Ground freeze dig for Berlin metro U5 final step 6 Dec 2018

In Berlin, the city’s transport agency, Berliner Verkehrsbetriebe (BVG), is building a missing metro link to run east-to-west from Alexanderplatz to Berlin Central Station (Hauptbahnhof), in two stages.

Schematic section of Berlin metro’s Line U5 extension

The first stage of the project was completed in 2009, in the west end, and extended the metro’s U55 line from the main rail station to the east with two new stations. The U55 extension runs below the government and parks district, where Bundestag station was built, and stops where Brandenburger Tor (Brandenburg Gate) station was constructed. The running tunnels were constructed using a Herrenknecht Mixshield TBM, and Brandenburger Tor station built using the pipe arch technique to freeze the ground to be excavated.

Those works left the missing link only slightly shorter, still stretching 2.2km from Alexanderplatz to Brandenburger Tor. Over this remaining gap, the new metro line would have a different name – Line U5 because it is, in effect, an extension of the existing U5 running on the east side of Alexanderplatz.

Video: Overview of Berlin U-Bahn’s Line 5 extension project (In German)

The route of the U5 extension passes below urban, historic and cultural buildings as well as bodies of water, complicating the challenges. The U5 extension involves construction of three new underground stations from east to west.

The techniques employed to build the entire missing link are many and varied. A mixshield was employed to bore the U5 twin tube tunnels. Station construction has been achieved by top-down method, except for Museumsinsel, sitting in saturated soils below both a water body and building, and consequently calling for ground freezing to hollow out the space for the station.

Progress is going well with works in their final stages. The difficult nature of the ground freezing challenge has placed the sequential tasks at Museumsinsel firmly on the critical path of the construction programme. With a milestone just achieved in building the central gallery, excavations are about to start on the platform tunnels, keeping the U5 extension looking for successful finish in 2019 with line opening in 2020.

Museumsinsel station’s three-cell cavern with freezing pipes

Initial Tunnelling

BVG, through its subsidiary Projektrealisierungs GmbH U5, was awarded the largest lot on the U5 extension to Bilfinger Construction in 2012. Subsequently, by late 2014, the contractor was acquired by Implenia. For this project, the acquisition included take over of all resources, technologies and personnel, and then pushing on with the construction challenge. Consultant Amberg Engineering has worked on the underground design.

The tunnelling challenge on the project has involved contending with glacial geology of massive deposits of sand and gravel, and groundwater level is 3m below the surface.

Construction has required sinking station foundations deeper than the sand deposits to anchor into underlying marl, undertaking dewatering, and the TBM drives contended with abrasive quartz in the sand.

Bracing the running tunnels before ground freezing

The twin tube, 5.7m i.d. running tunnels were bored by a 6.67m diameter Mixshield below overburden of 5-17m. The segmental lining was erected with closed elastomeric gaskets designed to withstand pressures up to 3 bar. To deal with driving in areas of low cover, a high-density support medium was employed, drawing upon experience on Klang Valley metro project in Kuala Lumpur, Malaysia. Ground limestone was included, helping to form a denser filter cake rapidly on the tunnel face.

Launched in 2013, the TBM sequentially completed each 1700m of tunnelling per tube and finished the last drive in 2015. Only the delicate break-in to the existing, operational Brandenburg Tor station was later required and achieved in 2017. Progress rates of about 20m/day were achieved.

Construction start at Museumsinsel

Museumsinsel station is near key national cultural venues, including the State Opera. The particular location found on the tunnel alignment for the station places it partly below the canal branch of the River Spree and also a building, respectively. The combination prevents excavation from the surface.

The chosen construction solution for the underground station below the building and canal is to freeze the saturated soil and excavate the ice plug in stages to form the full station cavern. The ice plug would be about 28,000m3, said Niehoff.

Excavating central cell of Museumsinsel station cavern

Excavating frozen ground at central cell

The first step in the works for Museumsinsel was to construct the concrete structures at each end of the station. These are shaft structures to provide both future access to the U5 metro but first to be the working sites from which the ground freezing operation and excavations are undertaken in the saturated soil separating them.

Each shaft was sunk from the surface to reach depths of up to 43m into marl. Diaphragm walls were sunk and the shaft pits were secured with jet grouted floors. For the walls, steel reinforcement was used except for the GRP rebar zones where the TBM drives would later pass through the shafts, building the running tunnels. Construction preparations for one shaft, beside the canal, required protection by coffer dam, and outside this area scour protection was laid as a bed of stones.

Having one mixshield on the project, the TBM bored the running tunnels in sequence. Based on test tunnel results, the TBM drives used a minimum high-density support medium of 1.3 tonne/m3 under the Spree and Spree Canal, driving with cover of about 6.5m. Up on the surface, on the canal bed, concrete slabs were placed for extra weight before the passage of the TBMs. Both drives had to be pass Museumsinsel before main excavation of the station with ground freezing between the shafts could start.

Ground freezing at Museumsinsel

The station cavern is arranged in a three-cell cross-section with the segmentally-lined running tunnels located in the outer cells. The construction sequence would be to, first, excavate the central cell holding the frozen soil between the running tunnels, and then break out from the existing tunnels to build the lateral cavern cells.

Having constructed the shafts and running tunnels, a further step was needed before ground freezing work could get underway. The running tunnels needed to be braced.

Fully excavated central gallery of cavern in frozen ground

“The lined tunnels need special support against the surrounding frozen ground,” said managing tunnelling director at Implenia Jens Classen. A frame concept was developed in-house for the task as a box frame with internal struts in an A-shape. Numerous steel frames were installed down the length of the tubes.

With the bracing support in place, the contractor moved next to install the pipes needed to freeze the ground. The works design calls for a pipe array enveloping a greater volume of soil than the station cavern itself, and ensuring a minimum cover of approximately 4.5m from the top of the ice mass to the canal bed overhead.

To install the pipes, a drill rig concept was developed, again in-house. Directional drilling was used to tightly control placement of the pipes over the 105m long distance between the shafts at the ends of the cavern. Drilling began in 2016 and the pipes were installed in a series of drilling campaigns. A further demanding aspect of the challenge was installation of the freezing items, starting from the pipe heads, said Classen.

Inner lining of central gallery before excavating rest of cavern

With the pipes in place, ground freezing was able to start in late February 2018. Timing of the excavations would then ‘depend on the development of the ice body,’ he said. By mid-May, excavations in the frozen ground were able to get underway, advancing the heading for the central cell. Breakthrough of the central gallery was achieved in late July.

Work then moved to install the inner lining, starting with the concrete invert onto which colonnades of temporary, circular steel, columns were placed down each side of the central gallery, and they in turn support the reinforced concrete crown, explained Classen.

The milestone of completing the central tunnel was achieved in late November. Next, the contractor is set to start opening up the two side cells. The task calls for ‘continuous demolishing of the TBM tunnels,’ he said, involving removal of the 350mm thick segments from the 1.5m wide concrete rings, currently braced against the frozen ground by the special strut frames. The lateral cells will be opened up in stages with headings, side and invert excavations to the full size of the cavern height and width. The last digging could be over by March, to be followed by lining works, said Classen.