Mineralogical Study of Landslide in Monden Area, Southwest Okayama Prefecture, Japan

Landslides, occurring in Tertiary Nyokan formation of the Monden area, at the southwestern part of Okayama Prefecture, southwest Japan were investigated to clarify the occurrence mechanism of Tertiary mudstone landslide from mineralogical viewpoint. The landslide of the area is characterized by slow creep movement at the rate of 2 to 10 cm per year, and can be considered to be a typical creeping landslide.
Three zones are distinguished in the landslide profiles based on the constituent clay minerals, i.e., upper, slip and lower zones, respectively. The slip zone develops dose to the boundary of the upper zone and the lower zone. Detailed examinations of microtexture indicate that the smectite rich landslide clays have undergone shearing deformation because the resulting texture is characterized by highly oriented parallel FF contacts. The landslide clays with highly oriented parallel FF contact texture have impermeable nature and the materials containing the clays are ready to start creep movement. Furthermore, cations such as Ca2+, Mg2^ and Na+ derived from feldspar in the host rock are highly concentrated in the landslide clays, causing the increase of smectite content and at the same time promoting the replacement of Ca2+ and Mg2+ by Na+. This reaction promotes the formation of Na-smectite or (Na, Ca)-smectite characterized by high expandability. This process further causes the decrease of soil strengths and the formation of characteristic microtexture, i.e., lubrication plane. When the lubricating plane develops to a certain extent, the creeping landslide takes place.
The displacement rate of the creeping landslide mechanically depends mostly on the pore water pressure (groundwater level) in the landslide profiles. High pore water pressure always corresponds to the large displacement rate. Taking into consideration of groundwater levels and the mineralogical characteristics, a rheological model is proposed. Applying obtained soil constants to the model, the possibility of prediction of the creeping landslide is discussed, especially on the deformation process.