Rock behavior estimated by field measurements during large underground cavern excavation

The behavior of the rock mass was monitored during the excavation of a large underground cavern for a power station in a mountain area in Hyogo Prefecture, Japan. The results by three monitoring techniques, which are the observation of discontinuities in the borehole, the measurements of the horizontal displacement and the vertical stress increment, are presented here. It was found that the rock deformation was dominated by the extending apertures caused with the local sliding in the discontinuity planes. The stress redistribution caused by the excavation did not indicate regular distribution as expected by an elastic theory and made alternately the increased zones and the decreased zones in the vertical stress toward deeper portion from the cavern wall. The loosening region was extended to be more than three fifths of the cavern width at the last stage of the excavation. However, the region was prevented from failing down by the support system.