In estimating the shear strength of rock mass, the weak planes contained in the rock mass must be taken into consideration. Therefore, a direct shear testing machine has been developed to perform the experiment. A preliminary research has been done which is described in the previous report.
In this study, a practical research is carried out by using the direct shear testing machine and the results obtained are summarized as follows:
(1) In the direct shear test, the shear load is applied at the desired normal stress. The shear stress becomes maximum at the point of failure strength and reaches finally a value of residual shear strength. The normal stress is then increased as predetermined, and again the shear load is applied. This procedure is repeated, and the testing method is called as multistage shear test.
The values of residual shear strength obtained from the multistage shear tests have little difference to those obtained from the usual shear tests.
(2) The direct shear tests are conducted on the rock specimens collected in the vicinity of places where the in-situ block shear tests have been done, and the results obtained from the direct shear tests are compared with those obtained from the in-situ tests. Consequently, it is found that the values of peak shear strength obtained from the in-situ tests lie between the values of peak shear strength and residual shar strength obtained from the direct shear tests. From this finding, it is clear that the direct shear tests is useful for estimating the shear strength of rock mass.
(3) The rock specimens are collected from many working sites, and the tests are carried out.
The experimental values indicating the relationship between residual shear strength and normal stress are scattered in the same rock type, because the rock specimens collected from different places are classified into the same rock type, and those collected from the same place are not homogeneous. But, it is recognized that the experimental values in the same rock type have a tendency to be expressed by Jaeger's equation.

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Five sandstones, one shale, one limestone, and one marble in Japan were deformed experimentally for a study of the intermediate principal stress effect with a servo-controlled Mogi-type true triaxial compressional apparatus which allow us to control the three principal stresses, independently.
The intermediate principal stress changes the strength of rocks up to 30% but affects little on the residual strength. It was found that the increase of the intermediate principal stress induces the transition from the ductile to the brittle fracturing or the increase of brittleness with reduction of the plastic component of volumetric strain. It was newly observed that the effect of the intermediate principal stress on the strength increased with increase of the minimum principal stress, and that ultimate strength decreased with increase of the intermediate principal stress beyond a certain value.

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The most important problem in the risk analysis of slope is how to predict an unstable slope. In this paper, a method of evaluation for the risk degree of slope failures is described by the application of Hayashi's quantification method III, which is a mathematical-statistical method in order to convert qualitative data into quantitative relations. The investigated area is Sakurajima volcano located in the center of Kagoshima Bay, southern Kyushu, Japan. Taking engineering aspects into consideration, the basic data relevant to slope stability were obtained on topographic and geotechnical conditions, land utilization and vegetation in a fixed unit area overlaied with the mesh of 250m square.
8 items and 33 categories are chosen as the factors of characterizing the unit area. The collected data are categorical, that is, non-numerical and qualitative, data, which can be quantified by the use of a suitable computer program based on the theory of quantification. Numerical values given to each category for the quantification are elements of eigenvectors, which are obtained from vector equation, corresponding to eigenvalues which are obtained from characteristic matrix. According to the relative distance between a couple of categories which can be calculated by elements of eigenvectors, each category is grouped into five classes, which are given the score of risk degree estimated by the values from 0 to 4. The rank of risk level is determined from the total scores of risk degree and finally a classification map of risk level of slope is presented.
The remarkable feature of this paper is that the score of risk degree for categorical data of slopes was determined theoretically by the grouping classification using a mathematical-statistical approach. Moreover, it is greatly interested to note that the informations of slope failures are not always required in applying the method III to the prediction of risk degree of slope.

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Both the coastal and undersea portions of the Hokkaido-side Kunnui Formation have been classified lithologically into 5 layers, (from Kn-1 to Kn-5).
The undersea portion of the Honshu-side Kunnui Formation is largely tuffaceous. Accordingly, it is somewhat difficult to compare it stratigraphically to the Hokkaido-side formations. However, based on established theory, analysis reveals that the foraminifera fossil-bearing muddy layers of the Honshu-side undersea formations and the Hokkaido Kn2 layers can both be correlated to the Middle Miocene Period.
The Yakumo Formation, whose special lithological feature include hard shale formations and a minimum of foraminifera fossils, can only be presumed to the Middle Miocene Period.
Utilizing the sonic prospecting method (sparker) prior to construction, a syncline structure was presumed to exist at the central undersea portion of the Kuromatsunai Formation. Numerous correlations were made between the thin layers of tuffaceous rock takes chiefly from exploratory boring core samples which were extracted periodically as the excavation progressed towards the syncline center from both the Hokkaido and Honshu sides. They revealed much about the syncline's geological structure.
Also, the upper layer of the axial part of the syncline was presumed to be from the Early Pliocene Period judging from the identification of the foraminif era fossils.
Meanwhile, a K-Ar age determination of biotite obtained from the thin tuffaceous layers of the Kuromatsunai Formation was carried out.
The result is a 7.6±0.4Ma reading at the upper regions of the strata.
Based on the chronological test and the foraminiferae analysis, the Kuromatsunai Formation depositional phase has been dated from the later Miocene Period to the Early Pliocene Period.
2. Unconformities
Three unconformities have been observed between the following pairs of formations: 1) the Kunnui and the Fukuyama Formations, 2) the Kunnui and the Yakumo Formations, and 3) the Kunnui and Kuromatsunai Formations respectively.
At the contact surfaces the lower strata exhibit a fragile nature due in part to weathering, while the basal layer of the overlying formation consists of various kinds of fine pebble together with thin intercalation including glauconite.

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