Steel core piles for Norwegian tunnel By Dominik Gächter, Dipl. Ing., Keller This article was originally published in DFI’s bi-monthly member magazine, Deep Foundations, Nov/Dec 2020 issue. DFI is an international technical association of firms and individuals in the deep foundations and related industries. To join DFI and receive the magazine, go to www.dfi.org. The largest bottleneck for railway traffic in Norway is in the Follo district between Oslo and Ski, a suburb of the capital city. As part of the Norwegian National Rail Administration’s plan to increase rail capacity out of Oslo, the Follo Line Project was begun in 2014 as the largest infrastructure project in Norway’s history. This project is the focus of intercity development that is planned south of Oslo, Norway’s largest city. It includes the Nordic region’s longest railway tunnel, which extends over 12.5 miles (20 kilometres). The Follo Line Project involves a twintube tunnel that is mostly embedded in solid granite, and that will allow trains to reach a speed of 150 mph (250 kmh). Keller began serving as the subcontractor on the project in 2016, mobilizing more than 70 specialists from nine European countries for this major project using steel core piles (SCPs) for the foundation. The geotechnical challenge centered on stabilizing sensitive clay and quick clay where it transitions into hard rock along sections of the railway tunnels. To do so, Keller used a combination of dry deep soil mixing and jet grouting. Besides the development of a novel jet grouting an18 PIC Magazine • June 2021
chor system, the subcontractor also used a backflow treatment plant to minimize the environmental impact of jet grouting — the first successful application of this approach in Norway. The foundation work that is the focus of this article relates to Oslo’s main railway station (Oslo S). The main construction work started in 2015 and is expected to be finished within 2020 so that the railway and signaling works can start. The first train is expected to pass through the new tunnel by December 2021.
GEOLOGY The soil in which the tunnels needed to be constructed is often characterized by different types of clay. In some parts of Norway, the clay is soft and might even be characterized as quick clay that can become unstable under stress; but in other areas, the clay is described as stiff and medium in its sensitivity. Above the clay there is typically a layer of dry crust and fill that mostly varies in thickness between 3.3 feet (one metre) to 16.4 feet (five metres), sometimes thickening up to 33 feet (10 metres). Underneath the clay, there is often a moraine layer between 3.3 feet
(one metre) and 66 feet (20 metres) thick, before reaching the bedrock of diabase and syenite porphyry. In the Oslo area, there are layers of alum shale between the moraine and bedrock. This is a black shale that resembles species of rock containing pyrites (sulphur ore) and pyrrhotite (magnetic pyrites). Quick clay in its remolded state has a shear strength of < 0.07 psi (0.5 kPa) and will be a liquid. This soil has brittle fracture properties (a considerable reduction of strength) when experiencing strains larger than strains at its maximum strength. Therefore, quick clay may cause progressive landslides, where a minor landslide evolves into a considerably larger landslide. Such landslides due to quick clay tend to occur in Norway, Sweden, Finland, Russia, Canada, and Alaska.
FOUNDATIONS WORKS For the tunnel foundation, an SCP was chosen, which is the most executed foundation type in Scandinavia. An SCP typically has a permanent casing of 3.5 to 16 inches (90 to 406 millimetres) for corrosion protection and a centered, solid steel core of three to 12.5 inches (80 to
