SLOW MOVEMENT OF SLOPE MATERIALS THROUGH THE PROCESS-ES OF FREEZING AND THAWING IN THE NORTHERN PART OF ASHIO MOUNTAINS, JAPAN

Abstract

The purpose of this research is to make clear the processes of movement of slope materials occuring through freezing and thawing. A studied area belongs to Nikko National Park in the central part of Honshu, Japan. Seven slopes were selected for the observation and measurement in the area. The followings were observed and measured : 1) longitudinal profiles of slopes, 2) features and depth of frozen slope materials, 3) water content of slope materials before and during the period of freezing, 4) displacement caused by the processes of freezing-thawing and by other factors such as changes in soil temperature, soil moisture, etc. In addition, a laboratory experiment was carried out in order to carry out a further examination of the processes of movement of slope materials caused by, freezing-thawing. The seven slopes are all in the natural conditions. Two of them have a convex profile while the others have a convex-concave profile. The mean inclination of these slopes ranges from 28 to 41 degrees. The frozen materials can be classified into following seven types : 1) ice layer, 2) ice layer bearing soil lens, 3) frozen soil bearing lens-like ice crystals, 4) frozen soil bearing short needle-like ice crystals, 5) concrete-like frozen soil, 6) frozen soil bearing cubic ice crystals in pores, 7) frozen soil bearing jam-like ice. The frost penetration into the slope materials generally shows a trend of increase in depth downslope. The water content has no change in the directions normal to and parallel to the slope surface during the non-frost period.It was observed that water increment had occurred toward the surface from inward during the frost period, and this phenomena had taken place up to the depth less than 12.5 cm. An improved Young-pit method was used to measure the displacement of slope material. This is different from the original Young-pit method in such a way that the top of pin was painted and displacement was traced on white paper. The rate of downslope displacement in each slope increases in general at the middle and/or the foot of slope. The depth in which displacement can be recognized is less than 12.5 cm on each slope. Furthermore, the dis-placement distribution is classified into four types. From the field observation and the laboratry experiment, several considerations were given to the processes of movement of slope materials occurring through freezing and thawing. These are summarized as follows. 1) Comparing the result obtained in the period of frost with that in the non-frost period, the movement of the slope mantle originated by freezing and thawing is different from that caused by other factors in the direction and the rate of displacement. 2) According to the observation of changes in soil moisture in the process of thawing of a frozen soil, the downslope displacement can be explained by the inter movement of soil aggregates in the thawing layers. 3) It is anticipated that the slope material would move downslope until soil aggregates make contact with each other in the process of thawing. 4) The maximum rate of the downslope displacement of slope mantles through the proces-ses of freezing and thawing appears at a certain range of slope angles, from 32 to 37 degrees. This range of slope angle is considered to be a critical angle in this study.