HydroCen researcers recently installed a state-of-art rig to monitor
in-situ development of swelling pressure and deformation in a hydropower tunnel. This is likely the first time researchers are able to these measurments in a flooded tunnel.
By Krishna Kanta Panthi, Professor,
Department of Geoscience and Petroleum, NTNU
Unlined / shotcrete lined water tunnels that pass through weak rocks like shale, slate, mudstone, clay stone, serpentinite and flysch are vulnerable to tunnel collapses during operation caused by swelling, slaking and disintegration of rock mass.
The main reason for this is that these rocks consists substantial amount of clay minerals. Statkraft and its subsidiaries have experienced instability caused by swelling, disintegration, loss of strength and deformability properties in tunnels built through weak rock mass.
Moglicë Hydropower Project is among them where swelling and disintegration of the rock mass can be experienced. The project is in its completion phase of construction. The project utilizes 300 m head where a reservoir is created by constructing approximately 150 m high asphalt core rock fill dam. The 10.7 km long medium to high-pressure headrace tunnel conveys water from intake to the underground powerhouse cavern located at the north bank of Devoll River to generate 172 MW electrical energy.
Over 8 km headrace tunnel length passes through very weak rock mass consisting mainly flysch and serpentinite rocks. The drill and blast part of the headrace tunnel is supported with systematic bolting and ribs of reinforced shotcrete. Since shotcrete (sprayed concrete) is permeable material the rock mass behind the shotcrete of the tunnel wall will be fully saturated during operation.
Therefore, Statkraft wished that a comprehensive research on the behavior of flysch and serpentinite rocks is carried out, including a PhD research titled “Effect of swelling rocks in water tunnels”. PhD fellow Lena Selen is engaged to full fill this challenging task.
After carrying out comprehensive laboratory investigations at NTNU Geolab it was found that the flysch rock has both swelling and disintegrating behavior. Hence, it was decided to develop a long-term monitoring rig and installed them between headrace tunnel chainage 7489 and 7510 where main rock type is flysch. This rig shall monitor and record swelling pressure and deformation development during first year of operation of the hydropower plant at its full hydrostatic head.
The installation of state-of-art monitoring rig, which is to our knowledge used for first time in the world to monitor in-situ development of swelling pressure and deformation in a hydropower tunnel, was completed on 27th of January 2019. The tunnel is expected to be water filled by April 2019. The researcher will visit the tunnel next year together with Statkraft to collect the stored data from the data loggers.