In designing a foundation of dam, a tunnel or an underground structure, it is necessary to estimate the mechanical properties of rock mass. However, rock mass is not a uniform body, but contains joints and other weak planes, and these weak planes reflect on the mechanical properties of the rock mass. The strength value obtained in laboratory leads to an over-estimation of strength of rock mass, because rock specimens are almost collected from rock mass between weak planes.
This study was undertaken for the purpose of estimating shear characteristics of rock mass containing weak planes, and a direct shear testing machine, which is applicable in the study, has been developed. As a preliminary research, the process of forming shear surface and the shear behaviours of rock specimens were investigated on OGINO tuff.
The results in this study are summarized as follows;
1) A distribution of the principal stresses in rock specimen was analyzed by F.E.M.. In this analysis, the tensile stresses appear near the shear edge of the shear testing machine. From this result, it is thought that a crack initiation in rock specimen is originated by the tensile stress, and the shear stress is then concentrated at the tip of the tension crack. Consequently, the crack may progress along the direction of shear load and the shear surface becomes undulated.
Also, as normal stress is inceased in the direct shear test, peak and residual shear strengths take higher values and shear displacement at the point of the peak shear strength is larger.
2) The direct shear tests were conducted on the rock specimens containing the artificial discontinuities.
From the results of the shear tests which were carried out at a constant normal stress, it is found that peak shear strength takes remarkably low value in the region of θ=45°.
On the other hand, it is found from the results of the shear tests done under the confined normal displacement that peak shear strength takes high value in the region of θ=45°.
From the above findings, it is clear that shear behaviours of rock containing discontinuities are affected by the angle (θ) between the direction of shear load and that of discontinuities, and the condition of restraining normal displacement.

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It is often troublesome to tunnel through the areas occupied by serpentinite which is usually dealt with as fragile earth for civil engineering. This paper presents the physioal and chemical properties of serpentinite in Himekawa, Niigata Prefecture from the standpoint of engineering geology. Many tunnels penetrating serpentinite masses in other parts of Japan are also referred.
Serpentinite in Himekawa is divided into two bodies, the Asaoyama and Hiraiwa. The latter shows petrologically more marginal facies than the former and both include chlorite minerals. The degree of alteration of serpentinite is fairly consistent with an index (F_c′) which is designated by the average ratio of hardportion within every ten meters interval of drill core. Chemical analyses have shown that aluminum oxide (Al₂O₃) and calcium oxide (CaO) tend to increase as the alteration of serpentinite proceeds. Talc and carbonate minerals also tend to increase in that course. Determined physical properties are as follows; ranging from 2.6 to 2.8 for apparent density, less than 3% for water absorption rate, and less than 5cm³ for swelling degree. The primary ultrasonic wave velocity (V_pc) for drill core has the following relationship with the secondary one (V_sc) :
V_pc=2.075V_sc-0.259
Serpentinite itself does not have any expanding nature but when it undergoes serpentinization along fractures, talc is readily formed followed by foliation. Eventually upon clay mineralization, serpentinite mass has a tendency to swell and slide due to the earth pressure and the leaching of ground water.

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The Seikan Tunnel is a super-long undersea tunnel which passes under the Western Passage of the Tsugaru Strait, an area subject to rapid tidal currents and an extremely changeable climate which have presented difficult conditions for geological survey works on site. The tunnel has a total length of 53.850km. (with an undersea length of 23.300 km.)
Utilizing the best marine geological survey methods at our disposal our survey was conducted both before and during excavation of the tunnel. Great labor and toil were required for this task. In the tunnel itself, we have undertaken long horizontal exploratory borings to investigate the geologic conditions before proceeding with the excavation of each new section. After twenty years since the start of the excavation, the pilot tunnel was broken through and completed on January 27, 1983.
At present, the construction works are focusing upon the remaining section of 2 km of the main Tunnel. Consequently, the outline of the geologic structure and characteristics of the undersea portion has become clear as a result of the marine and underground investigation hitherto carried out. Here the authors have put together the geologic outline of the so-called “Green Tuff” area, particularly those geologic conditions which have posed the most difficulty for grouting and excavation work.
This report will be presented in four serials as follows:
1. The outline of geological survey and geology.
2. Stratigraphy and unconformity.
3. Fault and seepage water.
4. Pyroclastic dyke and slightly cemented sandy strata.

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