1:45 PM - 3:15 PM
[HCG21-P09] Prediction of natural reduction of inflow into the Horonobe Underground Research Laboratory based on the flow dimensions in the faulted host rock
Keywords:flow dimension, inflow, in-situ measurements
Introduction
For geological disposal of high-level radioactive waste, inflows from faults/fractures in rock masses into the underground facilities after tunnel excavations may adversely affect the workability of installation and performance of bentonite buffer-materials and/or bentonite sealing-plugs.
Based on the diffusion equation for water pressure, inflows from faults/fractures may naturally reduce depending on the flow dimension in the rock masses1). The flow dimension depends on the site and its distribution can be estimated by performing packer tests in surface-based boreholes. Although it is theoretically possible to estimate the natural reduction in inflow from the locations after tunnel excavations on the basis of the distribution of the estimated flow dimensions, this has not been explicitly verified at actual excavation sites.
This study measured the current inflow rates along a drainage system of the gallery at 350 m depth in the Horonobe Underground Research Laboratory with the aim of comparing the natural reduction in inflow after tunnel excavations estimated from the flow dimensions in the faulted host rock and the actual natural reduction in inflow at a total of eight significant inflow points. The amounts of natural reduction were determined by comparing the inflow rates at the time of excavation with the current inflow rates.
Existing information on the inflow points
The flow dimension in the faulted rock mass around the gallery at 350 m depth has been estimated to be approximately 1.5 (relatively poor fracture connectivity and large expected natural reduction in inflow) in the east area and 2.0 (relatively high fracture connectivity and small expected natural reduction in inflow) in the west area, based on the results of surface-based borehole investigations1). From these values and the results of simulation, it is estimated that the inflow would reduce to 0.08–0.1 times in the east and 0.4–0.6 times in the west 10 years after excavation of the tunnels.
There were five inflow points in the east (E1–E5) and three in the west (W1–W3), and the inflow rates during their excavations was ~70 L/min at E1, ~700 L/min at E3, ~110 L/min at E41), 10–50 L/min at E2 and E5, and 5–10 L/min at W1–W3. All inflow points except for E1 were pre-excavation grouted. E1 and E3 were also post-excavation grouted, but there are no observed post-grouting inflow rates for comparison. Considering the change of inflow from the entire underground facility and the practical limits of reduction in inflow by grouting, the inflows at E1 and E3 were estimated to have been in the range of 1–10 L/min.
Method
Observation holes (22 holes in total) for measuring groundwater levels were drilled in the drainage system consisting of gravels below the floor of tunnels to determine flow rates along the tunnels. The flow rates were estimated based on flows directly measured at three drain pits in the tunnels, the distances between two adjacent observation holes, the differences in groundwater levels, and Darcy's law, then the current inflow rates at the flow points were evaluated.
Results and Discussion
The current inflow rate at each inflow point was 0.5–0.7 L/min in total for E1–E3, 0.07–0.09 L/min for E4, 0.2–0.3 L/min for E5, 2.6–3.4 L/min in total for W1–W2, and 0–0.6 L/min for W3. Compared to the inflow rates at the time of excavation, the current inflow rates have been reduced to (0.7–6) × 10−2 times for E1–E3, (6–8) × 10−4 times for E4, (0.4–3) × 10−2 times for E5, 0.1–0.3 times for W1–W2, and 0–0.1 times for W3. These ratios are similar to or lower than the ratios from the simulation results described above. Thus, the present study confirms that the natural reduction in inflow estimated from the water pressure diffusion equation is fully applicable to actual excavation sites.
1) Ishii (in press) Hydrogeol. Jour.
For geological disposal of high-level radioactive waste, inflows from faults/fractures in rock masses into the underground facilities after tunnel excavations may adversely affect the workability of installation and performance of bentonite buffer-materials and/or bentonite sealing-plugs.
Based on the diffusion equation for water pressure, inflows from faults/fractures may naturally reduce depending on the flow dimension in the rock masses1). The flow dimension depends on the site and its distribution can be estimated by performing packer tests in surface-based boreholes. Although it is theoretically possible to estimate the natural reduction in inflow from the locations after tunnel excavations on the basis of the distribution of the estimated flow dimensions, this has not been explicitly verified at actual excavation sites.
This study measured the current inflow rates along a drainage system of the gallery at 350 m depth in the Horonobe Underground Research Laboratory with the aim of comparing the natural reduction in inflow after tunnel excavations estimated from the flow dimensions in the faulted host rock and the actual natural reduction in inflow at a total of eight significant inflow points. The amounts of natural reduction were determined by comparing the inflow rates at the time of excavation with the current inflow rates.
Existing information on the inflow points
The flow dimension in the faulted rock mass around the gallery at 350 m depth has been estimated to be approximately 1.5 (relatively poor fracture connectivity and large expected natural reduction in inflow) in the east area and 2.0 (relatively high fracture connectivity and small expected natural reduction in inflow) in the west area, based on the results of surface-based borehole investigations1). From these values and the results of simulation, it is estimated that the inflow would reduce to 0.08–0.1 times in the east and 0.4–0.6 times in the west 10 years after excavation of the tunnels.
There were five inflow points in the east (E1–E5) and three in the west (W1–W3), and the inflow rates during their excavations was ~70 L/min at E1, ~700 L/min at E3, ~110 L/min at E41), 10–50 L/min at E2 and E5, and 5–10 L/min at W1–W3. All inflow points except for E1 were pre-excavation grouted. E1 and E3 were also post-excavation grouted, but there are no observed post-grouting inflow rates for comparison. Considering the change of inflow from the entire underground facility and the practical limits of reduction in inflow by grouting, the inflows at E1 and E3 were estimated to have been in the range of 1–10 L/min.
Method
Observation holes (22 holes in total) for measuring groundwater levels were drilled in the drainage system consisting of gravels below the floor of tunnels to determine flow rates along the tunnels. The flow rates were estimated based on flows directly measured at three drain pits in the tunnels, the distances between two adjacent observation holes, the differences in groundwater levels, and Darcy's law, then the current inflow rates at the flow points were evaluated.
Results and Discussion
The current inflow rate at each inflow point was 0.5–0.7 L/min in total for E1–E3, 0.07–0.09 L/min for E4, 0.2–0.3 L/min for E5, 2.6–3.4 L/min in total for W1–W2, and 0–0.6 L/min for W3. Compared to the inflow rates at the time of excavation, the current inflow rates have been reduced to (0.7–6) × 10−2 times for E1–E3, (6–8) × 10−4 times for E4, (0.4–3) × 10−2 times for E5, 0.1–0.3 times for W1–W2, and 0–0.1 times for W3. These ratios are similar to or lower than the ratios from the simulation results described above. Thus, the present study confirms that the natural reduction in inflow estimated from the water pressure diffusion equation is fully applicable to actual excavation sites.
1) Ishii (in press) Hydrogeol. Jour.