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.