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.

Background: Pumped-storage hydropower (PSH) plants remain a cornerstone of grid stability and renewable-energy integration in Europe, yet a large share of the European fleet is ageing and was built under design codes that predate modern geotechnical standards. The Dychów pumped-storage power plant (ESP Dychów, 88 MW, Lubusz Voivodeship, western Poland), in service since the 1930s, is classified as part of the national critical energy infrastructure. Its earthen retaining structures have a documented history of surface mass movements – including the 1997 landslide on the frontal dam, which damaged around 60 m of the crest road and the powerhouse – and require recurrent safety assessment in complex glaciotectonic settings. Methods: Electrical resistivity tomography (ERT) performed in a gradient array was combined with hydrogeological observations and finite-element slope-stability analyses in ZSoil. Two cross-sections were examined – the earthen frontal dam of the upper reservoir and the embankment of the derivation channel feeding the reservoir – each with four calculation variants and two groundwater scenarios. The shear-strength reduction technique was used to obtain the “global” safety factor SF. Results: SF equals 1.75 for the frontal dam and ranges from 1.80 to 2.10 for the channel embankment. Parametric reduction of the internal friction angle of saturated medium sand yields limit values of φ = 12.3° (dam) and φ = 20.3° (embankment), clearly below realistic in-situ values. Conclusions: Both structures comply with the safety threshold F ≥ 1.50 specified for Class I hydraulic engineering structures. The presented non-invasive ERT–FEM workflow offers a cost-effective tool for the periodic reassessment of ageing PSH infrastructure, supporting its continued role in balancing variable renewable generation in decarbonising power systems.