This paper summarizes the results of the hydraulic jacking investigations conducted at a hydroelectric project and presents the numerical simulations undertaken to evaluate the penstocks steel liner length. The project is located on the Romaine River north of Havre-St-Pierre (Québec, Canada). The power generation component comprises a 1.65 km long unlined pressure tunnel which conducts water from the reservoir to a surface excavated powerhouse. Of particular interest is the effect of a 121 m water head internal pressure applied to the rock mass near the powerhouse.

During a two year period, 82 standard hydraulic jacking tests were conducted in the downstream part of the power tunnel, manifold, penstocks and powerhouse areas to optimize the required steel liner length. However, as the powerhouse requires a deep 85 m excavation in the rock mass, consideration has been given to the magnitude of the stress relief due to the excavation and has been simulated to evaluate its impact on the final length of the steel liner.

Rock mass parameters (deformability, resistance, unit weight, joints sets) have been estimated from laboratory tests on rock cores and geocamera survey in boreholes. The parameters have been adjusted according to the rock mass rating. The Phase2Ä stress modelling software and the hydraulic jacking test results have been used to simulate the in-situ stress conditions and calibrate the model. The results of a sensitivity analysis showed that the stress relief is mostly affected by the joint and joint sets parameters such as orientation, dip, spacing and persistence. Final results showed that the effect of the joint network on the stress relief due to the excavation decreased the in-situ stresses by 15% and the length of the steel liner had to be increased by 10 m.

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