Rock Mass Stress Relief at Shallow Depth Due to Surface Powerhouse Excavation

The construction of the Romaine-3 surface powerhouse requires an important excavation of approximately 98 m (left to right bank direction) by 64 m (upstream-downstream direction) and 80 m deep in the rock mass. The water intake tunnel as a length of approximately 1.8 km including the lined portion and sits at an average depth of 150 m below the natural ground level. The unlined portion of the power tunnel is located in a rock mass that can sustain a hydraulic pressure of about 1.7 MPa as to prevent hydraulic jacking of existing joints. During the design phase, in-situ stresses have been measured with hydraulic jacking tests done in several boreholes. These boreholes are aligned and distributed along the axis of the future lined tunnel, starting from the powerhouse site to a distance of 350 m upstream. The tests were conducted in an undisturbed rock mass at variable depths corresponding to about 20 m above and below the future tunnel elevation. To acquire a better understanding of the stress relief caused by the powerhouse large excavation, a new testing program, with 2 boreholes, was conducted after the completion of the excavation. The first borehole was located about 0.6 m beside a previously tested borehole. Also, with a borehole televiewer, it was possible to identify, in this new borehole, the same joints sets that were tested in the existing borehole. The test intervals elevation has been adjusted in order to test these specific joints so as to compare the stress conditions pre and post-excavation. A second borehole was drilled at about 44 m upstream of the powerhouse to evaluate if the effect of excavation on in-situ stresses was observed closer to the excavation. Based on these hydraulic jacking results, it is shown that no substantial stress relief has been measured in the rock mass at 56 m upstream of the powerhouse after its excavation.