Influence of geological and hydrogeological settings in assessing the stability of a landslide slope in the Tutrakan area, Northeastern Bulgaria

The study is focused on the assessment of the stability of the lower part of the right slope of the Danube River. The research area is situated in the town of Tutrakan, Northeastern Bulgaria. The present study investigates the likelihood of activating landslide processes due to construction works on the water supply and sewerage network of the town, as well as the influence of the natural geological and hydrogeological settings of the region. The research area is part of the Danubian Plain. The groundwater level lies between 7 and 26 m below the surface, in the sandy sediments and limestones of the Pliocene that form the semi-confined to unconfined aquifer. They are mainly formed by infiltration of precipitation through the loess layer. The groundwater discharge is carried out naturally and technogenically towards the Danube, through a herringbone system of drains built in the toe of the landslide cirques. The assessment of the slope stability was made for the different hypotheses – natural (momentous – including the existing buildings), constructional (including excavations), primary and specific (including seismic forces) combination of stresses. The different geodynamic models, taking into account the influence of groundwater, the influence of different destabilizing factors and seismic impact, show that the slope, as well as the studied sections of it – the landslide cirques, are in a stabilized state, with safety coefficients (Fs > 1). According to the mechanism of deformation, the landslides are classified as a translational slide, and deep seated according to the depth of the slip zone. They are contemporary and currently active. The study confirms the hypothesis of the origin of the landslide masses in the area, formed as a mixture of loess material and Pliocene clays. The groundwater level is directly related to the current state of the landslide slope stability.