A STUDY ON INVESTIGATIONS OF THE MECHANISM OF HOT-SPRINGS LANDSLIDE OCCURRENCE

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A lot of hot springs where the fountain quality and atmosphere were especially done spring up in Beppu City, Oita Prefecture, and the numbers of sources (hole) are 2, 847-places and amount of discharge is 137, 040 kiloliters a day. Thus, Beppu City is hot-spring sightseeing spot of the top of Japan in both, the numbers of sources, and amount of discharge. Myouban district is a famous hot-spring area in the acid hydrogen sulfide fountain and the green turn fountain. But, that area is a hot-spring landslide area where the landslide activity is seen from having peculiar geological features condition, and the composition of a near region of a so-called solfataric clay. Properties of groundwater including the pH and the temperature are greatly different from usual landslide areas because it is under a peculiar environment of hot-spring area, and even if it undertakes the examination of the countermeasure, the attention is required. Moreover, when the stability of the landslide slope is exam … More ined, it is a current state that experienced method is used.Then, we performed the site investigations at Myouban landslide area to clarify the occurrence mechanism of the hot-spring landslide and calculated the safety factor of the landslide using different slope stability analysis and strength parameters in factors influenced on the landslide stability.The result of obtaining is shown as follows. 1) The groundwater level is large the change, and rises especially greatly through year until the rainy season and the typhoon time of September from the middle of June. 2) Properties of groundwater are under the high corrosion environment like a high temperature, a low pH, and a high electric conductivity, etc., and the tendency is different according to the bore hole. 3) Clay in the vicinity of the slip surface receives the history of a different water temperature, the pH, and the electric conductivity by the season, and the range is water temperature 24-29℃, pH3-4.5, and electric conductivity 10-30mS/m. 4) When having calculated the strength parameters by the back calculation method, the internal friction angle ¢ obtained by the "Fellenius method" grows by about 1.3℃ than the ¢ of the Bishop method under lower slip surface. Less