Beyond debuttressing: Thermo-hydro-mechanical rock slope damage during glacial cycles

Abstract

Cycles of glaciation drive in-situ stress changes in underlying bedrock as glaciers advance, erode, and retreat, generating damage in adjacent rock slopes and influencing paraglacial slope stability. Glacial debuttressing is frequently implicated as a trigger for paraglacial rock slope failures, despite commonly observed large lag-times between deglaciation and the timing of failure and often without clear mechanical reasoning. Rock slope damage generated during glacial cycles is hypothesized to have a strong role in preparing rock slope failures, however, the mechanics of paraglacial rock slope damage remain poorly characterized. Glacial cycles mechanically load and unload proximal rock slopes by the changing weight of ice, and in addition produce strongly varying thermal and hydraulic rock-surface boundary conditions tied to the fluctuating glacier. Bedrock beneath temperate glacier ice maintains near isothermal surface temperatures at ~0 °C. Glacier retreat exposes rock walls to new thermal boundary conditions with strongly varying daily and seasonal cycles, a transition we term paraglacial thermal shock. Temperature changes generate thermal strain, inducing thermo-mechanical stresses capable of generating rock mass damage. In addition, high subglacial water pressures near the ice overburden level prevail at the base of temperate glaciers, and affect groundwater conditions in proximal valley flanks. Groundwater recharge by precipitation and snowmelt raises the water table seasonally, which is superposed on changes in hillslope groundwater tied to varying glacial ice elevations. Changing cleft water pressures control effective stresses and the strength of rock mass discontinuities. Together, these thermo-hydro- mechanical stresses act in concert with glacial loading cycles to generate rock slope damage, preparing slopes for future failure. We study thermo-hydro-mechanical induced stresses and resulting rock slope damage during repeat glacial cycles in the valley of the Great Aletsch Glacier in Switzerland. Following Lateglacial deglaciation, the surrounding valley rock slopes in the Aletsch region experienced several minor glacier cycles during the Holocene. The foliated gneissic rock mass of the Aletsch valley contains several large rock slope instabilities with a concentration around the retreating, present-day glacier tongue. Surface exposure dating of the Driest instability head scarp reveals a Mid-Holocene initialization age (7.4 ± 0.7 ky), matching post-Egesen / pre-Little Ice Age relative ages for the majority of other rock instabilities in the Aletsch Valley. To investigate progressive rock slope damage induced during glacier cycles, we used detailed, conceptual numerical models closely based on our Aletsch Valley study area. Modeled glacier scenarios represent mapped ice fluctuations at Aletsch, while rock mass strength parameters applied in our models are based on local rock mass characterization. Ground surface temperature measurements, monitoring of subglacial water pressures in ice boreholes, regional spring-line mapping, and monitoring of rock slope deformation at Aletsch each contribute to parameterizing and validating our thermal and hydraulic model boundary conditions. Our simulations reveal that purely mechanical loading and unloading of rock slopes by ice during glacial cycles generates relatively limited new damage. This result supports our view that glaciers make a poor buttress for adjacent slopes due to the ductile behavior of ice over long time scales. However, ice fluctuations in our models do increase the criticality of fractures in adjacent slopes (bringing them closer to the failure envelope), which may in turn increase the efficacy of additional fatigue processes. On the other hand, bedrock erosion during glaciation (i.e., rock debuttressing) promotes significant new rock slope damage during first deglaciation. The amount of initial damage, inherited from pre-glacial, ice-free topographic and in-situ stress conditions, strongly controls the susceptibility of the slope to new damage from ice loading. The slope response during glacial cycles is path-dependent and varies in damage kinematics: glacier advance in our models enhances toppling failure while glacial retreat promotes sliding. Changing thermal boundary conditions during glacier retreat and advance in our models affects the temperature regime in the adjacent rock slopes. Thermal strain from long-term temperature changes induces stresses at depths exceeding 100 m, generating significantly more rock slope damage than predicted for purely mechanical loading cycles. Thermal expansion of the rock mass due to warming after glacier retreat causes increased stresses propagating fractures, while cooling during glacier advance results in contraction, reducing joint normal stresses and promoting toppling. First time exposure to seasonal temperature cycles during deglaciation induces a strong but shallow damage front that follows the retreating ice margin. Glacial loading cycles in parallel with thermal stresses (i.e., thermo-mechanical fatigue) are capable of generating significant rock slope damage. We extend our models by accounting for changing groundwater conditions in proximal valley rock slopes tied to high subglacial water pressures. Glacier loading cycles in parallel with long-term mountain water table variations generate substantial fracture propagation. Major damage occurs during initial ice occupation and first glacier retreat, while subsequent readvances result in minor damage. Superposition of annual groundwater cycles (i.e., hydro-mechanical fatigue) strongly increases rock slope damage during glacial loading cycles, destabilizing the toppling-mode valley flank in our models. The kinematics and dimensions of the predicted instability closely resemble observed characteristics of major landslides in the field at Aletsch. Our results extend simplified assumptions of glacial debuttressing, demonstrating in detail how thermo-hydro-mechanical stresses acting in concert with glacier cycles drive progressive rock mass failure preparing future paraglacial slope instabilities.