Im März 2012 begann mit Herstellung der ersten Schlitzwände die Rohbauphase zum Weiterbau der U‐Bahnlinie U5 in Berlin‐Mitte. Mit diesem U‐Bahnbau erfolgt ein wichtiger Lückenschluss im Berliner Nahverkehrsnetz zwischen den derzeitigen Endhaltepunkten der U‐Bahnlinien U5 am U‐Bhf. Alexanderplatz und U55 am U‐Bhf. Brandenburger Tor. Der neue Streckenabschnitt wird die Erreichbarkeit der für Touristen und Bürger gleichermaßen bedeutsamen Ziele rund um die Museumsinsel, entlang der Straße Unter den Linden und im Regierungsviertel maßgeblich verbessern und erstmalig eine direkte U‐Bahn‐Anbindung dieser Ziele an den Berliner Hauptbahnhof schaffen.Bauwerksreste und Gründungselemente vergangener Bauepochen, eng gestufte Sande, Zonen mit Geschiebemergel und Steinen sowie ein hoher Grundwasserstand von rund 3 m unter Gelände prägen den Untergrund im Bereich des Bauvorhabens. Diese eiszeitlich geprägten und für das Berliner Urstromtal typischen Baugrundverhältnisse stellen eine große Herausforderung für die Planung und Ausführung der Baugruben und der Tunnelvortriebe dar.Spree und Spreekanal beidseits der Museumsinsel sowie Bauwerke, historische Gründungen und das in Bau befindliche Humboldtforum müssen im Rahmen der Tunnelvortriebe unterfahren werden. Ferner sind die zuvor hergestellten, jedoch dann noch nicht gelenzten und noch nicht ausgehobenen Baugruben ebenso zu durchfahren wie auch Störungszonen im Bereich von zuvor zu bergenden Spundwänden und Ankern.About the building works on the metroline U5 in Berlin‐MitteIn March 2012 preliminary building works on the metroline U5 in Berlin‐Mitte started with the construction of the first diaphragm walls. This metroline intends to close the gap between the existing metro stations Brandenburger Tor and Alexanderplatz. The new rout section will improve the accessibility of main destinations around Museumsinsel, Straße Unter den Linden and of the government quarter for tourists as well as for citizens of Berlin. Furthermore it offers a direct connection from the aforementioned places of interests to the central railway station.Structural remains and foundations of past epochs, poorly graded sands, zones of marly till, stones as well as a high groundwater level of around 3 m u gsf form the underground conditions of the project area. The glacial subsoil, which is typical for the galcial valley of Berlin, poses a great challenge for planning and execution of construction pits and tunnel excavation.Spree and Spreekanal at both sides of the Museumsinsel as well as buildings, historical foundations and the Humboldtforum, which is already under construction, have to be crossed underground by the metroline. Furthermore construction pits, accomplished before, but not deballasted and excavated at the moment of undercutting have to be crossed as well as fault lines around retrieved anchors and sheet piles

In the Irlahüll Tunnel section to Ingolstadt, the high-speed rail route Nuremberg-Ingolstadt-Munich passes through limestone and dolomite with pronounced karst character. Various karst structures were encountered during the driving of the Irlahüll Tunnel. These were classified into four karst types. In order to activate the load-bearing effect of the ring of rock around the tunnel, karst structures had to be stabilised. The required thickness of the treatment was determined by structural calculations. Four specialised civil engineering contractors carried the work out.The challenge was to appropriately stabilise slightly-to strongly-jointed rock, heterogeneously interspersed with karstification structures. The treatment ranged from filling cavities to grouting loose material and rock. The karst structures were stabilised according to karst type, with materials ranging from cement suspension to concrete. The karstified rock mass was systematically drilled, each individual borehole was geologically evaluated and, where required, further investigated by inserting a camera. The quantity, strength and pressure of the grout used were evaluated continuously by the team of experts for the site supervision and appropriate further steps were decided. The success of the stabilisation was demonstrated through quality control drillings and control grouting. The improvement of the rock mass was achieved to the required degree.

An approx. 2.2 km long section of the U5 in the city centre of Berlin is currently under construction to close the gap between the existing underground structures at the Berliner Rotes Rathaus and Brandenburger Tor stations. Construction started early 2012 and the connection between Alexanderplatz and Brandenburger Tor is planned to open in 2019. The alignment starts from the newly built Rotes Rathaus Metro station, passing under the River Spree, the planned new Berliner Schloss and the Spree channel, and then follows the street Unter den Linden before arriving at the Brandenburger Tor. Three new stations have to be constructed at Rotes Rathaus, Museumsinsel and Unter den Linden, with the centrepiece being the Museumsinsel station where ground freezing techniques are being used.

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