Variation in fault hydraulic connectivity with depth in mudstone: An analysis of poroelastic hydraulic response to excavation in the Horonobe URL

Abstract

Faulted mudstones have low effective hydraulic conductivity if the faults have limited hydraulic connectivity. Therefore, the hydraulic connectivity of faults is a crucial consideration in the geological disposal of high-level radioactive waste. There is a simple method based on single-borehole investigations to classify domains of faults in mudstone as having either high or low hydraulic connectivity. However, the nature of the hydraulic connectivity’s transition with depth around the boundary between domains of faults with high and low hydraulic connectivity remains poorly understood. At the Horonobe Underground Research Laboratory (Japan), three shafts have been excavated in a Neogene siliceous mudstone, and hydraulic pressure has been monitored at boreholes during the laboratory’s construction and operation. This study analyzed long-term hydraulic pressure data to estimate the variation of effective hydraulic conductivity and explore the nature of the variation of fault hydraulic connectivity with depth around the predicted boundary (at ∼400 m depth) between domains of faults with high and low hydraulic connectivity (with less connectivity below the boundary). As the observed hydraulic pressure was greatly affected by the Mandel–Cryer effect, numerical simulations considered poroelastic effects. They showed that the effective hydraulic conductivity gradually decreased from ∼400 to ∼500 m depth, becoming comparable with that of intact rock below ∼500 m. Theoretical analysis of the observed data also indicated the same variation with depth. These results suggest that the hydraulic connectivity of faults does not change abruptly, but instead varies gradually over several tens of meters around the domain boundary.