Slope Stability Evaluation of Earthen Hydraulic Structures at the Dychów Pumped-Storage Power Plant by Electrical Resistivity Tomography and Finite-Element Modelling

Pumped-storage hydropower (PSH) remains the main grid-scale energy storage technology in Europe, yet much of the fleet is ageing and requires periodic verification against current geotechnical standards. The Dychów plant (88 MW, western Poland), in service since the 1930s and classified as national critical energy infrastructure, has a documented history of surface mass movements, including a 1997 landslide on the frontal dam. To reassess its condition, two earthen sections were analysed: lateral section of the frontal dam of the upper reservoir and the embankment of the derivation channel. Electrical resistivity tomography (ERT) profiles, measured using a gradient array in 2023 and further detailed in 2024 along the same GNSS-fixed lines, imaged the internal structure of both sections. The resistivity cross-sections, verified against shallow control boreholes and archival geological data, supplied the geometry of the finite-element (FEM) models in ZSoil: the confirmed layer boundaries became the material zones, and piezometric observations set the groundwater boundary conditions. The safety factor SF was then computed with the shear-strength reduction technique for four calculation variants and two groundwater scenarios per section. The resulting SF equals 1.75 for the side section of the frontal dam area and ranges from 1.80 to 2.10 for the channel embankment. A parametric reduction in the friction angle of saturated medium sand gives limit values of φ = 12.3° (dam) and φ = 20.3° (embankment), which are clearly below realistic in situ values. Overall, both structures meet the SF ≥ 1.50 requirement for Class I hydraulic structures. The ERT-to-FEM workflow offers a non-invasive and repeatable tool for the periodic reassessment of ageing PSH infrastructure, which continues to balance variable renewable generation.