A Study of mechanical stability of support elements and surrounding rock mass during shaft sinking through a fault

Tsusaka, Kimikazu*; Inagaki, Daisuke*; Nago, Makito*; Ijiri, Yuji*

This study describes the analysis performed on the West Access Shaft of the Horonobe Underground Research Laboratory, which was expected to intersect a fault at the depth of approximately 320 m. Field observation, and measurement data were used to determine analysis conditions including magnitude and orientation of in-situ stress, boundary conditions, and rock mass properties. The fault was modeled as having a dip angle of 40 degrees and apparent thickness of 5 m (equivalent to the height of the excavated rock wall). The shaft sinking procedure was simulated using three-dimensional excavation analysis. The excavation involved installing concrete lining at every 2 m span. The analysis considered two cases of maximum in situ principal stress orientation: (1) perpendicular to and (2) parallel to the fault plane orientation. The results of the analysis indicate that the maximum excavation-induced stress, developed in a single-span lining concrete, was in the direction perpendicular to the maximum in situ principal stress orientation, unaffected by the fault plane orientation. The influence of the fault plane orientation on the excavation-induced stress state was found to be significant above and below the fault rather than in the fault. Another observation was that the excavation-induced stress magnitude appeared to be greater when the maximum in situ principal stress orientation was parallel to the fault plane orientation.