Study on groundwater flow characteristics in Horonobe Underground Research Laboratory project

To understand the deep underground geological environment around the Horonobe Underground Research Laboratory, and to predict the underground facility construction effect on the geological environment, the following items were carried out: 1) Setting of two analytical regions and Build up of two numerical simulation datasets. 2) Description of regional groundwater flow characteristics at Horonobe Project site using the results of the numerical simulation and obtained field data. 3) Estimation of underground facility excavation effect on the groundwater flow. By the region setting, the two analytical regions were set, one is for the regional scale, and another is for the facility scale focusing on the area surrounding the underground facility. And two numerical simulation datasets corresponding to the regions were built up. The description of characteristics of regional groundwater flow showed that the distribution of chlorine content and that of the potentials of groundwater were sensitive to the hydraulic conductivity. The groundwater age of sampled pore water was estimated to be in the range of thousand years to million years. The direction of groundwater flow at the underground facility was estimated to be in the southwest direction of groundwater flow at the underground facility was estimated to be in the southwest direction. The predictive simulations of underground facility excavation effects on transient groundwater flow for ten or twenty years were carried out, and the results showed that the quantity of inflow to the underground facility and the change of hydraulic heads around the underground facility were strongly influenced by the hydraulic conductivity settings, and that the chlorine content of groundwater were not changed so much. From the descriptions of the hydro geological characteristics and the simulations, desirable investigations and further simulation approaches were proposed. (author)