Foundation improvements for a tunnel under an antique city center and detailed 3D modelling of rock mass deformation during excavation are just two of the many tasks that Golder Associates faced during geotechnical design work on Stockholm’s City Line, Sweden. The project is one of Scandinavia’s largest infrastructure developments in recent years and Golder carried out the preliminary design for several sections of the tunnel and a thorough environmental impact assessment for the central bay area under which the tunnel will run.
In this opinion piece for Geodrilling International, Markus Kappling, business area manager for Golder Associates in Sweden, discusses best practice in geotechnical engineering and rock mechanics based on the work with two of the City Line tunnel’s most challenging sections – Söderströmstunneln and Vasatunneln.
Markus explains how Golder’s geotechnical design ensured a safe connection between the concrete and rock sections of Söderströmstunneln, as well as adding foundational improvements to safeguard the many antique buildings in the Söderström city area. He also highlights the importance of 3D modelling of rock mass tension and deformation as a result of excavations beneath highly trafficked highways on bridges founded on overlying rock, as exemplified by the work on Vasatunneln. As part of the City Line project, Golder also developed a specialist software called CGW (Citybanan Geoweb), which is now used by Swedish authorities for a whole range of projects.
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In Sweden, the new Stockholm City Line commuter train tunnel is nearing completion. Golder Associates discusses best practice in geotechnical design based on the work with two of the tunnel’s most challenging sections.
Foundation improvements for a tunnel under an antique city centre and detailed 3D modelling of rock mass deformation during excavation are just two of the many tasks Golder Associates faced during our geotechnical design work on Stockholm’s City Line. By 2017, the biggest upgrade to Stockholm’s rail system in decades will be finished and the 200,000-plus commuters who travel through the Stockholm Central Station daily will enjoy a doubled track capacity.
With its long-term relationship with the Swedish Transport Administration, Golder has been involved in the massive project since 2003. Golder carried out the preliminary design for several sections of the tunnel, as well as a thorough environmental impact assessment (EIA) for Riddarfjärden – the central bay area under which the tunnel will run – and was also involved in several other stages of the project.
During the detailed design stage, the consulting company was responsible for the geotechnical and rock tunnel design of Vasatunneln and Söderströmstunneln – two of the 6km City Line tunnel’s most challenging sections. This work exemplified many of the challenges infrastructure developers face in urban areas.
Söderströmstunneln, about 400m long, sits on the southern half of the Stockholm City Line. An immersed concrete tunnel under the city’s bay area here links to a rock tunnel going under the island of Södermalm. This is an especially tricky section of the City Line because the immersed concrete tunnel had to be connected to the rock tunnel in a steep hill coming up from the water and extending up to the landmass under antique buildings. Södermalm is also one of the most densely populated city districts in all of Scandinavia, so it was critical to ensure the geotechnical design was accurate.
Golder’s design ensured a safe connection between the concrete and rock tunnels, as well as adding foundation improvements to safeguard all the classical buildings in the city area.
Geometrik, a fully owned subsidiary to Golder, supplied one of the essential technological components for securing the foundation. Geometrik installed and commissioned a fully automatic measurement system for the Söderström project, which measures displacements and loads in different supporting structures. All movements the system registers are displayed online at once so they can be continually monitored.
The monitoring system includes – among other components – underwater inclinometers to measure soil displacements around the immersed tunnel, installed by divers. It also contains extensometers to measure rock movements around two new station halls at the Stockholm Central Train Station, on the other side of the lake. The proximity to the underground railways made these new station halls a particularly technically challenging development, with high demands on blasting works to minimize disruptions and noise. But with the help of Geometrik’s measurement technology, the necessary requirements were met.
3D rock modelling
Vasatunneln (1,900m long), meanwhile, is another part of the Stockholm City Line. It sits at the Northern end of the line. Here, Golder produced the geotechnical and rock mechanical design, and technical specifications for temporary as well as permanent constructions, excavations, reinforcements and grouting. Golder’s work also included 3-D modelling of rock mass tension and deformation as a result of excavation beneath highly trafficked highways on bridges founded on the overlying rock.
Rock modelling is generally used in conjunction with measured data and monitoring to continually verify the design performance. It requires a high level of technical expertise and an in-depth understanding of industry best practice. For Vasatunneln, 3-D rock mechanics modelling was successfully completed and a key part of ensuring optimal excavation and reinforcement.
Golder also designed a geometrically complex sheet-pile wall to support construction. During the later construction of Vasatunneln and Söderströmstunneln, the consultancy has advised on critical parts of the temporary constructions, as well as track tunnels with low rock coverage and near sensitive structures.
The CGW information tool
Another key part of Golder’s overall work with the City Line involved developing a new information management tool called CGW (Citybanan geoweb). CGW allows engineers to store large data sets and easily extract meaningful information.
This is an essential task on all large construction and infrastructure projects, and the Swedish Transport Administration uses CGW for many other projects.
For the City Link project, CGW stored huge amounts of environmental data on ground water levels, settlements, vibrations, property details, historic precipitation, temperature, barometric pressure and other variables. The tool allowed Golder and the client to closely monitor groundwater levels and movement during design and construction.
This ensured the project design and the contractors always took these important data into account.
Urban infrastructure challenges
Stockholm’s City Line is one of the biggest infrastructure developments in a city centre in the Nordics in recent times. Golder’s EIA and geotechnical and rock mechanical designs have played a part in ensuring the line will come together successfully.
Protecting vulnerable urban areas from the impacts of new infrastructure can be a tought task, but with the right technical expertise and software tools – like the CGW – it is manageable. Söderströmstunneln and Vasatunneln together comprise about a third of the 6km long Stockholm City Line, and the geotechnical challenges involved are good examples of the kind of issues infrastructure developers have to take into account on large-scale projects in all urban areas.
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