Abstract
Abstract. Understanding rock slope kinematics in steep, fractured bedrock permafrost is a challenging task. Recent laboratory studies have provided enhanced understanding of rock fatigue and fracturing in cold environments but were not successfully confirmed by field studies. This study presents a unique time series of fracture kinematics, rock temperatures and environmental conditions at 3500 m a. s. l. on the steep, strongly fractured Hörnligrat of the Matterhorn (Swiss Alps). Thanks to 8 years of continuous data, the longer-term evolution of fracture kinematics in permafrost can be analyzed with an unprecedented level of detail. Evidence for common trends in spatiotemporal pattern of fracture kinematics could be found: a partly reversible seasonal movement can be observed at all locations, with variable amplitudes. In the wider context of rock slope stability assessment, we propose separating reversible (elastic) components of fracture kinematics, caused by thermoelastic strains, from the irreversible (plastic) component due to other processes. A regression analysis between temperature and fracture displacement shows that all instrumented fractures exhibit reversible displacements that dominate fracture kinematics in winter. Furthermore, removing this reversible component from the observed displacement enables us to quantify the irreversible component. From this, a new metric – termed index of irreversibility – is proposed to quantify relative irreversibility of fracture kinematics. This new index can identify periods when fracture displacements are dominated by irreversible processes. For many sensors, irreversible enhanced fracture displacement is observed in summer and its initiation coincides with the onset of positive rock temperatures. This likely indicates thawing-related processes, such as meltwater percolation into fractures, as a forcing mechanism for irreversible displacements. For a few instrumented fractures, irreversible displacements were found at the onset of the freezing period, suggesting that cryogenic processes act as a driving factor through increasing ice pressure. The proposed analysis provides a tool for investigating and better understanding processes related to irreversible kinematics.
Highlights
On steep, high-alpine mountain slopes, the behavior of frozen rock masses is an important control of slope stability when permafrost warms or thaws and seasonal frost occurs
In this study and based on a new 8year continuous data set of fracture kinematics, we propose and apply a methodology for separating and quantifying such irreversible displacements
The intra- and interannual behavior of the fracture kinematics strongly varies between locations, but patterns at individual locations are consistent over the entire observation period
Summary
High-alpine mountain slopes, the behavior of frozen rock masses is an important control of slope stability when permafrost warms or thaws and seasonal frost occurs. During the summer heat wave of 2003, air temperatures across a large portion of Europe were 3 ◦C higher than the 1961–1990 average (Schär et al, 2004), causing deep thaw and coinciding with exceptional rockfall activity in the European Alps (Gruber et al, 2004). Assuming that warming will continue or even accelerate, rock slope instabilities are expected to become increasingly important for scientists, engineers and inhabitants in the vicinity of high mountain permafrost regions (Gruber and Haeberli, 2007; Keuschnig et al, 2015). Improved monitoring strategies and hazard assessment for the dynamics of frozen rock walls are needed and require better understanding of the processes and factors controlling the stability of potentially hazardous slopes
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