With wonderful historic buildings resting upon complex geology and a high water table, Copenhagen’s ambition to radically expand its metro network was a huge coupling of technical and environmental challenges for tunnellers, especially in deciding how best to manage the groundwater regime.
Entering the new century, the Danish capital had a metro vision to build. Many lines and stations in an expanding network were planned. The lines would call for large-scale construction projects, mainly as major tunnelling works, including TBM bored tunnels and many deep excavations.
Protection of the local environment and old buildings, founded on timber piles in saturated strata, were among the engineering complications to be solved, demanding a balancing act of pumping water out to keep the deep excavations dry and stable, while near the digs the water is put right back in, recharging to support the water, naturally generally close to the ground surface.
The mostly underground metro network was developed over the last two decades, built in phases by client Metroselskabet. The principal project, enlarging the early metro network, added a 15.5km long, entirely underground loop in the form of the Cityringen project (the Circle Line – line M3 on the network). Cityringen includes 17 underground stations constructed at depth in the saturated strata.
A branch line (M4), mostly in tunnel, would follow and includes the Nordhavn and Sydhavn projects – the latter still underway. Further transport improvement for the city includes an above ground, surface-only, light rail line named Hovedstadens Letbane. The project developer and owner of all the lines if the group holding sister companies Metroselskabet and Hovedstadens Letbane.
TRANSPORT HUB OF THE NORTH
Upon completion of the extensive tunnelling works for Cityringen, Copenhagen looks relatively untouched. The subterranean powerhouse only subtly shifts the surface scenery of the capital, even as the metro network helps the city become even more of a lynchpin in the transport network of northern Europe.
For Copenhagen sits at the juncture of Scandinavia and continental Europe. With such a strategic location, Denmark has been boosting the transport capacity within the capital while also looking farther, to move beyond ferry connections to its neighbouring countries by building colossal road and rail projects, as fixed links reaching across seas.
To the east of the Nordic coastal city, across the Oresund sea, the gap to Sweden feels small. Quickly, via the 16km long Øresund bridge and tunnel link, landfall is made and the city of Malmö reached. There is much commuting between the cities, and opportunity to easily move out into the Scandinavian rail and road networks.
On the other side of Copenhagen, to the west and south, Denmark spreads its own national transport networks, including lines toward the ferry connections to Germany. The next giant project for Denmark is to build a fixed link to Germany in the form of the world’s longest immersed tube tunnel. Main construction is underway on the road and rail link, which is to be opened before the end of the decade. Denmark, then, will be more tightly bound into the wider continental European transport network.
With more travel through Denmark, all paths will cross through Copenhagen. As ever, key to that transport hub’s success is its metro system, built after tunnellers took on the construction challenges of deep excavations along with sensitive groundwater management.
GEOLOGY AND GROUNDWATER
Under a fill layer of typically 1m-5m thickness, Copenhagen rests upon Quaternary deposits over limestone bedrock. The Quaternary deposits include 10m to 30m thickness of meltwater sands and gravels and clay tills, and random boulders are present. While highly variable with large heterogeneity, the deposits comprise a recognised sequence of sand/gravel and clay till layers, the client’s geotechnical and hydrology manager Enrico Paulatto explains to T&T.
In the northern area of Cityringen’s tunnel alignment, the deposits of meltwater sand gravels were deposited directly upon Paleogene layer of mainly Danian limestone. To the south west, the geology in that section of the metro loop is slightly different with the deposits mainly clay till on the limestone bedrock.
The underlying limestone is horizontally bedded with different induration and flint layers and other hard horizons, and bioturbated zones, notes Paulatto. Glacial processes disturbed the upper bedrock, which is usually heavily fractured. The Copenhagen Limestone is classified in three subdivisions, based on geophysical data from borehole logs, plus five degrees of hardness, or induration.
There was marked variability in permeability found from borehole data, which presented a challenge for hydrogeological profiling. The sand and gravel meltwater deposits are highly variable with different hydraulic properties. The Copenhagen Limestone may contain several flow zones separated by zones of no flow. Generally, the horizontal permeability is 3-10 times that of vertical.
Copenhagen’s terrain is mostly low level, relatively flat and by the coast. The limestone, and sand and gravel layers in direct hydraulic continuity with the bedrock, constitute the primary aquifer in the area. Minor secondary aquifers can be found in fill layers and sand deposits that are not in hydraulic continuity with the primary aquifer. Groundwater levels in the secondary aquifer may be influenced by water levels in the primary aquifer in some locations, while in other areas the groundwater levels are influenced by the harbour, says Paulatto.
The importance of a stable high water table is made even moreso with many buildings having wooden piles or founded on disturbed layers in the eastern zone of Cityringen, which create relatively greater risk sensitivity to changes in groundwater conditions. Other zones requiring management of groundwater to ensure protection to the potable water supply and avoid any potential risk of pollutant migration.
CONSTRUCTION
Copenhagen’s metro began with a single line of 4.9m i.d. twin tunnels, branching in two (M1 and M2 lines), running from the west of the city to the south east side and holding a total of 22 stations. The first part of the driverless metro system opened in 2002 and, while works continued, studies began to examine options for the Cityringen loop (M3).
Cityringen has two stations (Kongens Nytorv, Frederiksberg) tying into the earlier metro lines (M1, M2), and three other transfer stations (København H, Nørrebro, Østerport) on rail networks.
Design and procurement for Cityringen built upon lessons learned in the earlier tunnelling, including handling flint and dealing with a blowout below a lake. Main activities for the new, entirely underground, loop began in 2011, arranged in two main contract lots for completion well before the end of the decade. Both contracts were awarded to CMT, a JV of Salini (then became Salini-Impregilo), Tecnimont and Seli. Groundwater management for the JV was by Holscher Wasserbau.
Cityringen is 15.5km long loop with single-track, twin tube, 5.78m o.d. tunnels, about 20m apart at their centre lines. EPBMs were to bore the running tunnels through different ground conditions along the alignment, varying from limestone to mixed face (such as the Kongens Nytorv – Marmorkirken stretch) to soft ground (Vibenshus Runddel – Skjolds Plads), respectively.
The shields were manufactured to Kawasaki-Seli designs by Seli and Seli Tunneling Denmark. Cutter and ripper tools from Palmieri, cutter bits and bucket from Bewarder, and the 1.4m long, 300mm thick precast segments were produced by Mobilbaustoffe.
In addition the project would also build many deep stations, as open box structures, typically 64m by 20m in plan. Station construction would involve either sinking secant piles (at 15 work sites) of 1000mm diameter, spaced at about 800mm centres, to an average of 28m; or, diaphragm walls (six sites), made up of 2.8m long by 1.2m wide panels excavated to depths as much as 44m.
Further works included opening up caverns, sinking three construction and ventilation shafts, and performing major fissure grouting, jet grouting, including works from compensation grouting shafts.
GROUNDWATER MANAGEMENT
Underground construction in the saturated strata called for use of abstraction wells at the bottom of the deep station box excavations. The abstracted water was treated and most (95% or more) reinjected close to the deep excavations to recharge the groundwater body, says Paulatto. This pairing of dewatering on one side of the watertight walls and recharge on the other maintained the water table level and stability of the old foundations below buildings near the alignment.
In planning, a 3D software model helped prepare to keep the groundwater system stable during the construction works.
When and where dewatering was allowed for the new deep structures it was only during the construction stage. About 850 abstraction wells were drilled for groundwater lowering within excavations on the project’s 21 construction sites. The box structures were designed to be watertight and where needed extensive but pinpointed grouting sealant works were undertaken at planned key locations, such as at the cavern at Nørrebroparken, against groundwater ingress and commensurate flush out of solids.
Provision for possible use of some emergency pumping wells (some in basement of buildings) was made, though, along shallow parts of the alignment of the TBM bores, should boulders or other obstructions be met in limited cover zones that might prevent use of hyperbaric intervention at the face. Brief, local, emergency lowering of the water table would then be undertaken in such a case, as part of the rapid process of removing obstacles.
Fortunately, boulders generally did not cause problems for TBMs, says Paulatto, although excavations in areas such as Rådhuspladsen station revealed cobbles and boulders of up to 1m wide in a local glacial valley with meltwater deposits. Larger boulders were found in the original excavations for the existing Kongens Nytorv metro station, and piling for the new works at the site.
Extensive monitoring of the groundwater environment as well as buildings and structures was a key aspect of the major project, performed to collect real-time data to satisfy the city’s strict environmental controls and ensure building integrity. For groundwater, measurements were taken with hundreds of sensors and data loggers in monitoring wells, an automatic SCADA monitoring and control system, and infrastructure for handling the abstracted water, and its reinfiltration to the ground, included 21 treatment plants and approximately 25km of pipeline.
The monitoring showed that the desired outcomes were met, for the water table and settlement, and in doing so also that TBM boring resulted in almost negligible volume loss while tunnelling with cover depths higher than 3m in limestone and, typically, there was less than the 0.5% volume loss limit sin mixed face boring. There were also brief periods of handling contaminants.
TBM boring with four shields was completed over late 2016 – early 2017, by when all deep station excavations had been finished. Best progress rates were 174m/week and 734m for a month, respectively, for the Seli Tunneling Denmark machines, the firm being subcontracted for tunnelling for CMT JV. Crews generally worked three shifts for six day weeks, plus a typical maintenance day.
MORE LINES
Cityringen opened in late 2019. The client has built on that further tunnelling success to expand the metro network yet further, focusing on the new M4 line, which extends to the north and south sides of the harbour area, respectively, in the Nordhavn and Sydhavn sections.
Construction for the not entirely underground Nordhavn section began in 2014 and called for tunnels for approx 1.8km twin tunnel with Herrenknecht EPBMs, a cut and cover ramp, a surface section, and two stations. The civils contractor was MetNord, a venture of Hochtief and Zublin. The M4 section opened in early 2020, shortly after Cityringen entered service.
Main works on the 4.5km long Sydhavn section started in 2018. Tunnel boring for the twin tube section was performed over 2020-21 with CREG TBMs. Works on the lot also included construction of five new stations. Civil works are by Tunn3l, a venture of Hochtief and Vinci. The Sydhavn section of M4 is to be fully operational by 2024.
A further, but surface only, part of the integrated transport network will be a 28km long light rail line began construction in 2019 at the outskirts of the city. Running from Lyngby to Ishøj, the light rail is to open in 2025.
With so many new transport lines in the metro and rail network built in a major drive within three decades, Copenhagen has become a vital hub for travellers in northern Europe.
