(1) Geological investigations

In order to ascertain the correlation between (a) varieties/amounts of investigations, analyses/evaluation technique and (b) the accuracy of the results, geological models are constructed. For each of the Shobasama site and the study area for groundwater flow simulation (ca.4kmca.6km), two models are constructed. First model is based on the data from literature survey and other geoscientific researches. Second model is based on the data including the results of the Phase I-a.

In the first model, the granite is divided into only two parts (weathered part and the remaining rock mass) because of the lack of information of fractured zones in the granite.

In the second model, supplemented with the investigation results of the Phase I-a, new geological units such as the Biotite granite, Felsic granite, "Upper fracture zone", "Moderately fracture zone" and "Fracture zone along the fault" are newly extracted by geophysical loggings and borehole investigations.

However, the second models contain the uncertainty related to the lack of information on tectonics in the northeastern part of the Shobasama site, the deep underground exceeding 1,000m and the fractures with a high inclination.

(2) Hydrogeological investigations

In subsurface hydrological investigations, water balance observation and measurement of meteorological parameters are carried out. It aims to set the top boundary condition for groundwater flow simulation and to develop techniques to obtain the evaluation data of the simulation results. As a result, it is proven that groundwater recharge measures approximately 510% of the precipitation.

Also, hydraulic tests are carried out using the MIU-13. As a result, groundwater level and hydraulic conductivity of the rock mass are estimated. In addition, hydrogeological properties of highly permeable fractured zones (presumed to be water channels) and the Tsukiyoshi Fault (presumed to be a hydraulic barrier) are estimated.

In order to ascertain the correlation between (a) varieties/amounts of investigations, analyses/evaluation technique and (b) the accuracy of the results, hydrogeological models are constructed and groundwater flow simulation is carried out. Both are carried out based on the following two data sets : a) data from literature survey and other geoscientific researches, b) data including the results of the Phase I-a. A continuum model is applied to the model construction and groundwater flow simulation using the data a). One the other hand, an "equivalent discontinuum model" is applied to the model construction and groundwater flow simulation using the data b). As a result, it turns out that an "equivalent discontinuum model" predicts more realistically groundwater flow in the groundwater flow simulation in a medium-sized domain including such discontinuous structures as fractures and faults. Specifically, an irregular waterhead drawdown is formed along the distribution trend of fractures. Also, the Tsukiyoshi Fault is expressed as a hydraulic barrier by change in waterhead.

(3) Hydrochemical investigations

From the results of the RHS Project and other investigations, groundwater is gathered and its physicochemical parameters are analyzed. It reveals that the groundwater in the shallow part (shallower than 300m in depth) in the Shobasama site is of Ca⁺²-Na⁺-HCO^₃- type, neutral (pH=7) and oxidizing (Eh>0mV). It changes to a Na⁺-HCO^₃- type, weakly alkaline (pH=9), and reducing (Eh<-300mV) in the deeper part (deeper than 300m in depth).

To examine the groundwater evolution, the Fe³⁺/Fe²⁺ ratio in cores (granite) obtained from the boreholes is measured. It reveals predominance of Fe³⁺ over Fe²⁺ at shallower than 300m in depth. This indicates that oxidation-reduction environment in the granite varies at the depth.

In the future, groundwater will be collected and analyzed in the MIU-4 excavated in the Phase I-a. Also, using the data from the MIU-4 and the RHS Project, chemical and mineral compositions of rocks and the groundwater evolution will be examined.

(4) Rock mechanics

On the basis of the data on physical/mechanical properties and initial stresses obtained in the AN-1 and the MIU-13, rock mechanics conceptual models are constructed in the following three steps.

a) construction using the data from the AN-1 and MIU-1
b) revision using the data from the MIU-2 and used in a)
c) revision using the data from the MIU-3 and used in b).

As a result, three following zones with unique physical/mechanical properties as well as stress states are shown : ground surface300/400m, 300/400700m and 7001,000m in depth.

In the future, mechanical/physical properties of the Tsukiyoshi Fault and associated fractured zones should be understood. Also, the validity of the model should be evaluated by numerical analysis. In addition, mechanical/physical properties of discontinuity planes should be assessed by the results of joint shear test. Resultantly, the model is expected to be quantified.

(5) Investigation techniques and equipment

Existing investigation techniques and equipment are improved, including those developed by the TGC. In addition, investigation techniques and equipment that would be needed in and after the Phase II are also developed.

As for techniques and equipment for borehole investigations, a drilling system using reverse aerated wire-line method is designed. As for the development of the partial casing insertion equipment, the whole equipment is assembled and an operational application test is carried out.

As for techniques and equipment for geological investigations, the development of seismic tomography using 1,000m-class boreholes is in progress to understand the extent of discontinuity planes deep underground. So far, an application test is carried out in the existing boreholes aiming at down 1,000m in depth. In addition, the development of a data analysis technique called "full-wave inversion" is carried out with the purpose of improving the resolution.

As for techniques and equipment for hydrogeological/hydrochemical investigations, hydraulic test equipment and water sampling equipment for depths of up to 1,000m, assembled and utilized by 1998 FY, are improved to be applicable to bent boreholes.

As for techniques and equipment for rock mechanical investigations, a initial stress measuring equipment for depths of up to 1,000m is designed. Its manufacturing is being carried out.

As for techniques and equipment for in and after the Phase II, continuous-wave radar investigation techniques, a long-term monitoring system using boreholes, and and an investigation system of research drift walls are developed.

A data base is introduced and improved for management and utilization of the data. The system used for data analysis/visualization of data on geological environments, which is used for the construction of geological environment models and groundwater flow simulation, is improved to enhance the function and operation.

As for techniques and equipment for information disclosure, virtual reality (VR) technology is introduced, and VR software introducing the MIU Project is developed. Furthermore, the software is improved to allow visitors more realistic experience by using a head mount display (HMD).

(6) Evaluation of prediction

In order to evaluate prediction appropriately, it is required to clearly define what and how to evaluate, where and how to obtain data. Taking it into consideration, overseas precedent cases are examined.

(7) Development of the engineering technology for deep underground

Basic design concepts are examined, and domestic and overseas precedents and requirements for underground research laboratory by Atomic Energy Commission (1994)¹⁾ are refered to. According to the examination results, detailed shape and size of facilities are drawn up.