1) Overview of the investigations in the AN-1 and the MIU-1^{39),74),75),76)}

(1) Concept of the investigations

Investigations in the two boreholes are carried out in the earliest stage of 1,000m-class borehole investigations in the Shobasama site. Their main aim is to obtain comprehensive data on physical properties and the initial stresses of the rock mass between the ground surface and 1,000m in depth.

(2) Contents of the investigations

Based on the above-mentioned concept, physical property/mechanical property tests and initial stress measurements are carried out at intervals of about 100m and 50100m, respectively. Specifications and amounts of the physical/mechanical property tests in the AN-1 and the MIU-1 are shown in Tab.4.27. Depths of the initial stress measurements are shown in Tab.4.28.

^* : Sampling depth for AE/DRA
^** : Middle point depth of test interval for Hydraulic fracturing

(3) Results of the investigations

Results of the physical/mechanical property tests in the AN-1 and MIU-1 are shown in Tab.4.29 and Fig.4.61. The obtained data vary so widely that results approximated by quartic polynomial expression are also shown in Fig.4.27 to show the relationship between the individual physical property values and the depths. Vertical initial stress values obtained by AE/DRA tests in the MIU-1 are shown in Fig.4.62. Horizontal initial stress states obtained by hydraulic fracturing tests in the AN-1 are shown in Fig.4.63. Hydraulic fracturing tests carried out at four depths (97.0m, 156.0m, 439.0m and 749.0m) in the AN-1 produce no longitudinal fracture to satisfy a theoretical assumption. Therefore, these results are judged too unreliable to be used for assessment of stress states and shown in the figure. Also, pore water pressures of the rock mass at the depth exert a great influence on values of calculated maximum principal stress. Therefore, the both values of maximum principal stress with pore pressures (H_min) taken into consideration and without pore pressure (H_max) taken into consideration are shown in Fig.4.63. Concerning azimuths of maximum principal stress, the error ranges obtained by a linear approximation using least squares method of longitudinal fractures produced by hydraulic fracturing are also shown in Fig.4.63.

	Item	Unit	AN-1	MIU-1
Physical property	Apparent density	-	2.592.62	2.612.64
	Effective porosity	%	1.01.8	1.01.8
	Water ratio	%	0.150.30	0.300.70
	Seismic wave velocity (Vp)	km/sec	4.04.5	5.05.8
	Seismic wave velocity (Vs)	km/sec	2.22.7	2.83.0
Mechanical property	Uniaxial compression test	MPa	120240	130250
	Young's modulus (E50)	GPa	3460	4565
	Poisson's ratio	-	0.300.37	0.300.37
	Tensile strength (by Brazilian test)	MPa	311	411
	Cohesion	MPa	-	2026
	Internal friction angle	Degree	-	5060

(4) Evaluation of the results

Both physical/mechanical properties of the rock mass in the AN-1 show an uneven distribution. The relationships between these properties and the depths show different tendencies in the following sections : ground surface300m in depth, 300700m in depth and more than 700m in depth. The relationships at the MIU-1 are similar to those at the AN-1. Characteristically, specific gravity and tensile strength seem to show a cyclic change.

The value of vertical stress obtained by AE tests is thought to be equal to overburden pressure approximated by unit weight and overburden. However, the measured values at 590.64592.39m, 896.52896.78m and 990.04990.36m in depth actually range from a half to a fifth the estimated overburdens. At these depths, a lot of fractures occur and fracture surfaces are softened. Also, stress discontinuities are found in these depths in the results of hydraulic fracturing tests. These facts suggest that the stresses change locally at these depths. On the other hand, no clear critical point appears in any stress-strain difference curves produced by one to five-times repeated loading. Also, DRA test shows wide strain differences in the prediction of vertical stress. These facts indicate a low reliability of the estimated stresses. The causes remain unsolved at the moment.

Regarding the initial stress state in horizontal planes obtained by hydraulic fracturing tests, the maximum principal stress drops about 300m and 700m in depth and doesn't increase linearly with the depth. Given that vertical stress is equal to overburden pressure, 3-D stress states of reverse-fault-type (_H>_h>_v), transitional-type (_H>_hv), and lateral-fault-type (_H>_v>_h) are expected at a depth of 0300m, 300700m and 7001,000m, respectively. The maximum principal stress trends N-S at a depth of 0300m and NW-SE at a depth of 3001,000m. Thus, the initial stress states on horizontal planes around the AN-1 are divided into three sections : 0300m, 300700m and 7001,000m in depth.

Referring to the results of BTV investigations in the AN-1 and MIU-1, characteristics of the fracture distribution in the Shobasama site are examined. Distribution histograms of fractures counted by BTV investigations in each 50m of these boreholes between the ground surface and 1,000m in depth are shown in Fig.4.64. These histograms indicate that the fluctuating trends change at a depth of 300m and 700m in the AN-1 and 300m and 750m in the MIU-1, respectively. The results of the BTV investigations reaffirm the above-mentioned three sections divided by the results of the initial stress measurement.

2) Construction of rock mechanics conceptual model⁷⁷⁾

As previously stated, physical/mechanical properties, initial stress state and fracture distribution equally change at about 300m and 700m in depth. Consequently, a rock mechanics conceptual model can be constructed in which rock mass is devided into three zones with different physical/mechanical properties and stress states at above-mentioned depths.