Bibek Neupane and Krishna Panthi in the hydroelectric tunnel at Sira-Kvina Kraftverk.

In Norway, we have over 5,000 km of hydropower tunnels that are starting to age, without us really knowing anything about how long they can last. An important characteristic of most hydropower tunnels in Norway is that they are unlined; that is, the water is in direct contact with the surrounding rock. Since there is direct contact, the pressure that the water presses against the tunnel wall will affect the rock mass itself.

The water pressure in the tunnel pressing against the tunnel wall will drive water into cracks and further into pores in the rock. As the pressure rises or falls in the tunnel, the pore pressure will with a slight delay rise or fall inside the rock as well. As long as the water pressure in the tunnel is greater than the pore pressure in the rock, the system is stable, but if the pore pressure in the rock becomes higher than in the tunnel, there is a danger that pieces of the rock mass will be pushed out into the tunnel.

Figure showing the movement of water in the rock when pressure is applied in the tunnel.

New production regimes bring challenges

With traditional power plant operation, there have been relatively few collapses in the tunnels, as the hydropower plants have more or less always been kept running at the same pace, with small changes in pressure. After the deregulation of the power market in the 1990s, the tunnels are to a greater extent exposed to stress through frequent and short-term start/stop processes, due to the fact that production is now controlled by market prices that change rapidly.

Because of this, the water in the tunnel can suddenly be stopped or released. This leads to differences in the water pressure in the tunnel and the pore pressure in the rock masses. It is this pressure difference that can lead to problems both in the short and long term (exhaustion). In the worst case, parts of the tunnel may collapse.

A small change in operation pattern can extend the tunnel's lifespan

Through his work, Bibek Neupane has found that with small changes in how quickly one shuts off and increases the water flow during a start/stop process, the period of time where the pore pressure in the rock exceeds the water pressure in the tunnel, so-called "Hydraulic impacts", can be minimized.

The duration of the stress depends on how quickly the process is carried out, and the slower you turn off the water in the tunnel, the slower the pressure builds up, and the time delay between the pressure peak in the tunnel and in the rock will be shorter.

By using rapid start/stop processes, as is often done today, the hydraulic stress on the rock mass can increase by up to 10 times compared to halving the shutdown period. By increasing this closure period, the 'wear and tear' of the rock around the tunnel will be slowed down, thus reducing the chance of a landslide.

The figure shows the relationship between how the water pressure in the tunnel (blue line) and the pore pressure in various places in the rock (red, purple and green line) increase at different rates when the water flow in a power tunnel is shut off. The areas where the pore pressure exceeds the water pressure in the tunnel (yellow areas) are periods of hydraulic stress = periods of increased stress and risk of collapse in the tunnel.