Thermomechanical Stresses Drive Damage of Alpine Valley Rock Walls During Repeat Glacial Cycles

Abstract

Cycles of glaciation alter the temperature field in proximal alpine valley flanks, driving rock slope damage through thermomechanical stresses. Here we extend simplified assumptions of glacial debuttressing to quantitatively examine how paraglacial bedrock temperature changes, acting in concert with changing ice loads during Late Pleistocene and Holocene glacial cycles, create damage in adjacent rock slopes and prepare future slope instabilities. When in contact with temperate glacier ice, valley walls maintain near isothermal ~0 °C surface temperatures and are shielded from daily and annual cycles. With retreat, rock walls are rapidly exposed to strongly varying temperature boundary conditions, a transition we term “paraglacial thermal shock.” Using detailed, conceptual numerical models based on the Aletsch Glacier in Switzerland, we show that including thermomechanical stresses during simulated glacial cycles creates significantly more rock slope damage than predicted for purely mechanical ice loading and unloading. Glacier advances are especially effective in generating damage as rapid cooling drives contraction of the rock mass reducing joint normal stresses. First time exposure to annual temperature cycles during deglaciation induces a shallow damage front that follows the retreating ice margin, generating damage in a complementary process at shorter time scales. Acting on a reduced strength rock mass, modeled thermomechanical cycles enhance the development of a slope instability with similar attributes as observed in our study area. Our results demonstrate that thermomechanical stresses acting in conjunction with changing ice loads are capable of generating considerable rock slope damage with spatial and temporal patterns controlled by glacier extents.

Hydromechanical Rock Slope Damage During Late Pleistocene and Holocene Glacial Cycles in an Alpine Valley

“…We adjusted assumptions in Grämiger et al (2017), assuming that joint set F1 (see Table 2) contains 15% (instead of 10%) rock bridges, set F3 contains 25% (instead of 20%) rock bridges, and intact rock represented by Voronoi contacts contains 100% rock bridges for HM models. The calculated rock strength properties listed in Table 2 resulted in an initial damage field that was similar (albeit not identical) to the studies of Grämiger et al (2017Grämiger et al ( , 2018 also including a similar ratio between shear-and tensile-mode failure.…”

“…Our goal was to begin the simulations with similar initial damage conditions (here quantified as the cumulative length of failed joints) as in our preceding companion studies (Grämiger et al, 2017(Grämiger et al, , 2018 in order to facilitate comparison. Assuming the same rock strength properties while accounting for effective joint stresses, however, resulted in a greater amount of initial damage.…”

“…Stress changes during glacier cycles drive fracture propagation in alpine valley rock slopes (Grämiger et al, 2017(Grämiger et al, , 2018; therefore, it is crucial to explore the influence of mountain groundwater on effective stresses in rock joints (Figure 1). In a glaciated catchment, portions of the groundwater table may be directly linked to the subglacial hydrology and meltwater cycle, in contrast to unglaciated areas.…”

“…In this study, we explore the role of hydromechanical coupled stresses associated with glacial cycles in driving rock slope damage and preparing paraglacial rock slope failures. We expand on the mechanical and thermomechanical studies previously introduced by Grämiger et al (2017Grämiger et al ( , 2018 by including subglacial water pressure, seasonal groundwater fluctuations, and associated effective stress changes in conceptual numerical models closely based on our Aletsch Valley study site in Switzerland. We incorporate new subglacial water pressure data from ice boreholes in the Great Aletsch Glacier, as well as field mapping of springs to inform modeled local groundwater conditions, and use measured seasonal rock slope deformation from continuously operating Global Navigation Satellite Systems (GNSS) stations at the glacier margins to confirm modeled slope displacements.…”

Fracture dynamics in an unstable, deglaciating headwall, Kitzsteinhorn, Austria

“…As glaciers are wasting down due to recent atmospheric warming, the lowest headwall sections are exposed to direct atmospheric forcing. Their formerly constant thermal regime is now disturbed by diurnal to annual temperature cycles, a transition recently termed 'paraglacial thermal shock' (Graemiger et al, 2018). A related long-term terrestrial LiDAR study from the Kitzsteinhorn found dramatically increased rockfall activity in recently deglaciated headwall sections and thus confirmed a pronounced paraglacial response (Hartmeyer et al, in preparation).…”