Excavation analysis of vertical shaft in sedimentary rock taken into account strain softening and depth variation of rock properties

It is essential to evaluate EDZ (excavation disturbed zone or excavation damaged zone) in assessing performance and designing plugs in geological disposal. It is known that it remains possible that the large-scale EDZ is generated due to strain localization from boring investigations and tunnel excavation analyses and rock properties change with increase of depth. In addition the unorthodox short step method has been adopted as the method for constructing the Horonobe URL. Therefore, it is expected that stress in surrounding rock is complex status compared with usual construction method. Excavation analysis of vertical shaft in sedimentary rock taken into account strain softening, depth variation of rock properties and the actual construction procedure has been done in order to understand EDZ of Horonobe URL. The results are summarized as follows: When drilling analysis of borehole has been done in order to reproduce the borehole shape, the borehole shape obtained from numerical analysis was similar into the actual borehole shape. The large-scale EDZ due to strain localization was generated around the border between the Koetoi formation and the Wakkanai formation. As an interpretation to this phenomenon, it is considered that rock mass around the border became deformed intricately and properties in surrounding rock changed significantly because properties of Horonobe siliceous rocks changes significantly due to diagenetic effect. From result obtained from shaft excavation analysis, generated EDZ is evaluated to range from 60 cm to 120 cm. And it was estimated that seismic velocity changed by 20% , elastic modulus changed by 30% and hydraulic conductivity changed by one order of magnitude. (author)