Mechanical and hydrological variation of rock mass around a shaft excavated in Neogene sedimentary rocks

not registered ; not registered; Sato, Toshinori  ; Yoshioka, Naoya

As a result of underground excavation to construct an underground facility, an excavation disturbed zone (EDZ) is often generated in rock mass around a shaft and a drift. In order to design and construct an underground facility, and to evaluate its long-term stability, it is necessary to understand the phenomena related to excavation disturbance. PNC (Power Reacter and Nuclear Fuel Development Corporation) excavated a shaft of 6m diameter and 152.3m depth in Neogene sedimentary rocks, and studied the influence induced by the shaft excavation on rock mass and groundwater flow around the shaft. This project was named SEE (Shaft Excavation Effects) project, and was carried out from 1978 to 1991. This report described a part of the SEE Project, focusing on mechanical and hydrological variation of rock mass around the shaft. The current conclusions were su㎜arized as follows: (1)Mechanical and hydrological variation of rock mass around the shaft was induced by stress redistribution and fracturing around the shaft. (2)The zone where mechanical and hydrological characteristics were varied was extended about 1m from shaft wall. (3)It is considered that unrecoverable deformation is generated in rock mass around a shaft because of fracturing and plastic deformation. We introduced a concept of permanent deformation modulus to take account of the unrecoverable deformation, and estimated the results of borehole jack tests. Consequently, we found the permanent deformation modulus was useful to identify the EDZ.