All TBM boring complete for Follo Line 28 Feb 2019

Final breakthrough of the last two of four double shield TBMs employed on the Follo Line twin tube railway tunnel in Norway finalises 18.5km x 2 or a total 37km of hard rock tunnel boring within 30 months leaving behind a fully permanently lined infrastructure ready for mechanical and electrical fit-out. From start of the first set of TBMs in September and October in 2016 and the second two in November and December 2016, the first two machines completed a double TBM breakthrough at the north Oslo end of the alignment in September 2018 with the final two achieving their double-TBM breakthrough in February 2019 into the transition portal to the surface approach to the central station of the city of Ski in the south.

The final breakthroughs celebrate the dedicated work of the tunnelling crews and management of the Spanish/Italian Acciona/Ghella Joint Venture, and rewards the efforts invested by the Bane Nor Norwegian national railway authority in a successful hard rock TBM project.

The drives to the south have broken through according to schedule with the two TBMs driving north completing their drives much earlier than expected. “This could have been because slower rates were anticipated through the harder rock in the north drives,” said Anne Kathrine Kalager, Tunnel Production Manager on behalf of Bane NOR, “but we were prepared for that very hard rock and the TBMs have been tailor made for these tough Norwegian rock conditions. We gathered all the knowledge and experience we have of hard rock TBM tunnelling in Norway to work with our contractor and prepare these four ladies for their tasks.”

Experiences of tough TBM jobs in the 1980s and 1990s, particularly for hydro schemes in Norway, had resulted in an emergent preference during the early 2000s for drill+blast excavation through the tough conditions. The decision to adopt TBM boring, and with a one-pass precast segmental lining, for the Follo Line tunnels marked a return of TBM tunnelling to Norway and has brought with it many advantages.

“Had we selected drill+blast excavation, on the single tube double track configuration that was standard for drill+blast railway tunnels in Norway, we would have had to progress excavation from both portals and in both directions from seven intermediate adits to complete excavation in the same time as employing four TBMs,” explained Kalager. “The double shield method had the advantage also of erecting the precast segmental lining simultaneously with rock excavation and the machines worked in the double shield mode – gripping and thrusting off the tunnel walls rather than off the segments – for all of their progress.”

In preparing for the hard rock conditions, Bane NOR worked with the original tender submission by the contractor to increase the robustness and stiffness of the 10m diameter TBM cuttterheads, increase the capabilities and design life of their main bearings, develop cutters and extra wear plating on the cutterhead and shield bodies of the machine to counteract the abrasivity of the gneiss rocks, and equip them with the facilities to manage high volume water ingress through potential fractures and joints in the massive rock formations. Discussions with the contractor and with Herrenknecht, the manufacturer of the four TBMs, were that these additions to the tender design of the TBMs were necessary – and they were. The features on the four strong, powerful, and well equipped TBMs did incur added cost to supply of the machines but they have completed their drives with no unexpected stops. “The success of the machines is a good example of having a competent team on both sides – the cleint and the contractor,” said Kalager. “As owner we also had a vested interest in a successful project and, together with the contractor, we had our own team of inspectors stationed full time in the Herrenknecht factory during the manufacture of the machines. It has been a strategy that has paid dividends.”

In combination with the tough, strong rock conditions, water ingress was an aspect of the project that had to be anticipated and managed. The four machines are equipped with permanently fitted drilling rigs for both systematic probing and pre-excavation grouting. Every day, during the morning maintenance shift, a 45m probe hole was drilled from 15m behind the face to 30m into the rock to warn of water ingress conditions ahead. If water through the probe hit trigger levels, cycles of pre-excavation grouting was performed.

“We did not need to stop the water inflow completely,” said Kalager. “Our precast gasketed and effectively backfilled lining provides a water tight finish to the tunnels, but we needed to manage water table drawdown which could result in settlement on the surface. We have been boring under urban areas on the surface and control of settlement was a priority concern during excavation.”

In a departure from the norm, the Acciona/Ghella JV decided to backfill the annulus with a two component grout mortar, rather than using pea gravel and mortar. The two component grouting materials were mixes and prepared on the surface and pumped through long feeder lines to reservoirs on the TBM backups. Grout with the accelerator B component was injected through the tail seal as well as through the segments to ensure adequate lining backfill to keep pace with the double-shield mode excavation rates.

Another advantage of selecting TBM excavation over the drill+blast method was the possibility of operating and concentrating the entire excavations process at one central job site area and access point. The midpoint access adit and job site is located in an area shielded from surrounding urban areas and avoided the use by heavy construction trucks on public roads. All the material from the excavation process was also contained on the one construction site, with suitable rock being reprocessed through the on-site crushing plant to be used in the on-site segment casting yard and the remainder has been stockpiled for later use. Once the project is finished and the site demobilised, the stockpiled muck is to be used to prepare the foundation of a new urban development to be built on the site.

With breakthrough of the final two TBMs, work continues using drill+blast on other sections of the project and the Acciona/Ghella JV will now prepare for the electrical and mechanical installations of the railway, which are also part of its contract.

In total, about 18,000 working hours in four shifts on each TBM heading maintained the positive progress rates. On each of the last working machines, 112 employees have been involved in the production and 24 with the maintenance. At the peak of the tunnel production 1,000 persons went through the gates of the large rig area, including to the on-site segment casting factory.

“Thanks to the strong emphasis on quality, safety, innovation and expertise, we have delivered a world class result,” said Fernando Vara, Director of the Acciona/Ghella JV. “We faced tough Norwegian hard rock and some challenging ground conditions, but drawing on our experience and quality work, we have been able to deal with it and we are pleased to manage such a huge and demanding TBM project. We have produced more than 18km of quality rail tunnel infrastructure, on schedule as we promised, and we now move on into the new phase, and we are confident that we will do a good job also in the remaining M&E and rail technology installations.”

Having completed their work successfully, the four TBMs are being dismantled and shipped back to the Herrenknecht factory in Germany.

“The tunnel being celebrated is by far the largest part of the Follo Line Project,” said David Borenstein, Director of the Follo Line Project for Bane Nor. “Some sections did present challenges, but TBM excavation is executed on schedule. We are pleased with the work and with the cooperation we have had and will continue to have with the Acciona/Ghella JV.”

For Bane NOR, the new high-speed underground railway will reduce by 50% the travel time between Oslo and Ski from 22 to 11 minutes. The Line will provide increased capacity and stimulate growth and development in both the Oslo and the Follo regions. All civil works on the project are to be finalised by Spring 2021 and the new Oslo-Ski rail link ready for service in 2022.

For the tunnelling industry of Norway, other projects are being investigated and developed to apply the advantages of mechanical excavation by TBM design and equipped to be fit-for-purpose in the hard rocks that are the foundation of the mountainous Nordic country.