Instrumentation and pore pressure monitoring technique for unlined tunnels of Norwegian hydropower projects – a case study

Abstract

The power market in Norway was deregulated in 1991 and have caused power plants to operate with more frequently varying loads within short time periods. Such operation causes significant pressure fluctuations and unsteady flow along the waterway. Pore water pressure in the rock mass around an unlined tunnel is dependent on the water pressure in the tunnel itself. Hence, pressure transients during load changes cause the rock mass pore pressure to change as well, which may cause instabilities in the tunnel during its long-term operation. However, the relation between power plant operation and rock mass pore pressure variation is largely unknown. A field instrumentation is conducted inside the headrace tunnel of Roskrepp hydropower plant in southern Norway, to measure rock mass pore pressure during power plant operation. The methodology of instrumentation and preliminary results are presented. Results show that pressure transients of both short and long time periods i.e, water hammer and mass oscillations can travel deep into the rock mass. The monitoring is intended to be continued to measure the rock mass pore pressure behaviour over-long term.