRoc MP 650 microcement was additionally injected into rock fissures and cavities.
Fire protection with BASF
Fires are a great danger in tunnels, and the requirements placed on fire protection are strict. Tunnel concrete heated above 1,000 °C loses its loadbearing capacity and the tunnel collapses. For that reason, the Gotthard Base Tunnel wall was coated with a special fire protection cement from BASF: MasterRoc FP 1.350. “Thus protected, the tunnel walls withstand temperatures of up to 1,400 °C for at least 90 minutes. That is valuable time for the fire fighters,” says Frank Clement, BASF expert for fire protection solutions underground.
Construction of the Gotthard Tunnel, the longest railroad tunnel in the world, has cost around 9 billion euros. This Alpine crossing will reduce traveling time between Zurich and Milan by around one hour. The gradient on this route was notably reduced, with the highest point at a height of 550 m it is not quite 90 m higher than the northern tunnel portal in Erstfeld. Freight trains can therefore travel the distance with double the load and speed. In comparison: The old Gott-hard railroad passes through a height difference of 700 m between Erstfeld and the apex of the first Gotthard tunnel, which was opened in 1882.