Predicting the long-term natural decay of inflow from faults or fractures encountered during excavation of deep underground tunnels using the flow dimension

Sakuma, Keisuke ; Ishii, Eiichi   ; Murakami, Hiroaki  

The safe geological disposal of high-level radioactive waste requires consideration of the inflow of groundwater from faults or fractures encountered during excavation of deep underground tunnels, as this inflow may adversely affect the installation and performance of bentonite buffers. The water pressure diffusion equation, assuming uniform hydraulic conductivity and specific storage, predicts that the inflow rate from faults or fractures naturally decays at a rate depending on the flow dimension when the cross-sectional area of the flow areas in the fault or fracture system is proportional to the power of the distance from the inflow point. Although this prediction has been verified for decay ratios measured a few weeks after tunnel excavation, its applicability to long-term predictions several years after excavation has not yet been assessed. We investigated the natural inflow decay ratios (i.e., the inverse of the current inflow rate normalized to the initial inflow rate at excavation) at fault-related inflow points in a tunnel at 350 m depth in the Horonobe Underground Research Laboratory, Japan, 10 years after tunnel excavation, and compared them with those modelled using the diffusion equation and flow dimension. The natural inflow decay ratio measured at each inflow point was equal to or higher than the modelled ratio. Although in some cases the measured ratios may be up to four times as high as the maximum modelled ratio, this difference can be explained by the effect of hydraulic interference between adjacent inflow points. The diffusion equation and flow dimension can be used to predict the minimum long-term (several years or longer) natural decay ratio, providing useful insight to implications for repository design timelines or regulatory decisions (e.g., determining when and where to install waste and bentonite buffers in a repository).