放射性廃棄物地層処分 ＡＥ法とＤＲＡによる地下応力測定試験方法とケーススタディ

Abstract

In this paper, firstly, we describe the detail of the methodology for the stress measurement using the AE and DRA methods. Secondly, we show the two case studies of stress measurement using the AE and DRA methods to evaluate the possibility to measure in situ stresses using rock core samples obtained from underground excavation area.In one of the case studies, rock cores were obtained from horizontal boreholes drilled into the wall of a drift excavated in soft sedimentary rock. This allowed the vertical stress variation with distance from the drift wall to be determined up to a depth of 5 m. The vertical stresses determined using rock cores varied with distance and reached a maximum of 4 ∼ 5 MPa at a distance of 2 ∼ 2.5 m from the drift, when the highest concentration of stress was expected. The vertical stress variation was consistent with calculations by the three dimensional FEM program, ANSYS. Furthermore, the stress measured by the over coring method just nearly agreed with the stress determined by the AE and DRA methods. The AE and DRA methods described in this paper should be applicable to in situ stress measurement with reasonable accuracy even in soft sedimentary rock. A delay time of up to 101 days did not affect the determination of in situ stress in soft sedimentary rock.The other case study has been focused on full stress tensor determination from a single oriented cored rock. Core was obtained from an HI cell measurement hole at Cannington mine in Australia to establish whether the complete stress tensor could be determined from AE and DRA test data. The rockmass at Cannington mine is geologically very complex, and different stress magnitudes and orientation were calculated by the HI cell even within adjacent locations along a hole axis. Two test sites were analyzed for stress measurements using cored rock. The orientation of the estimated stresses appears to match the localized orebody orientation very well. The main principal stress was found to be parallel to the orebody strike. The intermediate principal stress was parallel to the dip, while the minor principal stress was normal to the orebody. Given that the predominant geological structures in the mine are a set of strike slip faults oriented 70 / 330 and that the general slickensides on these are sub-horizontal, the conclusion that the principal stress is parallel to the strike of the orebody is geologically sound.