Rock Wall Stability Research Articles

Assessing the rock failure return period on an unstable Alpine rock wall based on volume-frequency relationships: The Brenva Spur (3916 m a.s.l., Aosta Valley, Italy)

Defining the relationship between volume and return period is critical when estimating the risk of rockfalls and/or rock avalanche, especially during continued global warming at high altitudes that threatens rock wall stability. Characterizing the volume-frequency relationship based on historical datasets is, however, limited by observation and quantification biases, which have not received enough attention. Here, to monitor recent activities for the Brenva Spur (Mont-Blanc massif, Italy) that is also a rock avalanche scar and estimate the return period of future rock failures based on the volume-frequency relationship (and the corresponding uncertainty), a structure-from-motion photogrammetric survey was conducted from 2017 to 2021. 39 rockfall sources with volumes ranging from 11 to 13,250 m3 were identified within the scar. The total failure volume is 22,438 m3, with an associated erosion rate of 15.5 mm/year, indicating very active morphodynamics possibly linked to the permafrost evolution in the spur. The volumes were characterized by a negative power-law that fits significant two events in 2016 (3.4 × 104 m3) and one in 1997 (2.0 × 106 m3) remarkably well, and the randomness of the fit was evaluated by a Monte Carlo approach. 7 potential failure scenarios ranging from 3.1 × 104 m3 (S1) to 4.8 × 106 m3 (S7) were defined according to a structural analysis and the sloping local base level concept. Their extrapolated return periods derived by the power-law fit indicate a longer return period for the maximum failure scenario than for the smaller scenarios. S1 has a 50% chance of occurring every 3 years, while S7 has a 50% chance of occurring every 31 years. Though the median return period of S7 is 31 years, the 95% and 68.2% confidence intervals range from 8 to 399 years and 14 to 93 years, respectively, which reflects a high level of uncertainty but is realistic when considering global warming, progressive rock failure, etc. In addition to characterizing recent rock failure activities in high mountains, this study offers a preliminary examination of the return periods of some extreme scenarios and provides primary data for risk management in mountainous areas that are very sensitive to global warming.

Upper bound analysis of active and passive mechanisms of intermediate rock wall between two adjacent pressurised cavities

The construction of adjacent cavities is important for deep oil storage and tunnel engineering. However, studies on the failure mechanisms of intermediate rock walls are scarce. Rock walls for underground cavities, such as oil reservoirs and tunnels built near concealed karst caves, are exposed to pressure variations. Because the magnitude of pressure imposed on the inner surface of two cavities varies, different kinematic mechanisms and failure extents of rock walls can be observed. To derive the analytical expressions of the failure surface for rock walls, limit analysis is conducted within the framework of the upper bound theorem. The active and passive failure modes of rock walls are defined based on the variation in the magnitude of pressure on both sides of the walls. Furthermore, two failure states of rock walls are summarised according to the relationship between the pressure difference and weight of rock walls. Moreover, based on the upper-bound solutions, thenon-linearoptimisations for critical pressure and thickness of rock walls are implemented by assuming that both cavities are cylindrical (rectangles in two dimensions). The results of the presented methods and numerical simulations indicate that the magnitude of pressure difference determines the failure modes and shape characteristics of rock walls. Evidently, the effects of internal pressures on the stability of rock walls must be intently considered in the design of two adjacent underground cavities filled with fluid.

Water Flows in Rock Wall Permafrost: A Numerical Approach Coupling Hydrological and Thermal Processes

AbstractRock wall permafrost is highly sensitive to atmospheric warming. Its degradation is regarded as a primary factor accounting for the recent increase in periglacial rock slope failures. Investigations of rock wall permafrost have so far ignored hydrological processes acting in bedrock fractures. We propose a 2‐D coupled thermal and hydrological modeling approach applied to alpine rock wall permafrost. Simulations are carried out on the scale of the mountain flank for four case studies with variable saturation levels and hydrological forcings. Results show that cold water infiltration from the summit leads to a deeper permafrost body than with no water flows. Ice‐filled fractures first delay permafrost thawing due to latent heat consumption but then accelerate it when the ice starts to melt. Ice may thus subsist in fractures surrounded by thawed bedrock while thawing corridors may form in fractures embedded in frozen bedrock. As a result, temperature gradients are made steeper. When connected fractures thaw, bottom‐up permafrost degradation can occur through upward propagation of thawing wedges delineated by these fractures. High hydraulic head values are associated with the formation of perched water tables above or within the impermeable permafrost body, and correspond to hydrostatic pressures that could reach critical values in terms of rock wall stability. The proposed model setting is more appropriate for investigating long‐term permafrost changes, rock wall permafrost dynamics and hydrogeology across entire mountain flanks, but could be applied on a finer spatial scale to investigate short‐term permafrost response to climate signals and rock wall destabilization.

Seasonal Freeze‐Thaw Cycles and Permafrost Degradation on Mt. Zugspitze (German/Austrian Alps) Revealed by Single‐Station Seismic Monitoring

AbstractThawing of mountain permafrost in response to rising temperatures degrades the stability of rock walls and thereby affects infrastructure integrity in Alpine terrain. In this study, we use 15 yr of passive seismic data from a single station deployed near a known permafrost body on Mt. Zugspitze (Germany), to monitor freeze‐thaw processes. The recordings reveal a persistent cultural seismic noise source, which we utilize to compute single‐station cross‐correlations and extract relative seismic velocity changes. We find that parts of the cross‐correlations show seasonal velocity variations (≈3% peak‐to‐peak amplitude) and a long‐term velocity decrease (≈0.1%/yr). Comparison with meteorological data and a previous electrical resistivity tomography study suggests that these velocity changes are caused by freeze‐thaw cycles and by permafrost degradation, respectively. The results demonstrate the potential of passive seismology for permafrost monitoring and suggest that denser instrumentation will provide detailed spatio‐temporal insights on permafrost dynamics in future studies.

Impacts of the 2003 and 2015 summer heatwaves on permafrost-affected rock-walls in the Mont Blanc massif

Rockfall is one of the main geomorphological processes that affects the evolution and stability of rock-walls. At high elevations, rockfall is largely climate-driven, very probably because of the warming of rock-wall permafrost. So with the ongoing global warming that drives the degradation of permafrost, the related hazards for people and infrastructure could continue to increase.The heatwave of summer 2015, which affected Western Europe from the end of June to August, had a serious impact on the stability of high-altitude rock-walls, including those in the Mont Blanc massif. A network of observers allowed us to survey the frequency and intensity of rock-wall morphodynamics in 2015, and to verify its relationship with permafrost. These observations were compared with those of the 2003 summer heatwave, identified and quantified by remote sensing.A comparison between the two years shows a fairly similar rockfall pattern in respect of total volumes and high frequencies (about 160 rockfalls >100m3) but the total volume for 2003 is higher than the 2015 one (about 300,000m3 and 170,000m3 respectively). In both cases, rockfalls were numerous but with a low magnitude and occurred in permafrost-affected areas. This suggests a sudden and remarkable deepening of the active layer during these two summers, rather than a longer-term warming of the permafrost body.

Seasonally intermittent water flow through deep fractures in an Alpine Rock Ridge: Gemsstock, Central Swiss Alps

Geological investigations and seismic refraction tomography reveal a series of 70° steep, parallel and continuous fractures at 2950masl within the Gemsstock rock ridge (Central Swiss Alps), at the lower fringe of alpine permafrost. Temperature measurements in a 40m horizontal borehole through the base of the ridge show that whilst conductive heat transfer dominates within the rock mass, brief negative and positive temperature anomalies are registered in summer. These have very small amplitudes and coincide with summer rainfall events lasting longer than 12h. In contrast, a complete lack of anomalous thermal signals during spring snowmelt suggests that runoff does not penetrate the open joints, despite high snow water equivalents of around 400mm. This is attributed to the development of an approximately 20cm thick, continuous and impermeable basal ice layer which forms at the interface between the snow cover and the cold rock on the shady North facing rock wall during snowmelt. Spring snowmelt water therefore does not affect rock temperatures in the centre of the rock mass, despite the presence of deep open joints. The mechanical impact of snowmelt infiltration on rock wall stability at depth is thus assumed to be negligible at this site.

Thermal characteristics of permafrost in the steep alpine rock walls of the Aiguille du Midi (Mont Blanc Massif, 3842 m a.s.l)

Abstract. Permafrost and related thermo-hydro-mechanical processes are thought to influence high alpine rock wall stability, but a lack of field measurements means that the characteristics and processes of rock wall permafrost are poorly understood. To help remedy this situation, in 2005 work began to install a monitoring system at the Aiguille du Midi (3842 m a.s.l). This paper presents temperature records from nine surface sensors (eight years of records) and three 10 m deep boreholes (4 years of records), installed at locations with different surface and bedrock characteristics. In line with previous studies, our temperature data analyses showed that: micro-meteorology controls the surface temperature, active layer thicknesses are directly related to aspect and ranged from <2 m to nearly 6 m, and that thin accumulations of snow and open fractures are cooling factors. Thermal profiles empirically demonstrated the coexistence within a single rock peak of warm and cold permafrost (about −1.5 to −4.5 °C at 10 m depth) and the resulting lateral heat fluxes. Our results also extended current knowledge of the effect of snow, in that we found similar thermo-insulation effects as reported for gentle mountain areas. Thick snow warms shaded areas, and may reduce active layer refreezing in winter and delay its thawing in summer. However, thick snow thermo-insulation has little effect compared to the high albedo of snow which leads to cooler conditions at the rock surface in areas exposed to the sun. A consistent inflection in the thermal profiles reflected the cooling effect of an open fracture in the bedrock, which appeared to act as a thermal cutoff in the sub-surface thermal regime. Our field data are the first to be obtained from an Alpine permafrost site where borehole temperatures are below −4 °C, and represent a first step towards the development of strategies to investigate poorly known aspects in steep bedrock permafrost such as the effects of snow cover and fractures.

Influence of different digital terrain models (DTMs)on alpine permafrost modeling

The thawing of alpine permafrost due to changes in atmospheric conditions can have a severe impact, e.g., on the stability of rock walls. The energy balance model, PERMEBAL, was developed in order to simulate the changes and distribution of ground surface temperature (GST) in complex high-mountain topography. In such environments, the occurrence of permafrost depends greatly on the topography, and thus, the digital terrain model (DTM) is an important input of PERMEBAL. This study investigates the influence of the DTM on the modeling of the GST. For this purpose, PERMEBAL was run with six different DTMs. Five of the six DTMs are based on the same base data, but were generated using different interpolators. To ensure that only the topographic effect on the GST is calculated, the snow module was turned off and uniform conditions were assumed for the whole test area. The analyses showed that the majority of the deviations between the different model outputs related to a reference DTM had only small differences of up to 1 K, and only a few pixels deviated more than 1 K. However, we also observed that the use of different interpolators for the generation of a DTM can result in large deviations of the model output. These deviations were mainly found at topographically complex locations such as ridges and foot of slopes.

Assessment of geomorphic hazards in connection with permafrost occurrence in the Zugspitze area (Bavarian Alps, Germany)

In the Zugspitze area (Bavarian Alps, Germany), permafrost conditions are present in limestone bedrock and in regolith. Distribution is strongly dependent on topography in the east–west oriented mountain crest with steep north- and south-facing slopes. Numerous structures mainly for tourist purposes (cable car and recreation buildings, ski-lift masts, rack-railway tunnel, tunnel with supply facilities) are situated in the area, and several of them are placed on ground with permafrost. Results from a temperature measurement programme and distribution modelling show that for some of these constructions, the effects of permafrost degradation have to be considered in terms of stability of the foundations. The permafrost limit is close to the summit crest, and therefore, stability evaluations for the constructions in this area have to bear in mind the possible warming or even melting of ice within the bedrock crevasses caused by climate warming. Stability of the foundations as well as stability of rock walls in this area will probably be affected by a shifting of the permafrost limit. Constructions in the Zugspitzplatt area are already affected by the melting ground ice, and stabilizing measures have to be evaluated for several foundations where subsidence is likely to occur. Besides the local results, the study provides for the first time data on permafrost distribution in the northern Alpine margin based on standard methods of BTS measurements and numerical modelling.

Stabilization of the rock wall “Pastýřská sténa” by anchoring

The sandstone rock wall called “Pastýřska stena” at Děcin (northern Bohemia), threatens by its inferior stability the road and buildings situated along its foot. A detailed geotechnical investigation of the rock wall made 20 years ago and now completed, yielded the data necessary for the project of its stabilization by anchoring. The paper describes the principles of static analysis of the rock wall stability and of the individual unstable blocks, as well as the computation of the anchoring forces. The project is being realized at the present time. About 600 anchors of different type will be mounted on the rock wall.

Rock mass classification as a means of predicting rock wall stability : Denkhaus, HG CSIR, Pretoria, ZA In Polish. 6F, 6T, 6R. Przegl. Gorn. V30, N1, Jan. 1974, P10–19

Rock mass classification as a means of predicting rock wall stability : Denkhaus, HG CSIR, Pretoria, ZA In Polish. 6F, 6T, 6R. Przegl. Gorn. V30, N1, Jan. 1974, P10–19