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Copenhagen metro: Gently does it

Groundwater management and care for ancient buildings feature heavily in construction of Cityringen, the new metro line under Copenhagen’s historic centre. Jessica Rowson reports.

Preparations for Copenhagen’s Cityringen − roughly equivalent to London’s Circle Line − are of necessity, painstaking and thorough, with concern for ancient property heightened by the fatal collapse on a metro scheme in Cologne, Germany earlier this year. Much essential work has been done to ensure safety and with tender documents now issued, construction should begin late next year − all being well.

The new line is a significant addition to Copenhagan’s metro system, whose first two lines opened in 2002 to connect the city centre with the airport and open up outer suburbs for development. Cityringen will link major parts of the city centre to each other and to the original metro which bisects it.

“This one is made for the people living in Copenhagen,” says project manager of client Metroselskabet Jens Gravgaard. “The idea of Cityringen is to link people in the densest part of Copenhagen by rail and provide an alternative to the car.”

Copenhagen is situated on the islands of Zealand and Amager with water never far away. Most tunnelling projects demand that care be taken to avoid damage to property and this is true in the Danish capital.

Cityringen: in the running

Companies prequalified to bid for the civils works

  • Alpine Bau in consortium with FCC Construcción
  • Bilfinger Berger Ingenieurbau in consortium with VINCI Construction Grand Projets and Per Aarsleff
  • MTHøjgaard in consortium with Hochtief Construction, Ed Züblin and E Pihl og Søn (Denmark)
  • SALINI Construttori in consortium med TECNIMONT and SELI Società Esecuzione Lavori Idraulici.

Companies prequalified to bid for the transportation works

  • Ansaldo
  • Bombardier Transportation
  • Construcciones y Auxiliary de Ferrocarriles (CAF) in consortium with Thales Transportation, Signalling and Security Solutions, Balfour Beatty Rail Danmark and Keolis Nordic
  • Siemens in consortium with Siemens STS, Siemens and Hyundai ROTEM

In particular, protection of historic buildings means that the groundwater table needs to be closely monitored and maintained. “In old historic city parts, we are not allowed to lower the groundwater table,” says Gravgaard. “Houses sit on timber piles and if the top of the pile is exposed it would rot.”

Cityringen’s stations will be typically cut and cover box structures − around 64m long and 20m to 22m wide − constructed top down. Station platforms will be typically around 19m below ground level. The retaining walls need to be watertight and rigid. Groundwater will be pumped from the excavation to keep it dry, and then back into the aquifer outside the excavation to make sure that the groundwater table is not lowered.

“The idea of Cityringen is to link people in the densest part of Copenhagen by rail and provide an alternative to the car.”

Jens Gravgaard, Metroselskabet

“The walls will serve as a waterproof barrier,” explains Gravgaard. “Pressure relief wells will be installed in the bottom of the excavation, so water flowing in through the excavation floor is extracted and goes back into the acquifer. The inflow can be minimised by grouting or extending the cut off of the box, making the walls longer.”

As well as having to be very careful with groundwater management, the contractor will also have to deal with awkward sites in a congested city centre close to existing buildings that are several hundred years old. “All the stations are in narrow squares, which are difficult, and small sites,” says Gravgaard. “It will be a logistical challenge.

We expect bidders to compete by describing how they will protect the buildings. They had similar conditions in Leipzig and compensation grouting [which consolidates poor soil] was used to protect buildings. We have reserved areas for grouting wells.”

Public antipathy about Cityringen peaked earlier this year following the collapse of Cologne’s archive building into an excavation for a new subway, killing two. Danish politicians demanded assurances that the proposed construction method was safe (NCE 18 March 2009). There was also unhappiness with possible impacts of a planned station near the historic Frederikskirken Marble Church.

Metroselskabet agreed to the station being placed 2m deeper and 1.2m further away from the foundations of the church than originally planned. Permission to proceed with Cityringen followed the decision.

“We’re building the box on the doorstep of a structure which has one of the largest domes in northern Europe. There has been lots of nervousness.”

Jens Gravgaard, Metroselskabet

“Frederikskirken is highly challenging, technically,” says Gravgaard. “We’re building the box on the doorstep of a structure which has one of the largest domes in northern Europe. There has been lots of nervousness but we managed to convince Parliament by technical arguments. It’s not often politicians listen to engineers.”

Cityringen will have 17 stations covering the major parts of the city centre as well as the Østerbro, Nørrebro and Vesterbro districts and the Municipality of Frederiksberg currently not covered by S-train (the urban over-land railway) or Metro line services. It connects to the existing metro at two stations − Kongens Nytorv and Frederiksberg.

A joint venture of Cowi, Arup and Systra produced the tender design for the new line’s civils work but the selected contractors (see box) will be expected to carry out preliminary and detailed design under a design and build contract.

Construction of the cityringen

The new 15km long metro will run exclusively in tunnels, between 20m to 35m below ground.

Most of the tunnelling will be through limestone − underlying opper and intermediate Copenhagen limestone bedrock typically starts between 10m and 30m below ground level − but in the northern part of alignment it will run through overlying quaternary soils.

Around 13.6km of the new line will be constructed using four tunnel boring machines (TBMs) approximately 5.7m in diameter. Earth pressure balance TBMs are to be used through the limestone, perhaps with a switch to a slurry type TBM through the northern section with quaternary soils.

There will be three shafts − one between Trianglen and Østerport in the northeast, one between Nørrebro and Nørrebros Runddel in the northwest and one on the branch for the control and maintenance centre in the south.

The TBMs will work in parallel − two on each contract. For the northern contract, the TBM will enter at the north western shaft and work its way counter clockwise to Aksel Møllers Have.

Then it goes back to the north western shaft and works its way clockwise to the north eastern shaft. For the southern contract the TBMs will start at the control and maintenance centre and bore clockwise to Aksel Møllers Have. Then they will move to the north eastern shaft and bore clockwise to København H.

However, the fork section to København H will be sequentially excavated and secured using rock bolts and a sprayed concrete lining, as it will be easier to form the cross section for the branch line using this method. Initial support for the tunnel will be provided using a combination of sprayed concrete, rock bolts, lattice girders, grouted steel piles, soil grouting and freezing.

Tunnelling works should be complete in between 30 months and 36 months.

“There are between 1,000 and 1,300 tender drawings,” says Gravgaard. “We’ve got lots of knowledge from building a metro before.” This experience led to Metroselskabet placing a lot of importance on accurate geological information for the sensitive tunnelling projects.

Around 350 individual geotechnical holes were undertaken during 2007 and 2008 and the number of boreholes and geotechnical tests has since been increased. “We decided to extend and expand,” says Gravgaard. “We’re now testing 500 boreholes and supplementing this information with seismic testing.”

“Bidders can bid for either north or south or both. The size of the contract is large and we wanted the possibility to have more competition.”

Jens Gravgaard, Metroselskabet

For the seismic testing, geophones − which measure ground acceleration − were placed on the ground at approximately 5m spacings. A car fitted with vibration equipment was then used to create a shaking motion, generating shock waves. These waves travel through the ground and are reflected by hard strata. In this way it can be calculated at what depth hard strata lies.

“It’s a way of creating a long profile,” explains Gravgaard. “We can discover structures in the geology of hydraulic significance and find fracture zones. It’s like an x-ray. We did it during night time this spring as we had to close roads.”

Service diversions for the project have started around the city and the main construction contract is due to start next year. Metroselskabet issued tender documents this month for the three contracts that will make up the bulk of the work.

One contract covers the civil works on the northern part of Cityringen, another the civil works on the southern part and a third the transportation system and operation and maintenance. “We are letting south and north separately,” says Gravgaard.

“Bidders can bid for either north or south or both. The size of the contract is large and we wanted the possibility to have more competition. Bidders have the chance to win one or both contracts.”

The tender process is expected to be concluded during the autumn of 2010 and, according to Gravgaard, construction work should start around three to four months after that.

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