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

Norway has over five thousand kilometers of hydropower tunnels. A lot of them are starting to reach a respectable age but their actual durability is uncertain. An important characteristic of most of these tunnels is that they are unlined; meaning that the water is in direct contact with the rock surface of the tunnel. Because of this contact, the water pressure against the tunnel wall affects the rock mass itself. 

The water is pushed through cracks and further into pores in the bedrock. When the pressure inside the tunnel rises or falls, the pressure will do the same with a small delay inside the pores of the rock. As long as the water pressure inside the tunnel is higher than the pressure inside the pores in the bedrock, the system is stable. However, if the pressure inside the pores of the bedrock is higher than inside the tunnel, there is a risk that pieces of the bedrock 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 (fatigue). 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.