Investigation on the mechanism of Saein Strait Landslide (Nir-Sarab Road) in regard to the hydrological and geomorphological conditions

Saein Strait landslide took place along Nir-Sarab Road by the southern flank of Sabalan volcano on June 6, 2005, and damaged 350 m of the main road. The main factors which might affect the stability of this particular slope are the thickness and texture of the soil, extensive weathering, gentle slope to cause the accumulation of the material, and finally the regionally affected groundwater level fluctuations. A research was planned based on the field studies, laboratory tests and numerical modeling to identify the main factors affecting the stability of this slope. Landslide profile and engineering geological properties of materials were applied in the numerical models. The effect of rainfall in soil saturation process was simulated for 35 days prior to the failure. Modeling results shows that the infiltrated rainwater (during the 35 days) into the sliding mass cannot percolate more than 7 m and, thus, it must be followed by another triggering factor. The impact of fluctuation in the groundwater level on the stability of the slop was evaluated using limit equilibrium and finite element modeling methods. Previous studies showed that the local conditions of paleogeomorphologyand the existence of a depression might cause the accumulation of volcanic eruptions materials to create sliding mass today. This depression may drain the surface and the groundwater toward the sliding mass from upstream catchment. The materials are highly weathered and groundwater rise due to the exceptional precipitation (rain and snow) during 2004-2005 water year are the main factors bringing the slope to its yield point.