第三紀層地すべり地におけるすべり面の発達過程とすべり面粘土の生成について

Once landslides movement initiates, the sliding mass tends to move toward a more stable position. However, landslides that have occurred in Tertiary formations indicate recurrent movement. A possible reason for this behavior is that the mineral composition and properties of slip plane materials become more lubricante as the movement repeats . In order to determine the causes, five Tertiary landslides with different movement histories were studied. Based on a geological investigation, the results below were obtained by studying the development process of slip planes and clays of Tertiary landslides that forms the slip planes.<BR>The clay content of slip plane materials is directly related to the magnitude and history of slide movement . This study indicates that slip planes alter from: (1) striation type (showing only striation on bedrock without clay); to (2) brecciatd type (breccia with clay); then to (3) mylonite type (clay with breccia); and finally to (4) clay type (clay without breccia). If the slip plane area is composed of tuff or tuffaceous materials and subjected to increasing slide movement, the content of smectite in the slip plane clay increases while the content of silicate minerals decreases. Furthermore, the smectite content increases within the same landslide from the head to the toe portions of the landslide (direction of ground water flow). The ground water at the slip plane (above the impervious zone) contains an increased concentration of Ca²⁺ ions. As the concentration of HCO₃^- exceeds over 40 mg/l an environment is created which promotes the formation of smectite. This process indicates that there is an active ion exchange within the ground water near the slip plane that forms smectite. Thus, the formation of smectite in the slip plane requires the presence of tuff and sufficient concentration of HCO₃^- within the ground water.<BR>The study results indicate that frictional resistance decreases as clay content increases. The reason for the repeated movement of Tertiary landslides with tuff or tuffaceous composition at or in the vicinity of the slip plane is attributed to the increase in clay portions along the slip plane as the movement repeats, and the groundwater that promotes the formation of smectite which one of the clay minerals with lower frictional resistance.