A large-scale landslide and related mechanism: a case study in the Qazvin–Rasht freeway, Iran

Abstract

The Manjil landslide occurred after the slope cutting and the removal of the toe buttress (8 July 2013), between 41 + 400 and 42 + 200 km section of Manjil–Radbar in the Qazvin–Rasht freeway, Gilan Province, Iran. The occurrences of landslide led to blockage of the freeway. A cutting was designed with a width and height of 4 and 6 m, respectively, and a 5 to 6 m excavation depth within a 120 m line along the slope toe. Based on the site investigations, the landslide was characterized as a complex deep-seated landslide, displaying composite rotational-debris flow. Moreover, extensive field survey, subsurface investigation, laboratory and in situ tests show that although the landslide was triggered by an inappropriate cutting, geostructural, geological engineering properties and hydrology have also played a key role in its initiation and further aggravation. Therefore, the western and eastern boundaries of the landslide coincide with the two faults (F1 and F2). In addition, landslide sinisterly movement is controlled by a left lateral strike-slip fault (F2). It was observed that the scarp follows the N130°/75 direction of bedding rocks. The presence of marls and shale implies a high swelling under wet conditions, thus providing lubricating sliding surfaces. In addition, alteration and weathering of andesite, basalt, tuffite, and tuff have been reduced the shear strength of the slope. Engineering geological map (EGM) of the landslide was prepared on the basis of aerial interpretations, the orientation of ground cracks and fissures, geomaterial, geostructural and hydrology. Based on ground cracks and fissures, the displacement vectors are also determined and shown on the map. The relationship between the displacement vectors estimated from ground cracks and fissures with GPS observation and slickness slip surface is coherent.