PROJECT | STOCKHOLM BYPASS
Above: Grouting plan for water hazards in subsea passage between Sätra and Kungshatt
“But it has the secondary function as well, of
sealing the fractures and stopping water seepage which was useful but not the primary reason for grouting.”
RISK AND TOOLBOX OPTIONS Nothing in tunnelling is certain. Risk needs understood and managed. “Where the preliminary investigations gave us some
information about the rock, we did risk assessments, and that of course includes the risk of encountering poor conditions,” Stille says. “So, you look at those risks and define proper mitigations for if and when you come across them. “We came up with a sort of a toolbox of various
measures to how handle various types of risks,” Stille adds. He continues: “You never know exactly what you have
ahead of you. Sometimes you encounter poor rock even though you haven’t expected it. Then you adjust your support and maybe even the grouting scheme for water tightness, in accordance with the geological conditions. I think it’s important to note here that there were no really unforeseen conditions. Even though some parts were really poor we were prepared for them.” Passing below lake had more risk. “The risk level was very high because the
consequence of a tunnel failure could be enormous,” he says. “But we adjusted the technical solution and the design with regards to that the level of consequence.”
It was a fairly conservative approach, he comments. “Also, as we went forward, we included very
many steps and decision points,” he says. “One of the mitigations to make sure that we took adequate measures with regards to risk was to change the organisation during those passages of the tunnelling. “We included all the disciplines, all the specialists;
and, every week we would gather the contractor, the client and the designer, all sitting together and going through the past week’s work and seeing what it had told us of the geological conditions with regards to the identified risks: do we need to make any changes to the design, or to make some adjustments in procedure? Everything like that we discussed. Stille says: “This happened at specific times, and
when we reached specific areas. Sometimes we were excavating at the rate of less than 10 metres in a week; it takes a couple of days just to drill the pipes, and then you have to grout it and excavate. It was fairly slow progress in those parts but it meant that we had a lot of conferences for every metre dug, so that reduced the risk. Most tunnelling contracts in Scandinavia are build-
only, not Design and Build, he says, and this was the case also for Stockholm Bypass. “Long sections of the tunnels were quite basic. We
knew which types of failure mode to expect. There may have been a little more fractured rock or a little less, but we knew that rockbolts and shotcrete would
Top: Cross section of the proposed pipe umbrella support for the subsea passage Above: Cross section showing the designed procedure for the installation of shotcrete and rock bolts 18 | April 2025
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