Abstract
Most natural rock slope failures result from long-term strength degradation. It is often hypothesized that glacial retreat leads to enhanced progressive damage accumulation in adjacent rock slopes, due to mechanical unloading and changes to the thermal and hydraulic boundary conditions. However, direct observations of subsurface processes in a rock slope subject to glacial retreat are rare. In this paper, we present the design, implementation, and performance of a new borehole monitoring system installed on a rock slope in fractured crystalline rock located beside the glacier tongue of the retreating Great Aletsch Glacier (Valais, Switzerland). The three vertical, 50 m deep boreholes were instrumented to continuously monitor groundwater pressure, temperature and deformation at high resolution, in order to investigate thermo-hydro-mechanical coupled slope processes that drive progressive rock mass damage. We show that the system is capable of measuring both reversible and irreversible displacements along single fractures at magnitudes ranging between 0.001 mm to >2 mm in the studied rock slope, and that it is often possible to identify drivers of these deformation signals. The transient subsurface temperature field shows clear indications of former ice occupation in form of cold temperatures preserved at depth and superimposed annual temperature cycles penetrating down to a depth of about 17 m. The variability of the pressure head in the slope is driven by annual snowmelt infiltration cycles, rainfall events, and the assumed connection to englacial water of the temperate glacier. The new and continuously growing dataset presented here will enable us to relate the changing boundary conditions caused by glacial retreat and fatigue from daily to annual thermal and hydraulic loading cycles to progressive rock mass weakening, which may ultimately result in rock slope failure.
Highlights
Retreating glaciers induce progressive damage in rock slopes, leading to long-term strength degradation, and rock slope failure (Augustinus, 1995; Ballantyne, 2002; McColl, 2012)
The new and continuously growing dataset presented here will enable us to relate the changing boundary conditions caused by glacial retreat and fatigue from daily to annual thermal and hydraulic loading cycles to progressive rock mass weakening, which may result in rock slope failure
The monitoring system should enable quantification of displacements at the borehole and fracture scale caused by permanent changes in thermal and hydraulic boundary conditions due to ice retreat and cyclic thermo-hydro-mechanical loads originating from daily and seasonal fluctuations in the rock slope and the ice body
Summary
Retreating glaciers induce progressive damage in rock slopes, leading to long-term strength degradation, and rock slope failure (Augustinus, 1995; Ballantyne, 2002; McColl, 2012). Accurate in-situ monitoring dedicated to the study of subsurface thermo-hydro-mechanical processes driving damage in rock slopes in the context of ice retreat remains absent. Quantification of these thermo-hydro-mechanical coupled processes and their potential to induce damage is critical to understand the long-term evolution of initially stable paraglacial rock slopes that can potentially turn into active slope instabilities. The strength of an initially stable slope degrades through time due to stress perturbations caused by different fatigue processes that drive progressive damage. Once a slope is weakened enough, failure can be triggered by a discrete event such as heavy rainfall or an earthquake (e.g., Preisig et al, 2016)
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