GNSS-BASED DEFORMATION MONITORING OF EARTHFILL DAMS: METHODOLOGY AND APPLICATION AT THE TOPALANG DAM, UZBEKISTAN

ABSTRACT. Embankment dams undergo continuous, load-dependent deformation driven by reservoir level fluctuations, seepage, and temperature changes, yet classical geodetic surveying methods capture this behaviour only through periodic, discrete campaigns. This limits their ability to reflect the real-time dynamics of deformation processes and to provide timely warning of hazardous trends. This paper presents the methodology of continuous Global Navigation Satellite System (GNSS) deformation monitoring — covering the positioning principle, coordinate-difference and displacement-vector computation, atmospheric and multipath error mitigation, statistical reliability criteria, and velocity/acceleration-based hazard classification — and demonstrates its application to the permanent GNSS monitoring network installed at the Topalang earthfill dam (Surkhandarya region, Uzbekistan). The system, commissioned in 2024 by "Gidrotexmonitoring" LLC in cooperation with CHC Navigation, comprises ten multi-GNSS (GPS + GLONASS + BeiDou + Galileo) stations achieving ±2 mm horizontal and ±4 mm vertical accuracy in continuous static mode. Monitoring results show a consistent downstream displacement azimuth of 170–190° at all crest stations, confirming hydrostatic pressure as the dominant driving factor, with station GNSS-3 recording a horizontal displacement of 28 mm — approaching the 30 mm threshold recommended by the International Commission on Large Dams (ICOLD). These findings, compared with international case studies (Enguri, Sanalona, and Chinese reservoir dams), confirm that continuous multi-GNSS monitoring provides a reliable, complementary tool for the real-time safety assessment of embankment dams.