Norway selects drill+blast for next big rail link 29 Nov 2018

The Norwegian national rail company, Bane NOR, has seen significant success from its choice of using mechanised tunnelling on its most recent major projects for the Follo Line in Oslo and Ulriken project in Bergen, but despite the deserved plaudits, and opportunities naturally sought for more use of TBMs, the client has confirmed it is opting for large-scale drill+blast on its next major rail project. Located to the west of the capital Oslo, the Ringeriksbanen rail and E16 road scheme sits in a 40km long corridor extending from Sandvika to Sundvollen, and beyond to Vik and then Hønnefoss.

The Ringeriksbanen rail tunnel is almost 23km long and consists of parallel tubes, one large 123m2 profile for the double track railway and a 60m2 escape and service tunnel, the two connected by cross passages at regular intervals. Both are non-circular in cross-section, the larger tunnel being 8.5m high from the top of the tracks to the crown of the tunnel lining.

Tunnelling work for Ringeriksbanen will be procured in three lots, according to the preliminary contract structure:

• Lot TK-01: Located at the south end, as a junction near Jong, the package includes two 2.5km long single track rail tunnels, a 3km long double track tunnel for the large tunnel, 5.5km of the parallel escape service tunnel and access tunnels at Jong, Reverud and Kattås.
• Lot TK-02: Covering a major stretch of the middle of the tunnel, the works include 13.5km of the parallel large and smaller section tunnels and access tunnels at Rustan, Avtjern and Høgas.
• Lot TK-03: At the north end near Sundvollen. A 3.5km stretch of parallel rail tunnels.

The E16 road tunnels come in north of Sundvollen, over a short body of water at Vik, and are 3.2km twin tubes running beside a double-track stretch of railway tunnel also 3.2km long. Drill+blast and cut-and-cover methods are envisaged for the trio of tunnels. The project also includes various rail and road bridges, construction of a new rail station at Sundvollen, rebuild of Hønnefoss station, and 15km of new roadway. The national roads authority Statens Vegvesen (NPRA) is responsible for the E16 road as part of its co-development of the overall scheme.

Prequalification for the main contracts on the Ringeriksbanen rail tunnel is anticipated during 2019-20, by when preparatory works are due to start. The first main contract are expected to be awarded by 2020-21 with construction started by 2021-22.

"No design or construction alternatives will be entertained by Bane NOR to its selection of drill+blast which is made after extensive studies and debate. The choice of tunnelling method has been made and is set," said Lisbet Kierulf Botnen for Bane NOR. “It is final for the whole distance from Sandvika to Sundvollen.”

Getting to the decision on excavation method, however, was no simple matter.

Layout and excavation options

Geological and hydrogeological studies for the Ringeriksbanen rail tunnel drew upon a variety of investigations These have included core holes and geophysical investigations as well as studies for other tunnels in the area including the 5.2km long water supply tunnel constructed by drill+blast between Toverud ad Kattås.

The geology along the Sandvika-Sundvollen rail tunnel axis is divided into broad groups of: Cambro-Silurian sedimentary layers of limestone and claystone with some sandstone and schist; and, along much of the middle of the alignment, there is porphyry. The groups are separated by inclined adjacent bands of basalt of the Krokskogen Group of Permian volcanic rocks and the Asker Group of late the Carboniferous period comprising conglomerate with sand, silt and claystone.

The Asker Group is expected to exhibit challenging excavation conditions for about 10% of the tunnel length. Major uncertainties exist around the quality and conditions in the Krokskogen Group. Many fault zones cross the tunnel, except for the middle third, and there are high levels of groundwater over the full tunnel alignment. Water ingress is considered the greatest factor of uncertainty.

With an original layout concept of a large and a small tunnel running in parallel, the Ringeriksbanen/E16 project team considered the bigger tunnel perhaps too large for TBM excavation and was cautious on the geological conditions. Seeking other views, it called the Follo Line project team to get their view of using TBMs.The result was a discussion over three suggested layout options, each a concept of parallel tunnels but varying in size and excavation method.

The large/small parallel tunnels concept: The original and eventually chosen concept of parallel drill+blast tunnels of 123m2 and 60m2 ;
Twin TBM tunnels: Suggested by the Follo Line project team to adapt its Follo project concept onto Ringeriksbanen, the option considered using two double shield TBMs of 10.5m diameter for an 80.9m2, circular sections eachTwin drill+blast tunnels: A further suggestion by the Follo Line team for twin 75m2 single track tunnels with non-circular cross-sections optimised for drill+blast excavation

The differing options and views led to much considered debate. Subsequently, the Ringeriksbanen/E16 project team called upon further outside opinion from an Expert Engineering Group tunnelling consultants. The Group reviewed the geological and hydrogeological data, looked at the challenges and risks associated with the different tunnelling options, and reported with conclusions.

Weighing risks

According to the studies, both the geological and hydrogeological risks are much higher than on either the Follo Line or the Ulriken rail projects. About 62% of the rail tunnel, under an overburden of 170m-350m, is expected to be subjected to groundwater pressures approaching more than 17 bar.

The expert consultants reviewed the many project experiences considered in the studies, including the Hallandsås rail tunnel in Sweden and the Storebelt undersea rail project in Denmark and reviewed the experiences gained in limiting water ingress on projects such as the HATS2A sewer tunnels in Hong Kong, where extensive pre-excavation grouting was used, and on the Uma Oya multipurpose scheme in Sri Lanka.

It was calculated that numerous zones of weak rock plus high hydraulic heads on the new Ringeriksbanen tunnel would exceed capabilities of currently available gasket seals for segmentally lined TBM tunnelssuch as applied on the Follo Line project in Norway. The conditions would call for extensive, effective and flexible grouting regimes to provide the same injection density as for traditional drill+blast tunnelling.

The construction complications on the Ringeriksbanen project would be difficult for any excavation method and have been gauged as posing greater potential problems for TBM drives. Primarily, the challenge for TBM drives is seen in terms of required flexibility, adaptability and associated logistics of extensive drilling and grouting, and, hence, more risk to the construction schedule. With the two parallel tunnels plus many cross passages, it is expected that pre-excavation grouting would be required in some degree over almost the entire length of the project.

While the expert group noted the Ringeriksbanen/E16 project team and the Follo Line project team arrived at their views from different perspectives and approaches, the possibilities they considered for tunnelling systems have outturn implications for relative risk and cost. The Group concluded the core choice between the tunnelling methods is fundamentally linked to groundwater control. How each tunnelling method can control groundwater results in different risk profiles for their respective use over the length of this particular long rail tunnel would influence the differences in potential construction time and economics for Ringeriksbanen.

TBMs may have done well on rail projects in Norway recently and also previously in their reintroduction to the national hydropower sector after decades away, but the decision of the Ringeriksbanen/E16 project team is to employ drill+blast over large TBM excavation for this large rail project. The expert group adds that while drill+blast is argued as saving some time and cost in this project scenario, the actual margins depend on the tunnel size after any optimisation by Bane NOR.