Seasonal thermo-hydro-mechanical dynamics of permafrost rockwalls revealed by automated electrical resistivity monitoring

Abstract. Permafrost warming in rock slopes and the associated long-term increase in slope instability have been intensively studied in recent years, with most interpretations of electrical resistivity tomography (ERT) focusing on the thermal regime while assuming homogeneous rock conditions. Seasonal forcing by water and ice in fractures has often been neglected, even though hydrostatic and cryostatic processes are increasingly recognised as key mechanical drivers in the preparation and initiation of permafrost rock instabilities. In contrast to previous studies, we applied automated ERT monitoring to decipher temporary phases of massive hydrostatic water injection into previously frozen joints and the development of cryostatic pressures related to ice formation processes. ERT monitoring was performed at the north face of the Kitzsteinhorn (Hohe Tauern range, Austria) year-round from April 2024 to April 2025. These measurements integrated reciprocal error estimation and a resistivity–temperature relation calibrated using in-situ borehole temperature data and laboratory experiments. The ERT data set was combined with observations of the rockwall's hydro-mechanical response derived from load cells of two 25 m-long anchors and from piezometric measurements at 16.85 m depth. We identified five characteristic phases of seasonal forcing on permafrost rockwalls, driven by subsurface temperature, snow pack, and piezometric pressure: stable freezing from April–May (phase I), snow melt and subsurface warming from May–July (phase II), maximum active layer thickness from July–September (phase III), superficial cooling from September–November (phase IV), and deep freezing from November-April (phase V). Among these identified phases, two emerged as potentially preparing rock slope destabilisation and were temporally constrained using the ERT data. During peak meltwater infiltration from May to July (phase II), drastic decreases in resistivity from 140 to 9 kΩm and enhanced piezometric levels of up to 1.2 bar indicated high hydrostatic pressures, while simultaneous declines in anchor loads from 576 to 519 kN indicated stress redistribution within the jointed rock mass. A second critical phase was marked by increased resistivity in deeper layers and rising anchor loads during subsurface cooling from November to January (phase V), suggesting the onset of ice formation processes and high cryostatic pressures. Here, we show that temperature-calibrated automated ERT monitoring in high-alpine permafrost rockwalls can offer new insights into the coupled thermo-hydro-mechanical response of rock masses to seasonal forcing, potentially controlling stability.