[H-CG21] Nuclear Energy and Geoscience

[HCG21-02] Estimation of groundwater inflow into single borehole using the value of flow dimension
-A case study at the Horonobe Underground Research Laboratory-

*Hiroaki Murakami¹, Eiichi Ishii¹ (1.Japan Atomic Energy Agency)

Keywords:groundwater, Inflow, flow dimension

Natural reductions in inflow during and after tunnel excavations can be related to the flow dimension in the rock mass (Barker, 1995; Ishii, in press). However, there has not been sufficient discussion of the relationship between the natural changes of inflow and flow dimension at actual excavation sites. The Horonobe Underground Research Laboratory (URL) in Hokkaido is suitable for this study because the inflow data during and after the tunnel excavations and the flow dimension in the surrounding rock mass are available. In this context, we conducted a prediction and verification tests in a borehole drilled from the 350 m gallery at the Horonobe URL prior to the planed excavations of the 500 m gallery will start in 2023. The Neogene marine siliceous mudstone of the Wakkanai Formation occur around the borehole and 500 m gallery. Constant-rate injection tests (flow rate at 0.6 L/min for 4 days) for the test-section (467.7–481.8 meters below ground level (mbgl)) resulted in hydraulic transmissivity of 1.04 × 10⁻⁴ m²/s, storativity is 1.62 × 10⁻⁴, and flow dimension of 1.29. This low flow dimension is consistent with previous studies indicating that the hydraulic connectivity of fractures is poor in the deep part of the Wakkanai Formation below 379–479 mbgl in and around the URL (Ishii, 2018; Ozaki et al., 2022). Using the obtained hydraulic properties, we predicted the relative change in the amount of inflow after the test section is opened; the results showed that the inflow reduces with an elapsed time at a slope of −0.356 on the log-log plot (Fig. 1 left). Next, we opened the test section for 50 days and observed the change in inflow; the inflow reduced with an elapsed time at a slope of −0.359 on the log-log plot (Fig. 1 right). This slope is consistent with the prior prediction. Therefore, the applied prediction method considering flow dimension can be useful for the prediction of natural reduction in inflow, potentially even for tunnel excavations. In the future, we will confirm the applicability of this method to changes in inflow during the excavations of the 500 m gallery.

Reference: Barker (1988) Water Resour. Res., 24, 1796–1804; Ishii (in press) Hydrogeol. Jour.; Ishii (2018) Water Resour. Res., 54, 3335–3356; Ozaki et al. (2022) Geomech. Energy Environ., 31, 100311.