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

Abstract

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, the deformation characteristics, the controlling factors, and the failure mechanisms of this landslide based on the field investigation, aerial photographs and 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 km2. This landslide is considered as a partial reactivation of a large pre-existing one (0.606 km2), which represents 6.65% of the total urban area. Moreover, the analyses identified two types of causative factors: (1) the triggering factor related to the high intensity and antecedent rainfall as well as human activity through slope excavations and embankments; and (2) the 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, and basal undercutting erosion of the Iazoughen and Aboud river torrents. The inclinometer measurements and ERT imaging reveal a complex, deep-seated and rapidly moving landslide whose failure surfaces are located at a depth of 11–29 m with an average velocity of 1–29 cm year−1. The entire slip surface is located along the geotechnical interface between the flysch bedrock and the overlying scree. This comprehensive study provides useful information on rainfall-induced landslides and may constitute guidance for landslide hazard mitigation and prevention.