The Japanese Islands are situated at the north-east margin of Pacific Ocean, and thrusted by the Pacific and Philippine Plates. About 40% of total area of them is covered by Tertiary and Quarternary sedimentary rocks, most of which are classified into weak rock. Particularly, the north-east district of Honshu Island is widely covered by so-called "Green Tuff", which is one of the weak and incompetent miocene tuff colored in green or dark blue. In this Green Tuff formation, a peculiar and complicated lead-zinc-copper ore "Kuroko" deposits are burried, which are one of the most important ore deposit in Japan and mined presently in several mines.

In Japan, most of coal bearing formations belong to Tertiary system. Although coalification process has been accelerated by geothermal effect and severe tectonic activities to be bituminous coal, the coal seam and coal measure rocks are frequently weak and fractured. Furthermore, frequent volcanic activities and severe tectonic movements have caused many foldings and faults in geological formation in Japan.

All of these geological situations have made rock formations fractured and altered, which cause many difficulties related to severe rock pressure in mining, in turn.

In coal mine, about 75% of the total output is mined from the depth of over 500m from pit mouth. Although mean depth of faces is not greater than those in Ruhr coal field, the combined effect of severe tectonic stress, high gas content of coal and weak rock formations causes various difficulties in mining, which need various technical developments to control severe rock pressure and to prevent hazards of rock and gas outburst.

In Kuroko mine, although the mean depth of stops is about 300m, Cenonic clayey shale as country rock is usually weaker than Kuroko ore which is classified into the weak rock in itself. Therefore, the maintainance of cross cut and haulage level roadway has been encountered with many difficulties concerning with design of support, cost and frequency of repair work in roadways.

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In this paper, the author reviews some features of mechanical properties of rock formation and some technical developments to control severe rock pressure in the incompetent rock formation in mines

State of rock stress in mining area

The initial state of rock stress is often measured for the purpose of disigning the underground hydraulic power plant, preventing the rock burst hazard or disigning the mine pillars as well as stopes.

Most of Rock stress measurements are carried out by means of stress relief method, particularly door stopper type Strain cell and borehole deformation gage.

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Most of the maximum horizontal stress determined by means of stress relief method trend approximately E-W, and this trend approximately coincides with the trend of tectonic stress determined by geodesic triangulation survey (Oka, et al. 977).

Some examples of the initial state of rock stress measured in mining field, are shown in Table 1.

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