How healthy are the main arteries of the hydropower plant?

In Norwegian hydropower we mostly operate with unlined pressure tunnels, meaning that the water is in direct contact with the rock and that the water pressure is transferred to the rock.

Previously, it was believed that as long as the weight of the rock above the tunnel, the overburden, was greater than the water pressure inside the tunnel, the design was safe and the tunnel would not crack as a result of the pressure.

Unfortunately, it’s not that simple. Large variations in rock stress, not corresponding to the weight of overburden, has been observed at several power plants. In a few cases such stress variability was left undetected, due to lack of stress measurements, causing dramatic and very costly hydraulic failure of tunnels.

Even though it now is more or less standard to perform stress measurements, measurements are few and far apart, often leaving kilometres of tunnel essentially untested. In his research Henki Ødegaard is therefore trying to facilitate more stress measurements, without the prohibiting costs of doing so, by developing a cost-effective and simplified version of the hydraulic jacking rock stress measurements.

Read more about HydroCens research in our Annual report for 2020.

Further reading:

• Design of unlined pressure tunnels in Norway – limitations of empirical overburden criteria and significance of in-situ rock stress measurements

• Rock Stress Measurements for Unlined Pressure Tunnels: A True Triaxial Laboratory Experiment to Investigate the Ability of a Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress | SpringerLink

Contact:

Henki Ødegård