Comprehensive monitoring on the city railway

- "The instrumentation from Cautus Geo was absolutely crucial. We couldn't have done the job without it," says Fredrik Bakke Johansen, Head of Soil and Geodesy at Veidekke.

The Bergen Light Rail has been under construction for almost 15 years. Since the first section was opened in the summer of 2010, a further three construction stages have been opened. Most recently the extension to Flesland in 2016. The fourth is now underway. The railway is being extended to Fyllingsdalen via Haukeland. This construction phase will be completed before next summer. The line will then have been extended by nine kilometres. At the same time, two tunnels have been built for pedestrians and cyclists.

Cautus Geo has had several assignments during the current expansion programme. This includes instrumentation and monitoring of the ground during work on the shaft for the underground station at Haukeland Hospital. It is 30 metres deep. This is where the new station is being built. Travellers can take a lift or escalator to and from the station when it opens in the spring.

- But the greatest complexity and most instrumentation has been on the new Kronstadt tunnel for pedestrians and cyclists," says field manager Sigmund Brekke Langelid.

Railway and loose material

The Kronstadt tunnel for cyclists will be 470 metres long. It is partly the result of expansions and improvements to the old railway tunnel and partly a new tunnel, established in loose masses in a busy urban environment. It is scheduled to open in spring 2023. This will provide cyclists with a fast and safe route between Bergen city centre and Kronstad.

- "The tunnelling work in the excavations under Møllendal cemetery was professionally demanding and highly complex," says Bakke Johansen of Veidekke, who carried out the job of renovating and expanding the railway tunnel and new excavation tunnel. Møllendal cemetery is the largest cemetery in Bergen with 26,000 graves and a history dating back to 1874.

Safety and security

- "We carried out extensive monitoring during the work on the loose mass tunnel. Actual knowledge of what is happening is absolutely crucial for safety," says Johansen.

Four total stations, 120 prisms, inclinometers in the cemetery above ground level and in the underground tunnel were interconnected in a comprehensive network and linked to the Cautus Web visualisation solution. Alarms were triggered when tolerance limits were breached. The instruments were moved forward simultaneously with the progress of the tunnel project.

- "We instrumented the blasting salvos closely, under and above ground, and had close follow-up and good control of everything that happened underground based on the extensive data from the monitoring system and the experience we gained along the way," says Johansen.

Demanding

- It has definitely been one of the most technically demanding monitoring projects I've been involved in. I'm glad we had Cautus Geo with us. It gave us peace of mind," says Johansen.

The instrumentation in the tunnel was placed as close to the work as possible and was moved at the same time as the work progressed from Christmas 2021 until this autumn.

- Cautus Geo was able to dismantle the last instruments in the autumn. "We couldn't have done the job without the instrumentation. It gave us continuous insight and data that was invaluable in the execution phase," says Johansen.

Fact list:

• Veidekke renovated and operated a new pedestrian and cycle tunnel at Møllendalen.
• Driving a tunnel in the soil under Møllendal cemetery required extensive monitoring.
• Cautus Geo was responsible for monitoring with total stations, inclinometers and CautusWeb access solution.
• Up to four total stations in operation simultaneously with 120 prisms. The total stations automatically measure optically against prisms (mirrors), placed in a network on struts in the cemetery and inside construction pits and in renovated and new tunnels.
• Inclinometers measured movement out from the sheet pile/pipe wall and vertical change (settlement) in the tunnel roof (pipe screen). The inclinometers in the tunnel were moved as the tunnel was driven.