古期地すべり面判定のためのすべり層粘土の鉱物学的・化学的評価 岩手県明倉沢古期地すべり地を例として

Abstract

In stability analysis of paleo-landslide area, it is indispensable to have fully understandings of the past main slip surface (paleo-slip surface). In this paper, some mineralogical and chemical features of shear zone clay distributed in the Myoukurasawa paleo-landslide area were studied in order to make clear the efficiency of mineralogical/chemical method for evaluating paleo-slip surface.The shear zone of the Myoukurasawa landslide area is located at the boundary part between bed rock and weathered moving soil, and composed of layered clay which is divided into six sublayers on the basis of the color. The brown clay sublayer in the lower part of the shear zone is characterised by a slip surface with slickenside. This shear surface on the brown clay sublayer (slip surface clay) possibly resulted from the principal displacement shear in paleo-landslide.Clays and rocks from the landslide area were examined by means of X-ray powder diffraction (XRD), quantitative analysis of amorphous matter, quantitative chemical analysis by EPMA, CEC measurement, and exchangeable cation analysis. As a result, some mineralogical/chemical characteristics of shear zone clay were shown as follows; (1) quartz index values obtained by XRD for minerals in the shear zone suggest that vermiculitic minerals increases at the upper part relative to the slip surface, while illite increases at the lower part relative to the slip surface, (2) the XRD quantitative analysis for quartz indicates the contents of quartz in the shear zone clay are lower than those of the gray mudstone and black mudstone, (3) the quantitative analysis of amorphous matter suggests that the slip surface clay have the maximum amount in shear zone clay, (4) EPMA analysis show that shear zone clay are poor in SiO2 but rich in Al2O3 as compared with the gray and black mudstones, (5) CEC value of the slip surface clay shows a maximum in shear zone clay.As mentioned above, mineralogy and chemistry of the clays are closely related to the occurrence of clays in the shear zone of the Myoukurasawa landslide area. Thus the mineralogical/chemical examination can be an usable method of characterizing shear zone clay also in other paleo-landslide areas. In particular, the amorphous matter formed in the shear zone during sliding can be an indicator for shear zone and slip surface. In addition, the decrease of quartz quantity in shear zone may be due to the development of shear cracks during sliding, suggesting that decrease of quartz can be an indicator for shear zone in the landslide area where weathering has been dominant.