Abstract
AbstractIn deglaciating environments, rock mass weakening and potential formation of rock slope instabilities is driven by long‐term and seasonal changes in thermal‐ and hydraulic‐ boundary conditions, combined with unloading due to ice melting. However, in‐situ observations are rare. In this study, we present new monitoring data from three highly instrumented boreholes, and numerical simulations to investigate rock slope temperature evolution and micrometer‐scale deformation during deglaciation. Our results show that the subsurface temperatures are adjusting to a new, warmer surface temperature following ice retreat. Heat conduction is identified as the dominant heat transfer process at sites with intact rock. Observed non‐conductive processes are related to groundwater exchange with cold subglacial water, snowmelt infiltration, or creek water infiltration. Our strain data shows that annual surface temperature cycles cause thermoelastic deformation that dominate the strain signals in the shallow thermally active layer at our stable rock slope locations. At deeper sensors, reversible strain signals correlating with pore pressure fluctuations dominate. Irreversible deformation, which we relate with progressive rock mass damage, occurs as short‐term (hours to weeks) strain events and as slower, continuous strain trends. The majority of the short‐term irreversible strain events coincides with precipitation events or pore pressure changes. Longer‐term trends in the strain time series and a minority of short‐term strain events cannot directly be related to any of the investigated drivers. We propose that the observed increased damage accumulation close to the glacier margin can significantly contribute to the long‐term formation of paraglacial rock slope instabilities during multiple glacial cycles.
Highlights
Retreating and advancing valley glaciers in alpine regions can induce both reversible and irreversible deformations in adjacent valley flanks
We expand our finite element models to compute the expected thermoelastic deformation resulting from annual temperature cycles. These analyses provide crucial information regarding the relative importance of thermo-mechanical deformation on the total strain recorded at our instrumented rock slope, and its relevance for the overall observed deformation and progressive rock mass damage
Detailed analysis and decomposition of the subsurface deformation signals allow to identify various potential drivers for reversible and irreversible deformation. This knowledge is important for understanding the main processes contributing to short- and long-term progressive rock mass damage that potentially lead to the formation of paraglacial rock slope instabilities
Summary
Retreating and advancing valley glaciers in alpine regions can induce both reversible and irreversible deformations in adjacent valley flanks. The present work is focused on field investigations of the transient thermal regime induced by glacier retreat in adjacent valley slopes, and the related subsurface deformations This data set is key to the understanding of heat transfer mechanisms in the subsurface, the timescales over which valley flanks adjust to the new thermal boundary conditions following deglaciation, as well as the magnitudes of thermo-mechanical stresses, which can lead to long term rock mass damage. Investigating these phenomena requires high resolution measurements of subsurface temperatures and deformation, which have never been recorded in alpine regions that feature temperate valley glaciers. Thermo-mechanical deformation signals can be overprinted e.g. by hydromechanical deformation (cf. Grämiger et al, 2020), and disentangling the two requires an understanding of slope hydrology, as well as measurements of pore pressure and precipitation events
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