1) Objectives

Based on thegoals set for the entire MIU Project and its Phases, the objectives of rock mechanical investigations are set as follows⁷⁾.
Data acquisition of the data on mechanical properties of the rock mass distributed between the ground surface and the depth in the Shobasama site
Understanding of the initial stress state, construction of rock mechanical model and verification of its validity by examining the data
Prediction of change in mechanical stability of the adjacent rock mass with the advance of the shaft excavation
Development of methodologies for systematic investigations and analyses of rock mechanical properties of the rock mass

2) Ways to forward the study

This investigations aim at granite. In general, properties of discontinuity planes and their distribution patterns exert a great influence on in-situ mechanical properties of the rock mass. It is assumed that the Tsukiyoshi Fault has a great effect on the stress state of rock mass distributed in the Shobasama site. In this investigations, construction and revision of rock mechanics conceptual model is planned in the Shobasama site following analysis of the survey results obtained in the AN-1 and the MIU-13 while taking above-mentioned influence factors into consideration. The rock mechanics conceptual model refers to a model in which the rock mass is divided into several zones according to physical/mechanical properties and initial stresses. A rock mechanical model will be constructed by quantitative setting of the distribution of physical values based on the rock mechanics conceptual model. And, according to the rock mechanical model, a predictive simulation is carried out on mechanical effects of drift excavation.

Specifically, core descriptions, BTV investigations, geophysical loggings, laboratory tests and initial stress measurements are carried out in the individual boreholes to obtain rock mechanical data. Based on the results, a rock mechanics conceptual model for the Shobasama site is constructed. Subsequently, a new borehole is drilled to obtain new rock mechanical data, with which the above-mentioned conceptual model is compared. As a result, the the conceptual model is revised based on results of the comparative study.

In this report, concepts, contents, results and evaluation of investigations in the AN-1, MIU-13 are described. Also, the rock mechanics conceptual model is shown.

3) Overview of the investigations

The contents of the investigations are devided into physical properties of the rock mass and initial stress.

Physical properties of the rock mass
The distribution of physical properties of rock matrix between the ground surface and the depth of 1,000m is understood by physical property/mechanical property tests of core samples. The physical property tests measure apparent specific gravity, water content, effective porosity and elastic wave (P-wave, S-wave) velocity. The mechanical tests consist of uniaxial compression test, Brazilian test and triaxial compression test. All of the tests are carried out based on ISRM guidelines or JIS standards.

Initial stress
The distribution of initial stress between the ground surface and the depth of 1,000m is examined by AE/DRA test using core samples and hydraulic fracturing test using boreholes. The AE/DRA test is aimed at obtaining data of the vertical distribution of initial stress. The hydraulic fracturing test is aimed at obtaining data of the distribution of principal stress and directions of maximum principal stress on horizontal planes. The hydraulic fracturing test is employed not only because there is no other technique to measure the initial stress state from the ground surface to a depth of 1,000m but because the vertical stress could be regarded as one of the principal stresses in this area according to the previous works^72),73).

For comparing the investigation results among boreholes, the same items are applied to the all boreholes. However, different amounts and depths are allotted to the individual boreholes. The items of tests carried out in the individual boreholes are shown in Tab.4.26. Physical/mechanical property tests, AE/DRA tests and hydraulic fracturing tests number some 20180, 2090, 10 and 1020, respectively. The Shobasama site is underlain by granite with a number of fractures, therefore, results of BTV investigations are used to choose the depths for hydraulic fracturing tests.

	Item	AN-1	MIU-1	MIU-2	MIU-3
Physical property	Apparent density
	Effective porosity
	Water ratio
	Seismic wave velocity (P-wave and S-wave)
Mechanical property	Uniaxial compressive test
	Brazilian test
	Triaxial compressive test	-		-
Initial stress measurement	AE/DRA	-
	Hydraulic fracturing		-