Sliding Tbilisi: Why Georgia’s Capital is Haunted by Multiple Landslides

The capital city of Georgia, Tbilisi, is located in the Southern Caucasus and reaches from the valley of Mtkvari river (~350–420 masl) to the mountain ranges of the Lesser Caucasus (up to 1500 masl). Tbilisi is home to more than 30% of Georgia’s population and due to the city’s growth over the last years the city is reaching out more and more to the surrounding slopes. Landslides occur on a regular basis in the city’s territory and are often, but not exclusively, associated with construction activity. Processes range from rather frequent minor rock and soil slides to serious slope failures and even large-scale compound landslides, that can involve mass movements of various types and even flash-flood processes like in the disaster of 2015. In many cases the geological setting is similar. The municipality of Tbilisi is located in folded Paleogene and Neogene sedimentary formations of varying composition and strength, with a predominance of alternating sandstone-claystone layers of Eocene–Oligocene age with often unfavourable bedding orientation parallel to the slope inclination. Failures and sliding are often triggered or associated with undercutting of the slope toes, but can also occur during high precipitation events or due to ongoing weathering or earthquake shaking. Mitigation of these landslides requires detailed and profound geo- and hydrological investigation, research of triggers and thoughtful planning. Individual solutions have to be developed. The effects of earthquakes in this seismically highly active region require special attention during design, often making stability calculations a challenge for geologists and engineers. During construction of mitigation measures close monitoring and supervision and the implementation of the “observation method” according to European Codes (EC7) are key issues to successfully handle these instable and hazardous slopes and to prevent landslides. In this contribution, we will give an overview of geology and landslide susceptibility of the Tbilisi region from our experience and provide examples of textbook-like landslides, their reconnaissance and mitigation.