Constant-head step-injection tests using a conventional straddle-sliding-packer system for investigating the shear capabilities of minor faults

Ishii, Eiichi   

Low-permeability rock is suitable as a host rock of an underground repository for radioactive waste disposal, but small faults may develop there. Investigating the shear capabilities (= shear compliances) of those faults is crucial as they could elastically shear by the waste's thermal effect to damage to the waste's engineered barriers. The present study performed constant-head step injection tests combined with a recently developed packer-pressure-based extensometer method for assessing the applicability of this method as a method to investigate the shear capabilities of small faults. Two neighboring small faults (faults A and B) in a siliceous mudstone were tested. The results showed that the shear capability is high for fault A with centimeters-thick fault breccia but low for fault B with millimeters or less-thick faut breccia despite containing an incohesive fault rock. As for fault A, an elastic shear displacement occurred during injection and reached to 15-66 mm when the test-section pressure increased from 4.1 MPa to 4.3 MPa, where the shear capability was on the order of 10 mm/MPa or more. Fault B had a cohesion, and no shear displacement was detected even when the test-section pressure increased from 4.0 MPa to 6.0 MPa; the shear capability was on the order of 10 mm/MPa or less. The estimated shear capabilities were consistent with results from previous laboratory experiments, and the applied method is useful to investigate the shear capabilities of small faults.