Building the albabstieg TUNNEL is a major project on its own. Over 4.5 years the contracting team joint venture of Züblin and Max Bögl will drill and blast 12km of a new single tube tunnel for client Deutsche Bahn AG that will take new high speed trains beneath the network of roads that lead into the city of Ulm. “We are building two single track tubes each around 6km long. At the north end there is a small village called Dornstadt and on the south end it goes down to the main railway station of Ulm,” explains Matthias Abele, the project manager for Züblin.

The two 5.94km tunnels are part of a larger scheme to build a 59.6km new high-speed rail line between Wendlingen (Neckar) close to Stuttgart and Ulm to the southeast.

A total of nine tunnels are required accounting for 30.4km of the new and improved public transport connection, which will increase train speeds along the route from 70km/h in some sections to 250km/h. Along this route a further 40 structures are required as road and rail viaducts along with one of Germany’s tallest railway bridges in the Fils Valley in Mühlhausen im Täle. These measures will not only improve local connectivity, the line is a critical link in the Central European east-west rail corridor from Paris through France, Germany, Austria and Slovakia and on to Budapest in Hungary.

This enormous EUR 3.26bn (USD 3.46bn) project sits alongside the even larger EUR 6.53bn (USD 6.92bn) Stuttgart 21 project, which is transforming the city’s rail network. At the heart of this is the reconstruction of the central railway station, which will no longer be a traditional terminus style station where trains must enter and leave along the same line leading to congestion at busy times. Instead Stuttgart will have a modern drive through arrangement where trains may stop or pass by the station, which also has waiting areas and allows operators the flexibility to manage more trains effectively. As well as the station upgrade a new central railway ring will be constructed to allow more trains to connect into the city including a new station for Stuttgart Airport allowing it to connect in to the regional and long distance rail system.

Abele explains that the Albabstieg contract started in August 2013, with work beginning on site in December 2013. “We started with the excavation in April 2014 and actually we ended the excavation this week with the breakthrough in UIm,” he says (talking to T&TI in early December).

The location of the tunnel in the low-lying mountain range of the Swabian Alps naturally determined the need for drill and blast tunnelling.

“There are mostly geological formations of massive limestone (about 60 or 70 per cent) and in between is the possibility of cavities (voids) and also there is a second geological formation, which is a sedimentary rock, which in German is called Süsswassermolasse,” explains Abele.

To conquer this, the contractor used EUR 32M (USD 33.9M) of tunnelling equipment including 10 Sandvik DT 1130i drilling jumbos each with two drilling-arms and one lifting platform; eight CAT 328 excavators, eight Volvo L 150 tunnel loaders and 18 Volvo A 30F dump trucks for rock removal.

“Around 30 per cent of the rock is reused in this project and the other 70 per cent is taken away and used as fill in different places,” says Abele.

The drilling strategy required using the rigs in three locations in each tunnel. Four began at the north of the bore at Dornstadt (two in each tube), two excavated the mid-section and two more pairs worked south from the middle of the tunnels to Ulm. “In 24 hours we moved about 4.5m in one direction. For drill and blast we are working 24 hours, seven days per week. A complete cycle takes four to five hours. With excavation and transport and bringing out the shotcrete into the tunnel we do four times in one day,” says Abele explaining that the maximum rate was 6.5m [double] in 24 hours.

As is typical with drill and blast the team usde the rig to drill the holes, which were then loaded with explosives. The team then maintained a 150-200m distance back from the face as the charges were exploded. The removal of this rock then took another two hours. Following on from this shotcrete is minimally sprayed at a 200mm thickness.

Where the ground contains softer rock steel mesh reinforcement is required within the shotcrete along with steel ribs every 1.5m and the shotcrete increases to a thickness of 350mm. “The rock is difficult as it goes from very hard to soft and is at both extremes in the project,” says Abele.

“For very hard rock you have to drill 140 holes and at the other extreme you only use a tunnel excavator or around 40 holes with drill and blast.”

Once the second layer of shotcrete has been sprayed the team can immediately move on to the next advance. “The shotcrete quality means we have no time to wait and can start the drilling machine for the next 1.5m,” says Abele.

For the whole project team, which had a peak labour force of 450 people (drill and blast, inner lining concrete shell and groundwork outside the tunnel), the most challenging aspect of the bore was the passage beneath buildings in Ulm. “There were big houses four to five floors built in 1940s or 1950s, which were very difficult to excavate under,” says Abele explaining that the minimum cover here was just 11m. “We had to excavate very carefully,” he says explaining that the advance rate was slowed to just 0.5m in 24 hours.

As for the inner lining work this began in mid 2015 using large formwork rigs from sub-contractor Baystag to place the cast-in-situ concrete.

“We are using two formwork machines in each tunnel for the cast-in-place concrete. The inner lining is about 400mm to a maximum 500mm thick. The reinforcement requirement depends on the structural engineering on average 80 to 110 kg/m³,” he says.

Waterproof liner sheets are sealing the tunnel using 2mm thick membranes manufactured by Germany’s Naue, installed on site by Strabag’s waterproofing department.

Now that construction has moved into the lining and fit out phase the number of people on site has fallen to 200 and by mid-2018 the tunnel will be complete, providing a vital 6km link on the new Wendlingen to Ulm high-speed rail.