DFI Magazine November/December 2020 - 16

STORY

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INFRASTRUCTURE
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Piles being prepared to extend the Oslo S rail tunnels

Steel Core Piles for Norwegian Tunnel
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 mi (20 km).
The Follo Line Project involves a twin-tube tunnel that is mostly
embedded in solid granite, and that will allow trains to reach a
speed of 150 m/h (250 km/h). 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

Twin rail tunnel from Oslo S to Ski

AUTHOR

Dominik Gächter, Dipl. Ing., Keller

16 * DEEP FOUNDATIONS * NOV/DEC 2020

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
anchor 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 ft (1 m) to 16.4 ft (5 m), sometimes thickening up to
33 ft (10 m). Underneath the clay, there is often a moraine layer
between 3.3 ft (1 m) and 66 ft (20 m) 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



DFI Magazine November/December 2020

Table of Contents for the Digital Edition of DFI Magazine November/December 2020

TOC #1
TOC #2
DFI Magazine November/December 2020 - Intro
DFI Magazine November/December 2020 - 1
DFI Magazine November/December 2020 - 2
DFI Magazine November/December 2020 - TOC #1
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