Status of study on stress in the Mizunami Underground Research Laboratory Project

Nakama, Shigeo ; Sato, Toshinori  ; Kato, Harumi*

stress is an important factor for design and construction of deep, large underground structures, such as underground power stations and geological disposal facilities for high-level radioactive waste. In-situ stress is recognized as a primary factor for development of an excavation disturbed zone (EDZ). An EDZ will be less stable and essentially will be a highly permeable groundwater flow path, so an EDZ is recognized as a key issue not only for the design and construction of disposal facilities but also for safety assessment. Japan Nuclear Cycle Development Institute (JNC) is conducting underground research laboratory projects to establish a firm scientific basis for safe geological disposal; one is the Mizunami Underground Research Laboratory (MIU) project for crystalline rock, the other is the Horonobe Underground Research Laboratory (Hnb-URL) project for sedimentary rock. Rock mechanical studies including in-situ stress measurements are performed in these projects. The main elements of these studies are: (1) hydraulic fracturing tests in 1000 m deep boreholes and utilization of several laboratory core testing methods; (2) development of overcoring methods for vertical 1000 m deep boreholes; and (3) development of numerical techniques for estimation of regional stress state. This paper describes the current state of in-situ stress studies in the MIU project, especially hydraulic fracturing tests in 1000 m deep boreholes, and development of overcoring techniques for vertical 1000 m deep boreholes in saturated conditions.