As a contribution to a focused international-interdisciplinary learning process, permafrost conditions were assessed for the detachment zones of three large recent events. At Rasac arête (Cordillera Huayhuash, Peru), about 1.1 to 1.5 million m3 detached in February 2023 from cold/deep permafrost near 5800 m a.s.l. with mean annual surface temperatures estimated at -5 to -6°C and thermally protected by intact ice aprons. The resulting rock/ice avalanche impacted Lake Rasac but without damage to inhabited areas. The detachment after precursory mobility signs at 3800-3900 m a.s.l. of 1 million m3 of perennially frozen rocks at Glacier Tbilisa (Caucasus, Georgia) on 3 August 2023 caused severe damage and losses of lives at Shovi; it took place at a site where mean annual surface temperatures are estimated at -2 to -4°C. About 15 million m3 from the large perennially frozen left lateral moraine (5300 m a.s.l.) slid into the deep and rapidly growing proglacial South Lhonak Lake (Sikkim Himalaya, India) on 03 October 2023, causing a devastating long-distance down-valley flood with heavy damage and losses of lives in a transboundary situation including Bangladesh. With mean annual surface temperatures of about -1 to -3°C and a maximum slide depth of 88 m, the rupture may well have taken place in warm permafrost or even at its base. In accordance with information from research on viscous creep in mountain permafrost (rock-glaciers), ice content of the failed moraine is estimated at some 60-80%. This excess ice content or ice-supersaturation enabled rapid large-scale creep movements with a strikingly coherent flow field and annual displacement rates up to 15 m in the years before the event. Movements are ongoing, and massive bodies of buried ice have been exposed within the outlet channel of the lake deeply eroded in creeping ice-rich perennially frozen morainic material.

Spitze Stei is one of the largest, failure prone rock slope instabilities in the European Alps. Over the last 5 years, annual displacement rates of more than one metre have been recorded. The slope is located above Kandersteg in the Bernese Alps, Switzerland, in a region affected by several extremely large prehistoric rockfalls. The tourist site Öschinensee below the slope was formed by one of these. The current destabilisation was accompanied by thawing of large parts of the slope. Detailed investigations of slope kinematics, displacement velocities, ground temperatures, geohydrology and geotechnical properties of the rock mass indicate that permafrost may have been the decisive trigger for the reactivation of slope displacements in this area. After the initial destabilisation phase, the slope displacement rates are currently in a labile balance, characterized by a distinct seasonal signal but no clear long-term trend. Hydrological models and tracer tests indicate that the slope contains a large water reservoir. Moreover, the level of this reservoir turned out to be an almost perfect indicator of seasonal velocity changes and thus for the early recognition of critical acceleration events. The transition to such a critical phase could lead to catastrophic failure events. Although potential rock avalanches will not reach settlements, extensive debris flow activity is expected, causing a redeposition of rock mass into the settlement area of Kandersteg.

In the European Alps, thrust moraine complexes are relatively rare due to the comparatively extensive glacial advances during the Little Ice Age (LIA), usually below the local lower permafrost boundary. Due to the complex formation, which may also be partly attributable to multiple glacial advances, permafrost-related ground ice, but also large amounts of sedimentary ice of glacial origin are often incorporated into these moraine complexes. The high amounts of ice within their internal structures make them especially sensitive to external changes, particularly climate warming. High ice contents also enable permafrost creep and internal deformation of the moraine complexes resulting in distinct morphodynamics. General spatial patterns and their temporal variations, but also their relationships to subsurface structures are key components to understand their origin and the future development of these landforms.

In the current research project, we investigate the internal structures of several thrust moraine complexes in the Swiss Alps using geophysical methods. The use of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) enables the detection of massive ice bodies and differentiation between ice of glacial and periglacial origin (sedimentary and magmatic ice) in the subsurface of such landforms. Additionally, Differential SAR Interferometry (DInSAR) based on Sentinel-1 data provides information about recent morphodynamics. A comparison of the geophysical and remote-sensing data should help to understand the linkages between the subsurface structures and the surface dynamics of these landforms. Besides, the coexistence of thrust moraines and active rock glaciers at the study sites enables the comparison of morphodynamic similarities or differences between both landform types.

Recent studies have brought upon numerous evidence for enhanced rock slope failure from degrading permafrost rock walls. These failures have been thought to be subaerial and triggered by thermal heat propagation from rising air temperatures into the exposed rock faces. However, we have neglected that, at the same time, the dividing line between cold and warm basal states of polythermal glaciers has shifted some hundreds of meters upwards. This means that previously frozen and ice-filled fragmented rock walls under cold glaciers have suddenly and for the first time in thousands of years been exposed to (i) hydrostatic pressures, (ii) warming and degrading ice in fractures, and (iii) rock mechanical degradation in warming rocks. One of the best case studies is the 3.9 to 4.3 million m³ rock slide at Bliggspitze on 29~June 2007, which detached from a north-exposed, glacier-covered rock slope at 3200 m above sea level. In this paper, we hypothesize that the transition from cold- to warm-based glaciers, a scarcely observed but widespread phenomenon, caused the massive rock slide. Through intensive analysis of glacier/permafrost evolution and rock mechanical modeling, we demonstrate a new type of rock slope failure mechanism triggered by the uplift of the cold/warm dividing line in polythermal alpine glaciers, a widespread and currently underexplored phenomenon in alpine environments worldwide.

We present here a paper submitted recently, in which we investigate the Eyjafirði talus slope (Tröllaskagi peninsula, Iceland). This talus is located outside of the climatic boundaries of permafrost. However, a landslide (06/10/2020) originated from the Eyjafirði talus, and shows evidence of presence of ground ice in the source material. Indeed, specific air circulation (the “chimney effect”) can occur in talus, enhancing the persistence of intra-talus permafrost. Hence, the thermal dynamics of talus is currently poorly understood.

Therefore, we use the software FEFLOW to investigate the permafrost dynamics within the Eyjafirði talus slope, from – 20,000 years to present. In our case, FEFLOW uses the finite element method to solve equations of heat transfer within a two-dimensional cross-section of the Eyjafirði talus slope. The thermal boundary conditions of our models are obtained from field temperature measurements acquired in 2021-2022. We test the sensitivity of our model to the initial porosity/ice content of the talus (0.3, 0.5, 0.8), and to the thermal conductivity of the rock phase of the talus and bedrock (0.75, 1.1, 1.75 W.m-1.K-1).

Temperature measurements show that a chimney effect occurs within the Eyjafirði talus. In our approach, we do not explicitly model the air convection; however, permafrost persist at the base of the talus slope in all modelling scenarios. Increasing the initial porosity/ice content and decreasing the thermal conductivity of the rock phase enhances the persistence of permafrost.

Our modelling approach is unconventional: we initially know that ice was present in the Eyjafirði talus slope at the time of the landslide, which enables additional interpretations of our modelling results. Thus, we can attest that the permafrost dynamics in the talus must be closer to our most ice-conservative scenario – with a thermal conductivity of the rock phase of 0.75 W.m-1.K-1 and an initial porosity/ice content of 0.8.

In recent years, large-scale rock slope failures in permafrost regions have been documented, notably at Fluchthorn (Tyrol, 2023) and Piz Scerscen (Graubünden, 2024). Nonlinear fracture mechanics and discontinuum modelling have proven to be promising approaches for simulat-ing these large-scale failures. However, the latter models are not calibrated for the recently observed magnitudes.

This study introduces novel data to extend the Mohr-Coulomb failure criterion for rock-ice failures at high loads. Moreover, we define the brittle-ductile failure transition in permafrost rocks as a function of overload and temperature. Consequently, we propose a law for the ice creep and fracture transition, dependent on temperature, stress, and deformation rate, for permafrost rocks.

To investigate rock-ice failure mechanics, more than 100 shear experiments were conducted at high normal stresses, simulating rock overburden of up to 65 m (1600 kPa). The tests were performed at temperatures ranging from -0.5°C to -10°C, with strain rates consistently maintained at 10⁻³ s⁻¹.

The experiments show that the ductile behavior of ice is primarily temperature-dependent, while normal stress plays a subordinate role. Ductile deformation occurs at temperatures from -1°C to -0.5°C. At colder temperatures, brittle behavior dominates, with two characteristic modes: stick-slip sliding occurs at normal stresses above 400-800 kPa, while single brittle fracture prevails at lower stresses. Ductile material behavior is outside the scope of the Mohr-Coulomb fracture criterion. The integration of this constraint into a conceptual transient thermal model emphasizes the spatially limited applicability of mechanical models to simulate rock slope destabilization.

This study refines the Mohr-Coulomb failure criterion for ice-filled rock fractures by integrating high-load mechanisms and defining the brittle-ductile transition as a function of overload and temperature, offering critical insights for enhancing mechanical models of large-scale permafrost rock slope failures.