عنوان مقاله [English]
The huge landslide of Dashtegan---Rudbar occurred on the Rasht-Qazvin freeway of Guilan province in Iran. This region is located in the western part of the Alborz Mountains, in the northern area of Iran, which is prone to landslides, due to seismic and tectonic activity and specific geological conditions. The landslide of Dashtegan---Rudbar involved 1.5 million cubic meters mass volume of soil and rock, beginning in December 2004, and, finally, occurring, after six months, in June 2005. These events blocked the Rasht---Qazvin freeway route, and stopped the executive procedure of freeway construction. The upper part geometry is a mixture of soil and rock, underlain by a silty clay layer located on the bedrock. In this paper, after gathering geological and geotechnical information, stability control and back analyses were performed by GEOSLOPE software for an unsaturated state (before sliding). The back analysis shows a factor of safety at about 1.2 for the critical section of the unsaturated condition, which indicates a stable situation in advance of any sliding. Investigations denote that major factors for sliding are related to the great reduction of soil strength parameters in fine-grained portions, due to water infiltration from the upper part, and the consequences of excavations at the toe level of the slope, which removed the supporting passive zone. A couple of remediation methods, including excavation of the upper part, overburdened geomaterial mass, and different cantilever retaining wall systems, have been studied to prevent long-term delay in project blockage, and a decrease in costs and expenses prior to the sliding failure mechanism, so that the construction work of freeways may be continued. Stability analysis, regarding the retaining wall supported by guard piles, indicates reasonable earth work and less shear and moment forces at the wall base, which lead to optimum structural design. Realizing technical, economical and practical aspects, the cantilever retaining wall supported by guard piles, among other measures, is the superior alternative.