The Implication of Structural Discontinuities in Rock Slope Stability Analysis: Example of Katsuura-Ubara, SE-Japan.

Discontinuities in rocks such as faults, joints and bedding occur at different orientations with respect to the principal stress; knowing the type and characteristics of fractures will benefit our understanding of the bulk physical properties of rock masses. The present paper attempts to bridge a strong relationship between structural discontinuities and slope instabilities in the vicinity of Boso Peninsula SE Japan, marked by steep sedimentary rock forming slopes. The rocks are essentially constituted of sandstone, mudstone or their alternation of Miocene age. The bed generally strikes E\-W or NE\-SW and dips toward N or NW. Aerial photo\-interpretation backed up by intensive field mapping of geological and geomorphological features revealed four main stages of deformation with different mechanisms which can be inferred from microstructures at the scale of SEM (Scanning Electron Microscope) and thin section. The representation of discontinuities and the evaluation of their angular relationships with regard to the drainage map indicate that the fracture intersection and fracture network geometry greatly influence the fluid flow properties. It was verified that seepage is mainly noticeable in most recent origin discontinuities suggesting that the fracture age and origin are very important in order to access the water transmissivity. On the other hand a quantitative description of the discontinuities in term of orientation, spacing and density suggests that the fracture system in this area controls the orientation and the morphology of the slopes. These examples illustrate how a precise knowledge of the fracture network is essential for the analysis and design of slopes. The method used in this paper is relatively simple with acceptable degree of accuracy, and it shows a significative participation of the fracture analysis as an important tool for solving problems concerning the various stages of the slope stability analysis. Keywords: Structural discontinuities; Slope Stability; Drainage map