Unique measurements to determine tunnel durability

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Norway has 5000 km’s of hydropower tunnels, now HydroCen researchers seek to find out how robust they are, and if they have an expiry date.

— It is very exciting. We don’t really have a clue how long Norwegian hydropower tunnels can last. Now many of them are already 100 years, but do they have an expiry date? That is the big question, says Kaspar Vereide, Project developer at Sira-Kvina Kraftverk and Associate professor at NTNU.

— This research will help us find some answers towards determining the robustness of hydropower tunnels and if they have an expiry date, he says.

Once every 15 years Sira-Kvina stops the water at Roskrepp power station to do maintenance. This time they let the researchers have a good look around.

Lights from powerful flashlights roam the rough walls of the headrace tunnel at Roskrepp power station in Sirdal in Norway. An enormous iron gate separates the researchers from the force of water on the other side.

The intake gate separates researchers and engineers from the powerful masses of water. Photo: Juliet Landrø/HydroCen

When the gate is open, water flushes through with a flow rate of 70m3/second. It takes the researchers about 3 hours to walk the 4-kilometre long dry tunnel, wading through the pools of water that remain in rough surfaces.

Engineering geologist and Professor Krishna Panthi has already checked that the passage is safe.

— This tunnel is in very good shape, he reassures the less experienced members of the expedition.

Researchers got a rare opportunity to take a closer look at the Roskrepp hydropower tunnel when it was drained for maintenance in june 2018. Photo: Juliet Landrø.

He has worked for many years to be able to research how the pressure in hydropower tunnels affect the pore pressure in the rock mass. It has never been done before, neither in Norway nor internationally.

Measuring pressure change

Together with several researchers from Hydrocen and employees from Sira-Kvina kraftselskap, PhD candidate Bibek Neupane has installed high-tech measuring equipment in the rock mass inside the tunnel.

— We are measuring the pore pressure in the rock mass, says Neupane.

He has installed steel pipes inside 5 boreholes in the rock mass with lengths varying from 5m to 11m. The steel pipes will collect the pore water in the rockmass. These pipes are brought out of the water tunnel to a dry area downstream of a concrete plug.

Pressure sensors are connected to this end of the pipe, which measure the pore pressure and data is recorded using a data logger. These custom made pipes have rubber seals or ¨packers¨ fixed inside the boreholes. This will ensure that the water from the tunnel does not leak directly into the boreholes from where the pore pressure is measured. The borehole length outside the packer is also sealed with cement for additional safety.

A separete pipe is also installed in the tunnel to measure the water pressure in the tunnel simultaneously.

When you start or stop the production in the power station there is a pressure change in the tunnel. We want to measure if these pressure variations influence the pore pressure in the rock mass, he says. It is literally tons of water that experiences sudden changes in the flow, so the force can be enormous.

The sensors are connected to an advanced data logger, so the researchers can follow the pressure variation data in real time when the power production starts, stops or varies. 10 pressure readings every second from each sensors are being recorded by the data logger as we speak.

Will the tunnels handle the operation scenario in the new times?

The rough tunnel roofs towering over the researchers has kept well since the production started here in the 1980’s, but tunnels operating today are under a lot more stress than they were originally built to handle. In the 1990’s the power market was de-regulated, consequently leading to a lot more variable operation of the power stations.

Project developer at Sira-Kvina Kraftverk and Associate professor at NTNU, Kaspar Vereide facilitated and contributes to research at Roskrepp power station. Photo: Juliet Landrø/HydroCen.

— Historically the power production has been very even, and we haven’t stopped and started the water a lot, but since the 90’s the power systems have been operated more aggressively, says Vereide.

Even though the rock mass in Norway is strong and dense, hardly any tunnels are built for this type of variable speed operation.

— Turbines and tunnels experience much more stress, and it is this stress we are now trying to measure and survey to see how it influences the tunnel, says Vereide.

The pressure in the tunnel changes very fast when the water is stopped.

— It can go from 70m3/s to 0 m3/s in about ten seconds, so the water comes to a sudden stop and builds a massive force.

— It can be compared to a full freight train that comes full speed through the tunnel, and suddenly had to come to a full stop, so it is amazing that we get to do these tests, says Vereide.

Large and frequent pressure changes can lead to unstable rock mass.

— The goal is to see how robust the tunnels are, what pressure changes they can handle and how much we can vary the power production without severe consequences for the long-term stability of the rock mass, says Neupane.

Bibek Neupane
Krishna Panthi
Kaspar Vereide

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