Observational evidence of elastic shear displacement along a minor fault in soft siliceous mudstone

田村 友識 ; 石井 英一  ; 青柳 和平  ; 八木 啓介*

Tamura, Tomonori; Ishii, Eiichi; Aoyagi, Kazuhei; Yagi, Keisuke*

When high level radioactive waste is emplaced in a rock mass with minor faults, it is necessary to consider the risk of shear displacement along them as this might damage engineered barriers surrounding the waste materials. When the shear compliance of the faults is high, large elastic shear displacement might occur along them due to hydromechanical changes after the emplacement. However, their shear compliance is usually assumed to be low (10 mm/MPa). To verify such assumption, we investigated a minor fault at a depth of 480 m in soft siliceous mudstone. We grouted it and investigated drillcores and borehole wall images, which revealed that the borehole axis in the footwall was displaced by 26 mm relative to the hanging wall as a result of 44 mm of reverse motion on the 57-dipping fault. We interpret this displacement as backward elastic shear displacement because (1) the water pressure in the borehole was 0.83.1 MPa lower than the formation pressure; (2) a reduction in water pressure can reduce the shear compliance of a fault, decreasing previously generated elastic shear displacement; and (3) this interpretation is consistent with the normal faulting stress state around the fault. The induced shear displacement and calculated shear stress on the fault yield a high shear compliance (10 mm/MPa). This study demonstrates that the shear compliance of minor faults is not always low; therefore, it is necessary to evaluate it using in situ investigations so that elastic shear displacement after the emplacement does not severely damage the engineered barriers.