Automatic monitoring in tunnels

The Strindheim Tunnel is to be permanently secured. Cautus Geo has installed monitoring equipment for continuous surveillance. Any new movements in the rock will be detected immediately.

The Norwegian Public Roads Administration closed the ten-year-old Strindheim Tunnel in Trondheim a year ago in May. Deformations in the shotcrete and in the rock were discovered during a major geological inspection. The tunnel between Nyhavna and Sirkus Shopping is heavily used.

The 2.5-kilometre tunnel was completed in 2014 following four years of construction work. Cautus Geo was also involved at that time.

“We monitored stability during the construction phase,” says Lars Krangnes, CTO at Cautus Geo.

Monitoring is now automatic, continuous and permanent. Any new changes in the rock are reported immediately.

Adhesive brick

Special inspections revealed cracks and unstable rock masses in an area of claystone.

– We have carried out a thorough reinforcement of the rock support in both tunnel tubes. It has been a major undertaking, but absolutely essential to protect the tunnel against future deformation, says site manager Cecilie Hestås Hove of the Norwegian Public Roads Administration.

Work began in May last year and turned out to be more extensive and demanding than anticipated.

– We have dismantled the electrical installations, removed wall and ceiling elements, and surveyed the rock behind them. The tunnel has now been secured with reinforced shotcrete arches, shotcrete and rock bolts. In some areas, we have also blasted out rock to make room for the safety measures,” says Hove.

Closed at night

Cautus Geo has installed longitudinal and transverse settlement and deformation tubes behind the concrete in both tunnel tubes. It’s all about safety. The work was carried out over a few nights whilst the tunnel was closed to traffic in both the eastbound and westbound directions. All traffic was then diverted to the tunnel tube where no work was taking place.

– We worked behind the concrete elements. The monitoring equipment is positioned directly on the rock face. Once the stabilisation measures have been completed, the Strindheim Tunnel should be stable. If any new changes in the form of movements occur in the rock, they will be detected and reported in real time, says project engineer Torje Fostås from the project department at Cautus Geo.

– The rock support in the tunnel has been designed and dimensioned in accordance with current standards. Nevertheless, monitoring procedures and measures must be put in place for the rock mass class of the tunnel’s claystone zone. There will still be visual inspections, but many of the areas are cramped and difficult to access. This type of monitoring using instrumentation is a good measure and now provides us with continuous monitoring, says Hove.

Normal traffic

The tunnel reopened to normal traffic in October, with the usual traffic pattern of one lane for eastbound traffic and one lane for westbound traffic.

Before the closure, deformations had been observed in the weaker sections of the tunnel. These have now been repaired.

“The automatic monitoring system will immediately detect any potential future instability,” says Fostås.

– Now that the safety measures have been implemented, the tunnel is safe, but it is important to verify that the repairs remain effective over the years, as required by the relevant safety class. The instrumentation from Cautus Geo includes both a monitoring solution that allows us to keep an eye on things ourselves, and an alert system that is set up to notify us if anything happens, says Hove.

EQUIPMENT:

• 3 x SAA (settlement tubes/inclinometers) at 40 metres along the roof and 20 metres across the roof arches.
• 1 x 62-metre flexible pipe laid horizontally along the roof arch.
• Logger – which controls data collection and data transfer.
• Cautus Web – a monitoring solution with real-time data that triggers alerts if any movement is detected.