Henki Ødegaard tests the methodology in the field, in the pressure tunnel of Løkjesvatn power plant. Photo: Erlend Andreassen
In Norway, we mostly have solid mountains with hard rock and can therefore build water tunnels where the water is in direct contact with the rock. So-called "unlined pressure tunnels" are much cheaper to build than "lined pressure tunnels", which must be reinforced with steel, but then the rock masses must withstand the high pressure from the water.
Old-fashioned measurements have been standard
If the water pressure is higher than the minimum principal stress of the rock, the water will be able to penetrate cracks and jack them open. This can lead to uncontrolled loss of water, potentially large rebuilding costs and, in the worst case, danger to life and health if the water goes astray.
In order to gain knowledge of the rock's minimum principal stress, one used to previously look at how much rock lay above the tunnel and calculated whether it was heavier than the pressure from the water in the tunnel. This is an uncertain rule of thumb for estimating the magnitude of the rock's stresses, and far less certain than actual measurements.
Since the 1980s, measurements of rock stresses have increasingly been recognized as necessary in the design of unlined pressure tunnels. Nevertheless, current practice with such measurements is still not optimal, as the tests are considered expensive and time-consuming, which means that tests are often only carried out at a few selected locations in the unlined pressure tunnel. These measurements will only be valid for the locations in the tunnel where they are taken, and therefore still lead to possibly unsafe designs with the risk of large-scale hydraulic fractures in other parts of the tunnel.
In order to avoid unforeseen low rock stresses and potential hydraulic breaks, far more measurements should have been taken throughout the pressurized part of the power plant. However, this is not realistic with current measurement methods, as these are inefficient and expensive.
More user-friendly methods result in better design
Through his project, Henki Ødegaard has developed a simplified test method for faster and more cost-effective measurement of rock stresses. The test method is named Rapid Step-Rate Test (RSRT) or "Rask stagratetest" and has given good results both in the lab and in the field at Leikanger power plant.
By using a simple piston pump, you can put a known water pressure on the rock mass to form and close cracks in the rock, and in this way be able to indicate the stress on that crack. With enough such measurements along the entire pressurized tunnel, a more accurate estimate of the rock's minimum principal stress can be obtained.
By using RSRT, one can now measure rock stresses far more often in a more practical and economical way and ensure a safer design for the pressurized part of the hydropower plant.
The test protocol has already been put into use by the Norwegian company Injekjonsteknikk AS in collaboration with Multiconsult. Based on the project results, input has also been sent to the Norwegian Water Resources and Energy Directorate (NVE) to update the current Dam Safety Regulations, which set requirements for how investigations and the design of pressure tunnels must be carried out.
Three main measures that are proposed for improving the methodology for estimating rock stresses in connection with the final design of unlined pressure tunnels:
1. Stress measurements MUST be carried out at the final design of the unlined pressure tunnel, and coverage considerations can no longer be the only goal in the stress estimation.
2. The stress measurements must be carried out more frequently distributed along the entire pressurized part of the plant, and not just at certain locations.
3. The new test method RSRT can be a fast and cost-effective alternative to existing measurement methods.