The observation of crack development around an underground rock chamber by borehole television system

Summary

Phenomena caused by stress redistribution in a granitic rock as a result of the excavation of a large underground powerhouse were observed using a borehole television system. The observations were carried out five times at regular intervals and at different stages of the excavation. They clearly revealed that microcracks had been progressively generated in the wall area. The majority of these cracks were not initiated from the pre-existing mineral filled joints, but appeared at locations where no joints had been present earlier. It was also found that the summed-up value of the apertures of the microcracks has accounted for more than thirty percent of the resulting wall displacement toward the chamber. From the measured orientations of the generated cracks it can be concluded that the directions of the principal stress axes rotate during the progress of the excavation.