Stability of rock mass structures and flow of ground water through rock masses are influenced by discontinuities included in the rock masses rather than the rock materials. This paper describes a method of estimating the distribution of rock mass discontinuities, especially the orientation and frequency of sets of discontinuities.
The frequency of discontinuities, λ_s, defined as the number of discontinuities intersected by a scanline of unit length, depends on the direction of the scanline. When a λ_s-θ curve is drawn as the variation of λ_s in rotating a scanline with direction of θ, the orientation and frequency of sets of discontinuities can be found according to the downward cusps appearing in the λ_s-θ curve. On the contrary, in the RQD-θ curve which shows the variation of RQD values in rotating a scanline or drilling line, upward cusps give the orientation of sets of discontinuities. The ‘cusp’ method is also applied to anisotropy of elastic wave velocity in rock masses, because the slowness of elastic wave passing through rock mass discontinuities is proportional to the frequency of discontinuities. The validity of this method is checked by using data obtained by HUDSON and PRIEST, ODA et al., and so on.

抄録全体を表示

Four debris flow deposits and many intraformational folds were found in the late Pleistocene Takigoshi lacustrine sediments near the Ontake volcano, central Japan. The debris flow deposits have been regarded to be supplied from a large slope failure around the Kamikurosawa valley, which is identified in the present study. The size of the failure is estimated to be comparable with the Denjokuzure, which was caused by the Naganokenseibu earthquake on September 14th, 1984. Detailed stratigraphic study of the Takigoshi sediments revealed that debris flows took place three times in 500-2500 years. It is suggested that periodic inland earthquakes generated the debris flows, because a large fraction of historical large-scale debris flows was induced by them and active faults densely occur in central Japan. A brief description of other lacustrine sediments suggests that debris flow deposits and intraformational folds are ubiquitous. It is pointed out that a detailed stratigraphic study of rhythmical sediments provides a significant method for estimating recurrence interval of inland earthquakes.

抄録全体を表示