Outcrop-scale observations of tensile fractures developed in a deep geologic media at the Horonobe Underground Research Laboratory, Japan

Tamura, Tomonori ; Sakuma, Keisuke ; Murakami, Hiroaki ; Aoyagi, Kazuhei  ; Ishii, Eiichi  

Global flow-path connectivity in fault zones is strongly influenced by local connectivity within individual faults and fractures. Even where geological structures appear well connected, low local connectivity can significantly reduce the effective hydraulic conductivity of the rock mass. Previous studies at the Horonobe Underground Research Laboratory (URL), northern Japan, showed that hydraulic properties of faults in Neogene siliceous mudstone depend on the Ductility Index (DI), defined as the ratio of mean effective stress to tensile strength. When DI exceeds 2, flow-path connectivity decreases markedly and permeability approaches that of intact rock. To clarify the factors controlling connectivity under high-DI conditions, this study examined the distribution and morphology of tensile secondary fractures along faults exposed on the tunnel floor at ~500 m depth, where DI 2. A total of 784 fractures were measured, and three representative faults were analyzed in detail. Around these faults, tensile secondary fractures were identified based on their orientation, morphology, and distinction from excavation damage zone fractures. However, their development was spatially limited, forming damage zones only several tens of centimeters wide. No significant groundwater inflow was observed along these faults. These results indicate that, under high-DI conditions, tensile secondary fractures do not substantially enhance flow-path connectivity. Instead, hydraulic behavior in fault zones is primarily controlled by DI-dependent fracture closure rather than by the extent of secondary fracturing.