Influence of a fault system on rock mass response to shaft excavation in soft sedimentary rock, Horonobe area, northern Japan

This paper focuses on the stress field that produced a fault system in the Horonobe area of Japan, and considers the relationship between the fault system and the rock mass response to shaft excavation in soft sedimentary rocks. In this area, hydraulic fracturing investigations in boreholes and tectonic plate movements indicate that the major horizontal principal stress is oriented E–W; however, during shaft excavation, the greatest convergence in the shaft was oriented NNE–SSW. Therefore, it is necessary to understand the influence of the fault system on the rock mass response to shaft excavation.We performed a fault-slip analysis of faults in the shaft wall and reconstructed the paleostress field that produced the fault system. The maximum horizontal principal stress was oriented mainly NNE–SSW, and the minimum horizontal principal stress was oriented mainly WNW–ESE. These directions are similar to the directions of maximum and minimum convergence in the shaft, respectively. The results show that a fault system can affect the rock mass response to shaft excavation in soft rock, and it is considered that the rock mass deformation is controlled by the fault system. In addition, it may be possible to assess and predict the rock mass behavior by focusing on paleostress field that produced the fault system.

► Shaft convergence in the soft sedimentary rocks does not correspond to recent stress field.
► Maximum horizontal principal stress and minimum horizontal principal stress estimated by fault-slip analysis are similar to the directions of maximum and minimum convergence in the shaft, respectively.
► Fault system can affect the rock mass response to shaft excavation in soft rock.
► Paleostress field that produced the fault system provide useful information to assess and predict the rock mass behavior.