Seasonal ground temperature variation controls stress regime and rock anchor tension in warming permafrost rock slopes

In recent decades, widespread impacts of permafrost degradation and glacier recession on rock instability have been well-documented in the Alps and other mountain regions worldwide. While these changes have largely been attributed to shifts in resisting forces – such as the stability of rock, ice, and rock-ice interfaces – changes in stress regimes have received less attention, primarily due to the significant challenges in measuring and recording them.

In this contribution we study seasonal and multiannual changes in the stress regime of a warming permafrost rock slope based on a unique five-year dataset (2016-2020) of loads measured at the heads of three grouted anchors at the Kitzsteinhorn, Central Alps, Austria. The studied steel anchors (total length 25 m) are located in a recently deglaciated rock-face below a high-alpine cable car station and are interpreted as extensometers which register stress changes in the surrounding rock mass. Variations in recorded load are interpreted as proxies for deformation along the 18-meter free anchor length, offering valuable insights into the deep subsurface. In this zone, the effects of climate warming are often more discernible due to the reduced influence of strongly fluctuating atmospheric conditions.

In the five-year observation period loads ranged from 350-600 kN at the three studied anchors. A strong seasonal variation of 40-125 kN was observed (high winter loads, low summer loads), which translates to strains of 1.3 to 4.1 mm. Seasonal load increases were found to correlate strongly with negative thermal gradients in the subsurface, which drive cryosuction and ice segregation. Autumn and winter load increases are thus likely associated with the seasonal formation of segregated ice in the active layer, while the summertime load decreases are attributed to the seasonal melt of ground ice.

Small variations of the maximum thickness of the permafrost active layer – measured in a 20 m deep borehole situated just a few meters from the rock anchors – seem to have a critical effect on the observed loads, pointing to ice melt at the base of the active layer as a critical driving force for load (stress) variation. Recorded anchor loads decreased by up to 24 % in warm summers and recovered only partially in the following winters resulting in a gradual, long-term load decline in the observation period.