Abstract
Mass movements and landslides are important factors that have contributed to the destruction of natural mountainous forests in northern Iran. The phenomenon is compounded by road construction and other human-induced development projects. Factors mainly attributed to soil instability in such areas are topography; hydro-geology, geology, and pedology. The Chafroud forest district located in western Guilan was selected as the study area chiefly because of its high incidence of landmass movement and landslides. Areas with higher mass movement rates were selected (showing an average movement rate of 80~100 cm during the seven months of from August 2005 to March 2006) for further assessment of mass movement type and rate, as related to soil type and mechanical features. Leveling operations and soil mechanical tests including Atterberg limits pertaining to liquid, plastic, shrinkage limits, and related indices, soil grading, and texturing were carried out. Results revealed that the main movement type was landslides caused by a combination of high moisture content in a fine-grained soil texture which prevented drainage. The most amount of landslide occurred in winter that assume the winter rainfall has saturated the clay soil of the regions and eventually increased the landslide. The natural water content of the soil in the sliding region increases with the increase of soil depth. This fact indicates that water content at depth is higher than the surface water content. Hence, scattered clay layers present at deeper levels contain higher amounts of water. Since clay loses its water content slowly, high water content in deeper layers results in instability of the soil structure and, consequently, results in landslides creating a higher level of damage.
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