Spatial distribution, controlling factors and failure mechanisms of the large-scale landslides in the urban area of Azazga city (northern Algeria)

The city of Azazga is facing extensive and damaging landslide hazard. The aim of this study was to investigate the inventory mapping, the deformation characteristics, the controlling factors and the failure mechanisms of the landslides in the urban area of Azazga based on field surveys, aerial photographs/satellite images interpretation and the exploitation of available data including rainfall database, boreholes, piezometers, inclinometers and laboratory tests. The prepared landslide inventory map indicates that the unstable urban perimeter covers an area of 281.6 ha which represents about 31% of the urban area. Analysis of the inventory data highlighted the action of two types of conditioning factors: (i) susceptibility factors represented by the presence of clay, marl deposits with low mechanical resistance characteristics, the presence of shallow aquifers, the morphology and steep slope, the tectonic features network and a dense hydrographic network; (ii) the triggering factors corresponds to the high intensity rainfall and the uncontrolled human activity. Statistical analysis of the relationship between the landslide occurrence and the landslide-conditioning factors shows that the highest density of landslides occur in quaternary scree and cretaceous clay flysch located on slopes ranging between 5° to 20°. The statistical results indicate that the highest concentration of landslides occurred in the highest rainfall classes ranging between 950 to 1050 mm. Additionally, the results showed the important role of the fluvial incision with more of the landslides occur at a distance of 50 m from the nearest rivers. These data also indicated that the high density of the landslides is concentrated in the farming and urban zones. The inclinometer measurement results reveal a deep rupture surfaces between 4 and 28 m located at the scree–flysch bedrock interface. According to the mechanism modes, landslides are controlled by the following: (i) the dip of the flysch formation layers, the schistosity planes and fractures downward slope direction; and (iii) the interface contact between the quaternary scree and flysch substratum. This work presents an exhaustive understanding of the causes, deformation characteristics and failure mechanisms of the large-scale landslides as a useful information for landslide hazard and risk mitigation.