Abstract
Thermo‐cryogenic processes prepare and trigger rockfalls and rockslides in alpine environments. Temporal occurrence, controls, and applied stresses of Thermo‐cryogenic processes on rock ...masses are poorly understood. This paper reports annual crackmeter measurements with 3 h resolution across perennially ice‐filled fractures in an unstable rock permafrost crestline. Thermo‐cryogenic processes are controlled by snow cover onset and duration. Thermal changes in snow‐free periods control expansion and contraction coincident temperature gradients on a daily to seasonal scale. We can show how snow cover promotes sustained temperatures from −9 to −1°C and boosts ice segregation‐related fracture opening up to 1 cm in 8 months. During snowmelt, meltwater induces ice erosion and ice relaxation, which occur in the freeze‐thaw window close to the thawing point. We hypothesize that Thermo‐cryogenic processes and their cyclic repetition can lead to Thermo‐cryogenic fatigue preparing rock slope failure and can control type and location of rockfalls in a changing climate.
Key Points
Thermo‐cryogenic processes preparing and triggering rockfalls are controlled by snow cover
Thermal changes control temperature‐gradient‐dependent rockwall expansion in snow‐free periods
Snow cover boosts ice segregation‐related fracture opening
Degradation of permafrost rock wall decreases stability and can initiate rock slope instability of all magnitudes. Rock instability is controlled by the balance of shear forces and shear resistances. ...The sensitivity of slope stability to warming results from a complex interplay of shear forces and resistances. Conductive, convective and advective heat transport processes act to warm, degrade and thaw permafrost in rock walls. On a seasonal scale, snow cover changes are a poorly understood key control of the timing and extent of thawing and permafrost degradation. We identified two potential critical time windows where shear forces might exceed shear resistances of the rock. In early summer combined hydrostatic and cryostatic pressure can cause a peak in shear force exceeding high frozen shear resistance and in autumn fast increasing shear forces can exceed slower increasing shear resistance. On a multiannual system scale, shear resistances change from predominantly rock-mechanically to ice-mechanically controlled. Progressive rock bridge failure results in an increase of sensitivity to warming. Climate change alters snow cover and duration and, hereby, thermal and mechanical processes in the rock wall. Amplified thawing of permafrost will result in higher rock slope instability and rock fall activity. We present a holistic conceptual approach connecting thermal and mechanical processes, validate parts of the model with geophysical and kinematic data and develop future scenarios to enhance understanding on system scale.
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•We present a conceptual approach to explain instability in permafrost rockwalls.•The conceptual approach is validated with geophysical and kinematic data.•Two time windows of critical instability are identified on a seasonal scale.•Future scenarios enhance system understanding on system scale.
The proportional contribution of low-, mid- and high magnitude rock slope failure orchestrates rock slope erosion and rockfall hazard in Alpine and Arctic environments. In this study, we compare ...sediment yield, geomorphic work and rock wall retreat of carbonate dissolution and five different magnitudes of rock slope failure in the steep Alpine Reintal trough valley. We combine a four-year rockfall collector measurement of 140Mg of fragmental rockfall, a 20th century scientific record of mid-magnitude rockfall as well as carbon-dating and a historical record of 15th to 19th century rock avalanche activity. The total rockfall sediment yield of 8.6 (±3.4)∗103m3year−1 is dominated by high-magnitude rock avalanches (>106m3: 62%) and low-magnitude debris falls (<10m3: 18%), while mid-magnitude boulder, block and cliff falls are less important. The magnitude signal contradicts studies on siliceous rocks where mid-magnitude rockfall dominates rock slope erosion. The geomorphic work released in the 17.3km2 large catchment by rockfalls (123 (±47) Wkm−2, i.e. 0.38 (±0.15) mmyear−1) and solute transport (34 (±18) Wkm−2, i.e. 0.05 (±0.03) mmyear−1) exceeds previously published Alpine values by one to multiple orders of magnitude. We hypothesise that the magnitude signal of enhanced small and high magnitude rockfall is characteristic for carbonate cliffs. The elevated porosity-related susceptibility to fragmentation and persistent carbonate dissolution along potential sliding planes are likely to favour both low- and high-magnitude rock slope failures. Here we show how the magnitude signal of rock slope failure influences rock wall retreat, geomorphic work, rockfall deposition and sediment connectivity.
► We analyse all magnitudes of rock slope failure in a steep Alpine trough valley. ► Their geomorphic work exceeds previous Alpine values by ≥1 orders of magnitude. ► Small (<10m3) and high-magnitude (>1 mio. m3) rockfalls dominate rock slope erosion. ► This could be influenced by multiple-scale solution processes in carbonate cliffs. ► The magnitude signal modulates cliff retreat, deposition and sediment connectivity.
Stratified talus deposits are reported from many different mountain environments. Numerous possible explanations are discussed in the literature; however, the sediment stores are rarely accessible as ...exposures are sparse. We applied ground-penetrating radar (25, 50 and 100 MHz antennae) to gain insight into the internal sediment structures of 23 alpine scree slopes; ten examples are presented in this paper. The study areas are spread over the Eastern European Alps at altitudes ranging from 1500 to 2900 m. The bedrock type is primarily limestone and dolostone; one area is composed of gneiss and mica–schist.
GPR turned out to be highly suitable for investigating sediment structures of dry talus debris. The results showed that almost all of the deposits investigated are characterized by pronounced stratification. Several different types of layering were identified. Discordant layers which are restricted to confined parts of the talus are probably related to sediment redistribution processes like surficial debris flows or dry grain flows. These features frequently occur at the uppermost part of the slope caused by overland flow from the adjacent rock face, but may also develop in the downhill part of a talus. One talus in the Reintal area showed surface-parallel, persistent layers of different grain sizes which cannot be explained by any known models. We suggest a novel model of talus development which is driven by climatic fluctuations. In periods of enhanced freeze–thaw activity like the Little Ice Age, the delivery of coarse debris prevails. In warmer climate with a higher frequency of rainstorms, the depletion of finer-grained intermediate stores in less inclined rockwall positions leads to delivery of clasts smaller than 2 cm. The type of layering found within a talus is determined by rockwall parameters like height, steepness, topography and dissection of the rock face. The “storage depletion” model applies to high rockwalls with a considerable volume of intermediate storage.
Our concept of progressive rock slope failures is on the one hand embedded in aggregated subcritical crack growth mechanisms and on the other sensitive to environmental conditions, especially water. ...To anticipate failure dynamics in rock slopes, it is a key requirement to reveal the influence of water on subcritical crack growth mechanisms and material properties. We present experimental data on the time‐dependent deformation of an exemplary rock, Carrara marble. We employed inverted single‐edge notch bending creep tests on large Carrara marble samples to mimic an open joint system with controlled water supply. Constant stress was applied in two steps approaching 22–85% of a previously determined critical baseline stress. Introducing calcite‐saturated water to subcritical stressed samples caused an immediate increase in strain by up to an order of magnitude. Time‐dependent accumulation of inelastic damage at the notch tip occurred in wet and dry samples at all load levels. Subcritical crack growth and the evolution of localized intergranular fractures are enhanced if water is present and readily approach tertiary creep when loaded above 80%. The immediate strain response is attributed to the reduction of surface energy and diffusion of the water into the rock. The resultant more compliant and weaker rheology can even turn the subcritical stress into a critical state. Over time, subcritical and chemically enhanced mechanisms progressively alter especially grain boundaries, which become the key controls of progressive failure in Carrara marble.
Key Points
The introduction of water increases the magnitude and rate of strain in subcritically stressed marble
Brittle and plastic deformations are enhanced in wet samples in comparison to dry samples
Subcritical crack growth mechanisms preferentially exploit microstructural predispositions, especially grain boundaries
Abstract
Hydrostatic pressure is one of the most important but still not fully understood destabilising factors of bedrock slopes. Water presence has often been recorded in major rock failures like ...at Piz Cengalo in 2017 but still its quantification and its effective destabilizing role remain unsolved issues in rockfall forecasting. Intensification of rainstorms due to climate change will enhance hydrostatic pressures in fractured bedrock, which will likely lead to increase in rockfall activity and connected risks for humans and infrastructures. Here we present a hydro-mechanical stability analysis of the Hochvogel summit (2,592 m AA) in the Northern Calcareous Alps. At this site, an imminent high-magnitude rockfall could destabilise up to 260,000 m
3
and is therefore acutely monitored. Displacement measurements on the summit showed daily acceleration following intense precipitation. With the help of direct investigations and laboratory tests from previous studies, we implemented the Hochvogel SE slope and its mechanical parameters in the 2D Universal Distinct Element Code (UDEC). Our model shows that the presence of water columns of 10 m decreases the factor of safety (FoS) on average by 11 % and can increase the max displacement by up to 70 %. When including the effects of cleft weathering in the model, FoS < 1 can be reached. The friction angle of clefts has a key role in this destabilization process. This study provides key elements for interpreting the mechanical behaviour of this imminent rockfall in connection with hydrostatic pressures, helping to improve hazard forecasting at the Hochvogel and at similar sites.
Abstract
Monitoring and early warning systems based on process dynamics gain importance to cope with an increasing number of alpine hazards. The imminent Hochvogel rock slope failure (up to ca. ...260,000 m
3
) is paradigmatic of natural carbonate slope failure dynamics and a benchmark site for developing an effective monitoring and early warning system. The analysis of process dynamics shows constant movement rates (12 mm/a) over the last 3 years but also a response of specific cracks to heavy precipitation events resulting in factor 5 higher movement rates during wet periods. Here, we show valuable lessons learnt during the development of a reliable monitoring system under challenging environmental conditions. The insights into pre-failure slope dynamics acquired at the Hochvogel will help to detect precursors of a final failure and to warn early.
Debris flows are among the most important natural hazards. The Northern Calcareous Alps with their susceptible lithology are especially affected by a double digit number of major hazard events per ...year. It is hypothesised that debris-flow intensity has increased significantly in the last decades in the Northern Calcareous Alps coincident to increased rainstorm frequencies, but yet there is only limited evidence. The Plansee catchment exposes extreme debris-flow activity due to the intensely jointed Upper Triassic Hauptdolomit lithology, being responsible for most of the debris-flow activity in the Northern Calcareous Alps. The debris flows feed into a closed sediment system, the Plansee Lake, where Holocene/Lateglacial sedimentation rates, rates since the late 1940s and recent rates can be inferred accurately. Using aerial photos and field mapping, the temporal and spatial development of eight active debris-flow fans is reconstructed in six time intervals from 1947, 1952, 1971, 1979, 1987, 2000 and 2010 and mean annual debris-flow volumes are calculated. These are compared with mean Holocene/Lateglacial debris-flow volumes derived from the most prominent cone whose contact with the underlying till is revealed by electrical resistivity tomography (ERT). Debris-flow activity there increased by a factor of 10 from 1947–1952 (0.23±0.07·103m3/yr) to 1987–2000 (2.41±0.66·103m3/yr). Mean post-1980 rates from all eight fans exceed pre-1980 rates by a factor of more than three coinciding with enhanced rainstorm activity recorded at meteorological stations in the Northern Calcareous Alps. The frequency of rain storms (def. 35mm/d) has increased in the study area on average by 10% per decade and has nearly doubled since 1921. Recent debris-flow activity is also 2–3 times higher than mean Holocene/Lateglacial rates. The strong correlation between the non-vegetated catchment area and the annual debris-flow volume might indicate a decadal positive feedback between enhanced rainstorm activity and debris flows. Here we investigate the temporal and spatial development of debris-flow fans to better understand the sensitivity of alpine catchments to heavy rainfall events in the context of climate change.
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•Temporal and spatial development of eight debris-flow fans for the last 60years•Estimations for the mean Holocene/Lateglacial sediment volume by a 400m long ERT•A higher activity in the last 2–3 decades compared to the Holocene/Lateglacial mean•Relation of this enhanced debris-flow activity to enhanced heavy rainstorm frequency
Knowledge of the internal state of rock is key to anticipate its rheological response and susceptibility to external factors. Time‐dependent failure in rock is controlled by internal state changes, ...like damage accumulation or strength degradation. But assessing internal states and changes thereof, nondestructively and independent of external forcing is not straightforward. Residual strains, measured with neutron diffraction techniques are used as a proxy for the internal state in material sciences. We investigated its potential for progressive rock failure by measuring residual strain states of an untested and three mechanically and chemomechanically pretested Carrara marble samples. We collected neutron diffraction data for three crystal lattice planes {10̅14}, {0006}, and {11̅20}. Measurements showed an initial overall contractional spatially homogeneous residual unit cell volume strain state of about −400 μstrain, though magnitudes were strongly partitioned among measured crystal lattice planes. However, they are equal within the spatial orientations of the intact sample. For the pretested samples, the induction and relaxation of strains varied spatially with the pretesting stress field and environmental conditions. The vertical extent of superposition of the initial residual strain state was greatest in wet samples, the magnitude of induced extensional strain highest in the dry sample. This indicates chemomechanically enhanced subcritical crack growth with concomitant residual strain relaxation as well as the mitigation of extensional strain built up by the presence of water during pretesting. Our experiments show that residual strain has a significant potential to provide insights into past and actual internal states to anticipate progressive rock failure.
Key Points
Neutron diffraction techniques allow nondestructive assessment of internal strain states of rocks
Inherited residual strain state of Carrara marble is overall contractional
Residual strain is indicative of the strength and possible damage state of rocks
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