FAQ: Influence of permafrost on rock slope failure

The changes in Alpine permafrost are mostly invisible but have increasingly significant consequences for mountain regions. The decreasing stability of permafrost slopes affects high mountain infrastructures and leads to an increase in natural hazards such as rockfalls and debris flows. Here we summarize some basic information about permafrost in the Swiss Alps and its potential role in major rock slope instabilities. Answers to the most important questions.

What is permafrost and where does it occur in the Swiss Alps?

Permafrost refers to soil, rock and sediments in which the temperature never rises above zero degrees Celsius for at least two years (generally for much longer periods). In the Swiss Alps, permafrost is found in scree slopes above about 2200 m.a.s.l. and in rock walls above about 3000 m.a.s.l. On the shady side of the mountains, the permafrost boundary is a few hundred meters lower than on strongly insolated southern slopes. It extends to a depth of several tens of meters, and on the highest peaks up to several hundred meters. In Switzerland, permafrost occurs on around three percent of the country’s surface. Measured annual average permafrost temperatures range from as low as minus ten degrees Celsius in the highest north-facing locations to only a few degrees below zero near the permafrost boundary. Most of the permafrost in the Swiss Alps is warm, i.e. its temperature is only one to two degrees Celsius below the melting point of ice. A simple overview of the permafrost distribution can be obtained using the SLF permafrost map (PGIM) at maps.wsl.ch. In fractures and in the microscopically small pores (cavities) of permafrost rock there is ice that can be thousands of years old. This ice can become visible after a rock slope failure in the detachment zone, but often the small amounts are hardly recognizable.

How has the permafrost in Switzerland changed in the last decades?

Long-term measurements show that permafrost temperatures in Switzerland and worldwide have risen significantly in recent decades. In Switzerland, the measurements date back to 1988 and permafrost monitoring has been coordinated by the Swiss permafrost monitoring network PERMOS for 25 years. The latest evaluations show changes in permafrost ground temperatures of -0.1 to +1.1 degrees Celsius in the in the last decade 2015-2024 at a depth of ten meters. The greatest changes are observed at cold, high-elevation locations and in rocky areas. In contrast to scree slopes, permafrost rock contains little ice and can therefore warm faster. At a depth of twenty meters, the warming is only about half as strong. At even greater depths of fifty meters and more, the warming is still small, as the near-surface changes of recent decades only reach this depth with a long delay. An overview of the changes in permafrost distribution in the top twenty meters of the ground since the 1980s is also shown on maps.wsl.ch.

Which factors lead to a large rock slope failure?

A large rock slope failure (Bergsturz) is generally defined as having a volume of one million cubic meters or more. Whether a large rock slope failure is possible depends on the combined effect of three factors:

1. The topography: is the slope steep enough?
2. The orientation of the weak zones in the slope: in which directions do fractures or rock layers run?
3. The material properties of the mountain: how high are the strength and frictional forces within a rock mass?

If the combination of all three factors basically makes a rock slope failure possible, the stresses in the slope due to gravity will lead to rock fatigue over thousands of years and a rock slope failure will occur at some point. Other external factors can accelerate this fatigue process and influence the timing of a rock slope failure.

What are possible external influencing factors?

There are a large number of possible external influencing factors. Depending on the initial situation, their influence on the development of a rock slope instability can vary greatly. The following is an incomplete selection:

• Glacier erosion (steepening of the rock slope over thousands of years)
• Water infiltration (increase in pressure and temperature, weakening of some rock types due to saturation over decades)
• Glacier retreat (unloading and changes in water pressure over decades)
• Permafrost (warming and ice loss, see section below)
• Thermomechanical effects (seasonal stress changes over thousands of years)
• Earthquakes (dynamic destabilization in seconds)
• Chemical processes (erosion through chemical reactions in the rock)

What is the influence of permafrost on the stability of steep rock slopes?

Changes in the permafrost can influence both near-surface failures such as rock fall, as well as deep-seated slope movements of over one million cubic meters, which can lead to large rock slope failures. Here we focus on the influence of permafrost on the latter.

Permafrost ice influences the hydrology of mountain slopes. In cold permafrost rock is hardly permeable to water, as cleft ice seals the fractures. If the ice warms or thaws slowly, water can penetrate into fractures and cause high pressures. This increases the stresses in the slope and accelerates rock fatigue. This process was observed in the Pizzo Cengalo rock slope failure near Bondo in August 2017 and is currently playing an important role in the case of Spitze Stei above Kandersteg. Some rock types also lose significant strength when they become saturated with water. These effects vary greatly depending on the rock type, fracture distribution and hydrology.

Permafrost ice in fractures and microscopic pores in the rock can also influence rock slope movements. Ice is a plastic material, similar to plasticine. The strength of ice decreases with increasing temperature. In addition, the strength of ice depends on how quickly it is deformed. The faster ice is loaded, the greater its resistance. If it is loaded very slowly, it offers no resistance. Permafrost ice can therefore possibly limit the deformation speed of an existing slope instability and delay a failure by years or even decades.

The extent of this retarding effect also varies greatly from case to case. On Pizzo Cengalo, for example, intact rock structures broke progressively. Compared to their strength, the strength of ice is of little significance (in this case the above-mentioned increasing water permeability was relevant). At Spitze Stei above Kandersteg, it is an labile sliding process on a sliding plane with much higher deformation velocities. The strength of the ice can make a difference here. Monitoring data indicate that ice is limiting the deformation velocity and delaying a failure. This retarding effect diminishes as the permafrost warms or thaws.

Which influencing factors may have influenced the rock slope failure in Blatten?

In the case of the Kleines Nesthorn above Blatten, the rock structure was prone to fail. Glacier erosion over millennia has made the slope steeper than the rock structure allows in the long term. The failure zone is located in permafrost. Measurements at sites in similar locations and the SLF permafrost map (PGIM ) indicate permafrost temperatures of minus three to minus two degrees Celsius and a permafrost thickness of around one hundred meters in detachment zone. The rock slope failure occurred at great depth and permafrost in this area is influenced by long-term changes and not by the weather conditions of recent weeks or months. As everywhere in the Swiss Alps or the mountains of Europe, the permafrost has also warmed on the Kleines Nesthorn in recent decades. It is therefore possible that ice loss and increased water infiltration could have led to higher pressures and additional stresses in the slope. This in turn could have accelerated the failure of the slope. More detailed investigations of the rock structure, permafrost conditions and the destabilization process will follow and may provide clues as to what influence the changes in the permafrost had on the destabilization of the Kleines Nesthorn.