# Failure Assessment of Embankment Dam Elements: Case Study of the Pirot Reservoir System
reliable and does not fully represent the safety of the object [6]. The concept of global stability of the geotechnical structures is commonly used in the analysis of the safety of complex structures by determining the global safety factor [7; 8; 9; 10]. However, the conclusion that the structure is stable under the considered acting loads does not always mean the structure is safe, especially when it comes to structures such as dams and reservoirs with accompanying elements [11; 12; 13]. For this reason, in addition to the analysis of the global safety factor, it is necessary to conduct analyses of the impact of damage that in the short term does not endanger the stability of the structure, but in the long term can affect its stability. In order to overcome the previously noted shortcomings, authors propose the principle of partial stability assessment by introducing a new local stability measure called remaining load-bearing capacity and implemented in the PAK software package [14], used for the analysis of filtration and strength of geotechnical structures. This new measure provides an overview of zones whose partial stability is potentially compromised, regardless of the fact that the global stability of the facility is not impaired. This is especially important in complex systems such as a multi-purpose reservoir, where a local loss of stability not compromising the global stability of the facility may activate other mechanisms that would compromise the stability of the entire facility. To conduct this analysis, it is necessary to identify the critical (weak) elements of the system and then to analyze their impact on the functionality of the system for hypothetical scenarios and thus on the long-term safety and operation of the system. In this analysis, the finite-element based method [14; 15] was used to simulate the filtration processes and stress-strain behavior of the dam and the surrounding rock mass. The global short-term stability of the structure was analyzed using the strength reduction method [16; 17]. The analysis used the case study of the Zavoj dam and accumulation near the city of Pirot in the Republic of Serbia [18; 19]. The effect of an earthquake, used as a critical load, is modelled by applying the maximum value of seismic acceleration at the dam site in two orthogonal directions, for a 1000 year return period [20]. Occurrence of earthquakes in southeast Europe is causing growing concern to the operators of the large complex reservoir systems, as in the last decade multiple significant earthquakes were recorded: Serbia 2010--5.5 moment magnitude scale (MMS) [21], Albania 2019--6.4 MMS [22], Croatia 2020--6.9 MMS [23], etc. Apart from the apparent risk to the safety of the structures, significant risks related to the reduced functionality of these systems need to be addressed and analyzed in detail. In the case of the Pirot reservoir, three key elements whose damage, induced by earthquake, affects the functionality of the system have been identified. The results of the functionality analysis of the individual elements of this system represent the input data in the analysis of system dynamics whose implementation was carried out by applying the concept of dynamic resilience [24; 25]. The paper is structured in the following manner: first, in the Theoretical Basis section, recapitulation is provided for the equations governing filtration, strength analysis, and shear stress reduction. Additionally, the concept of the remaining load-bearing capacity is introduced and explained in detail. The section About Zavoj Dam covers basic technical data and description of the Zavoj Dam as a part of the Pirot complex reservoir system. It is followed by Section 4, where the basic information about the finite element model is provided, along with the subsections describing identification of the weak points in the structure, used material parameters, loads, and boundary conditions. Section 4 concludes with the description of the three analyzed scenarios.