The Large Deep-Seated Landslide Induced by the March 12th, 2012 Rainfall Event in the City of Azazga, Northern Algeria: Deformation Characteristics and Failure Mechanisms

A rainfall-induced deep large landslide occurred on March 12, 2012, in the city of Azazga (northern Algeria) causing heavy damages to residential buildings and public infrastructures. In this work, we investigate the inventory mapping, deformation characteristics, the controlling factors and the failure mechanisms of this landslide based on field investigation, aerial photographs/satellite images interpretation, deep boreholes equipped with piezometers and inclinometers as well as subsurface geophysical imaging by Electric Resistivity Tomography (ERT). The obtained landslide inventory map shows that the landslide of March 2012, affected an area of 0,40 km 2 . This landslide is considered as partial reactivations of large pre-existing one (0,606 km 2 ) which represents 6,65 % of the total urban area. Moreover, the analyses identified two types of involved causative factors: (i) the triggering factor related to the antecedent rainfall and human activity through slope excavations and embankment; (ii) susceptibility factors related to the lithological nature and the internal structure of the flysch deposits, their weak mechanical resistance characteristics, the presence of shallow aquifers, steep slope and basal erosion and undercutting of the Iazoughen and Aboud rivers torrents. The inclinometers measurements and ERT imaging reveal a complex, deep-seated and rapidly moving landslide whose failure surfaces located at the depth of 11 to 28 m and an average velocity of 1 to 28 cm yr-1 with a displacement in the Northwest direction toward the Aboud and Iazoughen rivers. The entire slip surface is located along the geotechnical interface between the flyschs bedrock and the overlying scree. This comprehensive study provides a useful information of rainfall-induced landslides as well as guidance for landslide hazard mitigation and prevention.