NGI chose an electronic measuring method that was originally developed for levelling and monitoring large structures on the seabed. The measuring equipment in the tunnel consists of a long water level with measurement columns. It is a self-contained system, with any movements in the tunnel being indicated by the liquid level in the tubes.
"The challenges of high-precision measurement like this include tunnel vibrations caused by the traffic, a lot of dust and dirt – and jetting with high-pressure washers whenever the tunnel is cleaned," says Dag Tollefsrud, Head of NGI's Instrumentation Department.
The measuring equipment has to be robust enough to withstand all such external influences, while being accurate enough to register the smallest change in settlement.
Continuous, automatic measurement of settlement in the Bjørvika Tunnel has revealed daily variations as small as 0.2 – 0.3 mm. (Photo: NGI)
1 – 10 mm of settlement
After 18 months of continuous measurement, it turns out that both the Public Roads Administration and motorists can rest easy. At the western end of the tunnel, where solid ground is just a short distance away, there has been settlement of up to 1 mm. At the other end with thick layers of soft soil above the rock, 10 mm of settlement has been measured, well within the range expected by the Public Roads Administration.
"The system is extremely accurate, has no downtime and gives an instant response," says Ola Natvig, Senior Engineer with the Public Roads Administration, Bridge Section.
He adds that both the developer and the Public Roads Administration have had good use out of the system. The developer's consultants, Multiconsult, have actively used the measurements, together with continuous pore pressure measurements, to monitor the building site for the new Munch Museum, close to the tunnel.
Both the section in question and the tunnel as a whole are measured using conventional surveying equipment at regular intervals in order to document settlement. The disadvantage of conventional surveying measurements is that they are not as accurate, the tunnel has to be closed to traffic, and monitoring measurements cannot be done continuously and automatically.
"The measurements with an electronic water level are so precise that we can see daily variations of 0.2 – 0.3 mm caused by the tide and temperature changes," says NGI's Kjell Karlsrud, a geotechnical engineer with long experience of calculating settlement both in Bjørvika and various other places around the globe.
It was also possible to detect a correlation between individual groundwork activities, drops in pore pressure and changes in settlement speed while the groundworks and foundation work for the Munch Museum were in progress.
Designed for offshore use
The measuring system employing an electronic water level was developed by NGI for levelling under water. This is necessary when installing pile foundations for offshore wind turbines. NGI's Dag Tollefsrud expects the measuring method to be put to use in other areas:
"The method is ideal for monitoring settlement and permanent deformations in long, horizontal structures such as bridges, dams, railway lines, quays and large, land-based construction projects, as well as for tunnels," says Tollefsrud.
Public Roads Administration continuing with measurements
"The system is still of value to us, as Bjørvika is a large area that has shown itself to be prone to settlement when construction projects are in progress. So we still want to monitor movements in the tunnel, even though the groundwork construction for the new Munch Museum now has been completed. There are also plans for other construction projects in the vicinity of the tunnel. We are therefore going to keep the system in operation until further notice," says Ola Natvig of the Public Roads Administration.
FACTS about the Bjørvika Tunnel
- One of four tunnels making up the Opera Tunnel in Oslo.
- It consists of six concrete sections, each 112 metres long.
- The sections were lowered to the seabed and laid directly on the clay at a depth of approximately 10 metres.
- The tunnel opened for traffic in 2010 and forms part of the E18 from south to west through Oslo.