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Predicting the fracture character of weak layers from snowpack penetrometer signals
Abstract
Digital penetrometers provide reliable assessments of snow penetration resistance with depth. However, extracting useful information from the signals relating to snow stability has proved to be challenging. In this study, penetrometer profiles were collected in close proximity to compression tests. A scheme for predicting the fracture character of weak layers in the compression tests from the penetrometer signals is presented. When a two-group classification between sudden (Q1) (an indicator of instability) and other fracture character groups was performed, potential failure layers were correctly classified 80% of the time. The variables offering the best discrimination between sudden and other categories were weak layer thickness, average force gradient above the weak layer, and both the average and the maximum force gradient below the weak layer. The effect of introducing randomly selected layers into the prediction scheme was also investigated. When such layers were introduced, the classification rate dropped to 67%, indicating that more effective fracture character prediction occurred when weak layers were manually pre-identified. This suggests that this scheme should be used in conjunction with a weak layer detection model rather than as a stand alone analytical technique for the purpose of critical weak layer identification. The classification rate dropped further to 55% when a more detailed, four-group classification scheme was used.
... However, discrimination power within the intermediate range was low. We then applied the index to gridded snow micro-penetrometer measurements from 11 snow slopes to 10 explore the spatial structure and possibly relate it to slope stability. Stability distributions on the 11 slopes reflected various possible strength and load (stress) distributions that naturally can occur. ...
... However, relating spatial variations as derived from point measurements to slope stability has so far not been successful. For example, Bellaire 10 and Schweizer (2011) stated that firm conclusions on the dependence of slope stability on spatial variations were not possible due to the limited range of snow conditions in the dataset, and the fact that the definition of slope stability is partly intangible. From a theoretical point of view, as supported by numerical modelling (e.g. ...
... For the analysis described below, we used three different datasets out of these data from 23 slopes. 10 a. From the concurrent observations of point stability (CT) and penetration resistance (SMP), we analysed, after quality control, in total 129 SMP profiles with corresponding CT score. ...
Snow slope stability evaluation requires considering weak layer as
well as slab properties – and in particular their interaction. We
developed a stability index from snow micro-penetrometer
measurements and compared it to 129 concurrent point observations
with the compression test (CT). The index considers the SMP-derived
micro-structural strength and the additional load which depends on
the hardness of the surface layers. The new quantitative measure of
stability discriminated well between point observations rated as
either "poor" or "fair" (CT < 19) and those rated as "good"
(CT ≥ 19). However, discrimination power within the
intermediate range was low. We then applied the index to gridded
snow micro-penetrometer measurements from 11 snow slopes to explore
the spatial structure and possibly relate it to slope
stability. Stability distributions on the 11 slopes reflected
various possible strength and load (stress) distributions that
naturally can occur. Their relation to slope stability was poor
possibly because the index does not consider crack
propagation. Hence, the relation between spatial patterns of point
stability and slope stability remains elusive. Whereas this is the
first attempt of a truly quantitative measure of stability, future
developments should consider a better reference of stability and
incorporate a measure of crack propagation.
... Similar analysis were also performed with the SABRE penetrometer: e.g. Floyer and Jamieson [2008] and Floyer and Jamieson [2009] These results provide an overview of the existing research on the SMP, but is in by no means exhaustive. There is another method of interpreting these signals and these are by means of inversion models, which try to derive micromechanical properties from the snow. ...
La prévision du risque d'avalanche, les prévisions hydrologiques et l'estimation du bilan énergétique de la Terre dépendent d'une connaissance précise de la stratigraphie du manteau neigeux. Le test de pénétration du cône, qui consiste à enregistrer la force nécessaire pour faire pénétrer un cône dans le matériau d'intérêt, est largement utilisé pour mesurer des profils de neige. La sonde de battage, développée et utilisée depuis 1930, a été continuellement améliorée. Aujourd'hui, des pénétromètres numériques, tel que le SnowMicroPenetrometer, permettent de mesurer la résistance à la pénétration à vitesse constante avec une résolution verticale de quelques microns. Les fluctuations de force mesurées à cette résolution contiennent des informations sur la microstructure essentielles pour compléter la connaissance de la dureté moyenne de chaque couche de neige. Néanmoins, le lien entre le profil de dureté et la microstructure de la neige n'est pas encore entièrement compris. En effet, les modèles d'inversion existants négligent certaines des interactions entre la pointe du cône et la neige, comme la formation d'une zone de compaction, et ils n'ont été évalués que par des relations empiriques avec des propriétés macroscopiques. L'objectif de cette thèse est d'étudier l'interaction entre le cône et la neige à une échelle microscopique, à l'aide de la tomographie à rayons X, afin d'extraire, de manière plus précise, les propriétés microstructurelles de la neige à partir des mesures de résistance à la pénétration. Dans ce travail, nous analysons des tests de pénétration du cône de quelques centimètres de profondeur, qui contiennent une partie transitoire non négligeable due à la formation progressive d'une zone de compaction. Afin de prendre en compte explicitement ce processus, nous avons développé un modèle statistitique non-homogène de Poisson, qui prend en compte une dépendance à la profondeur du taux d'occurrence de rupture entre grains de neige. Nous avons utilisé ce modèle pour caractériser le frittage de la neige par des tests de pénétration du cône sous conditions contrôlées dans une chambre froide. D'après le modèle, l'hétérogénéité verticale des profils de dureté était due aux variations du taux d'occurrence de rupture, tandis que l'évolution temporelle de la force macroscopique était contrôlée par un renforcement des ponts. Cette partition est cohérente avec les processus de frittage connus et fournit une validation indirecte du modèle proposé. Une deuxième partie de la thèse a porté sur des expériences en chambre froide combinant des tests de pénétration du cône et de l'imagerie par tomographie X. Des images tri-dimensionnelles à haute résolution d'un échantillon de neige ont été prises avant et après le test du cône. Sur ces images, un nouvel algorithme de suivi de grains a été appliqué pour déterminer les déplacements granulaires induits par le test. Nous avons quantifié avec précision la taille de la zone de compaction et sa relation avec les caractéristiques de la neige. Nous avons montré que les déplacements verticaux observés compliquent l'utilisation de modèles d'expansion de cavité comme modèles d'inversion. Enfin, nous avons lié les propriétés microstructurelles obtenues par tomographie, telles que la taille ou le nombre de ruptures de ponts, à des propriétés dérivées des profils de dureté. Nous avons montré que les propriétés estimées à partir des tests de pénétration du cône sont des approximations de la microstructure de la neige, mais restent trop conceptuelles pour espérer une relation directe. A l'avenir, ces études devraient permettre de dériver, de manière objective, la stratigraphie du manteau neigeux à partir d'une mesure de terrain simple et rapide.
... Pielmeier and Schweizer (2007) found that weak layer hardness and the difference in hardness between the slab and the weak layer, measured with the SMP, were indicators of instability. Later, Floyer and Jamieson (2009) predicted fracture character from SABRE-signals, also using non-microstructural properties. This shows that the SP2 signal, if accurate, might be used to assess not only the presence of a weak layer, but also structural properties relevant for stability. ...
The snow cover stratigraphy is an important factor in the present operational avalanche forecasting. Manual methods for recording snow profiles and snow hardness is widely used. These traditional methods lack objectivity and are observer dependent. The rammsonde proved to increase objectivity of hardness measurements, but the low vertical resolution leads to avalanche-prone features, such as thin weak layers, being overlooked. The SMP has shown that it can accurately, objectively and reliably provide detailed information on the snow stratigraphy and has greatly contributed to the study of snow. The recently developed SP2 tries to pack the same features into an affordable and compact design. This would allow rapid collection of quantitative and objective measurements of snowpack features associated with snow stability, and could potentially be an accommodation for observers working in avalanche forecasting. In this study, we have investigated the SP2s accuracy and repeatability by performing field and lab tests, and by comparing its ability to quantify snow stratigraphy against traditional methods such as manual snow profiles. The SP2 showed that it could record the main stratigraphic features of the snow covers and that it had consistent hardness measurements. The results are restricted to the tested snow conditions, but indications were given that the hardness measurements are too coarse to resolve measurements in soft snow. The penetrometer is limited by poor accuracy in the depth measurements, where possible contributing factor may include unprecise surface determination and push-rate variability. With increased accuracy and force sensitivity the SP2 or similar penetrometers have the potential of becoming valuable tools for avalanche observers.
... Lutz et al. (2009) and Bellaire and Schweizer (2011) also found the micro-structural strength of the weak layer to be related to stability. Floyer and Jamieson (2009) predicted the fracture character of compression tests (CT) from adjacent SABRE penetrometer profiles (Mackenzie and Payten, 2002), whereas van Herwijnen et al. (2009) found snow stratigraphy derived from microstructural properties of the SMP to be related to the fracture type in CTs. ...
Snow slope stability evaluation requires considering weak
layer as well as slab properties – and in particular their interaction. We
developed a stability index from snow micro-penetrometer (SMP) measurements and
compared it to 129 concurrent point observations with the compression test
(CT). The index considers the SMP-derived micro-structural strength and the
additional load, which depends on the hardness of the surface layers. The new
quantitative measure of stability discriminated well between point
observations rated as either "poor" or "fair" (CT < 19) and those
rated as "good" (CT ≥ 19). However, discrimination power within the
intermediate range was low. We then applied the index to gridded snow
micro-penetrometer measurements from 11 snow slopes to explore the spatial
structure and possibly relate it to slope stability. Stability index
distributions on the 11 slopes reflected various possible strength and load
(stress) distributions that can naturally occur. Their relation to slope
stability was poor, possibly because the index does not consider crack
propagation. Hence, the relation between spatial patterns of point stability
and slope stability remains elusive. Whereas this is the first attempt of a
truly quantitative measure of stability, future developments should consider
a better reference of stability and incorporate a measure of crack
propagation.
Snow is a porous disordered medium consisting of air and three water phases: ice, vapour and liquid. The ice phase consists of an assemblage of grains, ice matrix, initially arranged over a random load bearing skeleton. The quantitative relationship between density and morphological characteristics of different snow microstructures is still an open issue. In this work, a three-dimensional fractal description of density corresponding to different snow microstructure is put forward. First, snow density is simulated in terms of a generalized Menger sponge model. Then, a fully three-dimensional compact stochastic fractal model is adopted. The latter approach yields a quantitative map of the randomness of the snow texture, which is described as a three-dimensional fractional Brownian field with the Hurst exponent H varying as continuous parameter. The Hurst exponent is found to be strongly dependent on snow morphology and density. The approach might be applied to all those cases where the morphological evolution of snow cover or ice sheets should be conveniently described at a quantitative level.
Stability prediction from SnowMicroPen (SMP) profiles would support avalanche forecasting operations, since objective stability information could be gathered more quickly than with standard tests, thereby allowing sampling at higher resolution and over larger spatial scales. Previous studies have related the snow properties derived from the SMP to observed snow properties at Rutschblock (RB) and compression test failure planes. The goals of this study are to show to what extent snowpack stability for artificial triggering, based on RB, can be derived from SMP measurements and how multiple measurements at the RB scale improve the results. Measurements at 36 different sites were used for the development of a classification scheme. Each site included a RB test, a manual profile, and 6 to 10 adjacent SMP measurements, for a total of 262 SMP profiles. A recently improved SMP theory was applied to estimate the micro-structural and mechanical properties of manually defined weak layers and slab layers. SMP signal quality control and different noise treatment methods were taken into consideration in the analysis. The best and most robust predictor of RB stability was the weak layer micro-scale strength. In combination with the SMP-estimated mean density of the slab layer, the total accuracy of predicting RB stability classes was 85% over the entire dataset, and 88% when signals with obvious signal dampening (11% of the dataset) were removed. The total accuracy increased when multiple SMP measurements at the RB scale were used to calculate the mean weak layer strength, when compared to using just one SMP measurement at a site. The analysis was robust to trends and offsets in the absolute SMP force, which was a frequent signal error. However, it was sensitive to dampened or disturbed SMP force micro variance. The sensitivity analysis also showed that the best predictor of instability, the weak layer micro-scale strength, was robust to the choice of SMP signal noise removal method.
Compression tests are snow stability tests that are widely used by avalanche professionals and snow researchers to identify potential weak snowpack layers. Describing fracture character in addition to the number of taps required to initiate a fracture improves the interpretation of compression test results, since certain types of fractures, i.e. sudden fractures, are more often associated with skier-triggered avalanches. Digital snowpack penetrometers provide high resolution penetration resistance data of the snow cover with depth. The SnowMicroPen (SMP) was used to measure high resolution penetration resistance profiles in the snowpack next to compression tests. A reliable method to automatically detect the snow surface in the SMP signals was introduced. Furthermore, a method based on the autocorrelation of the penetration resistance signal was developed to identify the failure layers, identified using compression tests, in the penetration resistance profiles. Using field data from 190 penetration resistance measurements, each collected between two compression tests, micro-structural parameters associated with different types of fractures were identified. More than 550 fractures were classified as either Progressive Compression (1.3%), Resistant Planar (12.1%), Sudden Planar (50.4%), Sudden Collapse (26.8%) or non-planar Break (9.4%). Measurement and analysis were focussed on micro-structural properties of the failure layer, the layer adjacent to the failure layer and the slab above the failure layer. Sudden collapse fractures were found to have typical micro-structural snowpack parameters that are generally associated with unstable snow conditions, such as large differences in penetration resistance between the failure layer and the adjacent layer.
Snow is a porous disordered medium consisting of air and three water phases: ice, vapor, and liquid. The ice phase consists of an assemblage of grains, ice matrix, initially arranged over a random load bearing skeleton. The quantitative relationship between density and morphological characteristics of different snow microstructures is still an open issue. In this work, a three-dimensional fractal description of density corresponding to different snow microstructure is put forward. First, snow density is simulated in terms of a generalized Menger sponge model. Then, a fully three-dimensional compact stochastic fractal model is adopted. The latter approach yields a quantitative map of the randomness of the snow texture, which is described as a three-dimensional fractional Brownian field with the Hurst exponent H varying as continuous parameters. The Hurst exponent is found to be strongly dependent on snow morphology and density. The approach might be applied to all those cases where the morphological evolution of snow cover or ice sheets should be conveniently described at a quantitative level.
Snow stratigraphy and changes in snow hardness are amongst the most important parameters in the study of snow. In seven snow pits, three types of hardness tests were performed: these were hand, ram and micro penetrometer tests. The stratigraphy, in terms of layers and layer boundaries, was established from planar sections at a high spatial resolution, providing an objective reference for comparison with the analysed profiles. The snow layers and layer boundaries from the hardness profiles were compared with planar section stratigraphy. The hand test captured 80% of all layers and layer boundaries and the ram test 60%, and the micro penetrometer captured all the identified layers. Hand and ram profiles can only be used to determine an average hardness and stratigraphy. Important details are missed such as soft and thin hard layers, which are particularly significant for avalanche formation. Quantitative evaluation of mechanical and textural snowpack properties requires objective measurements with a spatial resolution of at least 1 mm. Since the predominant heat and mass fluxes are perpendicular to the snow surface, the more detailed stratification shown with the micro penetrometer has significant implications for mass and heat transfer within the snowpack. Numerical models not representing this small-scale layering will be misleading. Avalanche formation, snow hydrology and electromagnetic modelling (microwave emission and radar) are other examples of areas where a good resolution of the snowpack stratigraphy is important.
This study investigates whether collecting shear quality data in conjunction with stability test results improves snowpack evaluations. Over the past six seasons we have consistently evaluated shear quality when evaluating snowpack stability. Shear quality is subjectively evaluated on a 3-tiered scale from Q1 (clean, fast shears) to Q2 (average shears) to Q3 (irregular or dirty shears). Our method is a formalization of what ski patrollers and others have been doing in the U.S. and elsewhere for at least several decades. We used a dataset of nearly 700 individual stability tests (rutschblock, stuffblock and compression tests) collected by seven observers on slopes from Alaska to Chile. In addition to stability test results, observers noted whether slopes they felt were similar to their snowpit location had avalanches, or collapsing or cracking snowpacks, on that day. Results suggest that shear quality provides important stability information, especially when stability test results appear to indicate relatively stable conditions, but the shear quality is rated Q1. This might be because stability test results are often spatially variable, while our experience indicates that shear quality is more homogeneous. Given these results, we believe formally integrating some description of shear characteristics into stability assessments may be important for avalanche workers and backcountry enthusiasts.
Compression tests are snow stability tests that are widely used by avalanche professionals and snow researchers to identify potential weak snowpack layers. Describing fracture character in addition to the number of taps required to initiate a fracture improves the interpretation of compression test results, since certain types of fractures, i.e. sudden fractures, are more often associated with skier-triggered avalanches. Digital snowpack penetrometers provide high resolution penetration resistance data of the snow cover with depth. The SnowMicroPen (SMP) was used to measure high resolution penetration resistance profiles in the snowpack next to compression tests. A reliable method to automatically detect the snow surface in the SMP signals was introduced. Furthermore, a method based on the autocorrelation of the penetration resistance signal was developed to identify the failure layers, identified using compression tests, in the penetration resistance profiles. Using field data from 190 penetration resistance measurements, each collected between two compression tests, micro-structural parameters associated with different types of fractures were identified. More than 550 fractures were classified as either Progressive Compression (1.3%), Resistant Planar (12.1%), Sudden Planar (50.4%), Sudden Collapse (26.8%) or non-planar Break (9.4%). Measurement and analysis were focussed on micro-structural properties of the failure layer, the layer adjacent to the failure layer and the slab above the failure layer. Sudden collapse fractures were found to have typical micro-structural snowpack parameters that are generally associated with unstable snow conditions, such as large differences in penetration resistance between the failure layer and the adjacent layer.
Local snowpack measurements and local stability tests are currently the basis for the assessment of snowpack stability in most avalanche warning operations. The SnowMicroPen (SMP), a high-resolution penetrometer for snow, measures penetration resistance of snow. In order for SMP measurements to be useful for stability evaluation (or avalanche forecasting purposes), stability information needs to be derived from the SMP signal. It was shown that structural and mechanical parameters derived from the SMP signal for known (or manually observed) failure interfaces were related to snowpack stability. The dataset contained 66 parallel SMP and manual snow profiles with stability tests from five winter seasons 2001-02 to 2005-06 in Switzerland. It was balanced between stable (35) and unstable (31) profiles. The manual failure layer determination in the SMP signal was improved. Micro structural and mechanical parameters were derived from the SMP signal using two models describing the interaction of the SMP tip with an idealized snow structure. The parameters from the improved model are compared with snow stability data for the first time. The new model slightly improved the results of the statistical analysis and the classification accuracy of a failure interface from a SMP profile. The analysis confirms the potential of the SnowMicroPen operational use in avalanche forecasting services.
The rutschblock test is one of the most reliable snow stability tests. It is a mini slab avalanche that is tested with the appropriate dynamic load: the skier or snowboarder. The test method and its limitations are reviewed. The application of the rutschblock test as the standard stability test in the observational network of the Swiss Avalanche Warning Service is described.
A new constant-speed penetrometer for field and laboratory measurements has been developed. The new penetrometer has high rigidity and a high-resolution large dynamic range force sensor. It uses a much smaller sensing head (5 mm) than previous designs and has a constant speed drive. With this construction, the penetration resistance of very fine layers and the bonding strength between snow grains can be more accurately determined than is possible with the rammsonde. Artificial foam layers as thin as 2 mm and thin layers in snow have been detected by the penetrometer. Thin snow layers detected from penetration resistance profiles have been correlated to fine layering as determlned from plane-section mlcrophotographs of samples taken adjacent to the profile. The instrument's measurements are highly repeatable and the lack of subjective decisions when operating the penetrometer makes the penetration resistance a quantitative measure of snow stratigraphy.
The mechanisms leading to dry-snow slab release are influenced by the three-dimensional variability of the snow cover. We measured 113 profiles of penetration resistance with a snow micropenetrometer on an alpine snow slope. Seven distinct layers were visually identified in all snow micropenetrometer profiles. The penetration resistance of adjacent layers did not change abruptly, but gradually across layer boundaries that were typically 2 mm thick. In two layers, penetration resistance varied around 200% over the grid, possibly due to wind effects during or after layer deposition. Penetration resistance varied around 25% in five layers. Statistically significant slope-scale linear trends were found for all layers. The semivariogram was used to describe the spatial variation. Penetration resistance was autocorrelated, but the scale of variation was layer-specific. A buried layer of surface hoar was the most critical weak layer. It had little spatial variation. The layers in the slab above had higher spatial variation. The penetration resistance of each snow layer had distinct geostatistical properties, caused by the depositional processes.
RÉSUMÉ L'identification de la couche fragile est un aspect important de la prévision des avalanches. Les détails du profil de profondeur de la neige, tel que la résistance à la pénétration, mesuré par un pénétromètre numérique à haute résolution, peut faciliter l'identification rapide de la couche fragile. À présent, l'identification de la couche fragile se fait en personne par un spécialiste; ce processus peut être lent si plusieurs profiles doivent être mesurés. Cet article présente une méthode pour tracer une couche fragile qui a été identifiée dans un profil d'un transect ou dans une grille de profils, permettant un certain degré de changement dans la profondeur ou la forme de la couche tracée. En utilisant cette méthode, la technique de spiking déconvolution de Wiener sert a identifier les coordonnées le plus probables de la couche du profil en question, à l'intérieur du domaine spécifié relativement au profil original. De plus, nous présentons un plan pour augmenter cette méthode pour identifier les couches fragiles possibles dans un signal non-interprété. Nous proposons que le signal soit comparé à une base de donnés de signaux de couches fragiles possibles, en utilisant l'output de la méthode de déconvolution pour mesurer les chances qu'une couche fragile soit présente dans le signal. Un système expert est prévu qui combinerait cette probabilité avec les connaissances expertes des probabilités sur la profondeur de la couche fragile, de sa dureté, ainsi que la probabilité que la couche existe. ABSTRACT Weak layer identification is an important part of avalanche forecasting. Digital, high-resolution snow penetrometers can aid weak layer identification by giving a rapid and detailed depth profile for certain snow properties, such as force-resistance. Presently, weak layer identification is done by a skilled human interpreter; this process is slow if many profiles need to be interpreted. This paper presents a method for tracing a weak layer that has been identified in one profile across a transect or grid of profiles, allowing for a certain degree of change in the depth or the shape of the layer being traced. In this method, the technique of Wiener spiking deconvolution is used to isolate the most likely location of the layer in the profile being analysed within a user-specified range of depths relative to the original profile. In addition, we present a framework for extending this method to identify potential weak layers in an un-interpreted signal. We suggest that the signal to be interpreted could be tested against a database of potential weak layer signals, using the output from the deconvolution method to indicate the likelihood of each weak layer being present in the signal. An expert system is envisaged that combines this likelihood with a-priori (expert) knowledge of the weak layer's probable burial depth, hardness and probability of existence.
The stuffblock is a new snow stability test developed by the Gallatin National Forest Avalanche Center and used operationally since 1993. The test involves stressing an isolated column of snow 0.30 m2 by dropping a nylon sack filled with 4.5 kg onto the column from 0.10 m increments until weak layer failure occurs. Results over several winters correlate the stuffblock test with the more widely used rutschblock test, and validate the usefulness of the test for evaluating snow stability in several different climates. Further, the test provides results that are consistent between observers, a favorable attribute for regional avalanche forecasting operations which use numerous observers.
This study investigates snowpack properties associated with skier-triggered dry slab avalanches, with a particular view on snowpack conditions favoring fracture propagation. This was done by analyzing a data set of over 500 snow profiles observed next to skier-triggered slabs (including remotely triggered slab avalanches and whumpfs) and on skier-tested slopes that did not release a slab avalanche. The relation of the snowpack variables with fracture initiation and fracture propagation, both of which are required for skier-triggering, was investigated. Specific snowpack characteristics, including hardness difference and difference in crystal size across the failure layer, associated with skier-triggered dry slab avalanches were identified and the frequency of skier-triggering was determined. In order to assess snowpack variables favouring fracture propagation, variables from failure layers associated with skier-triggered slabs that were not remotely triggered and relatively small were contrasted with snowpack variables from failure layers of remotely triggered slab avalanches, whumpfs and relatively large slab avalanches. The properties of the slab overlying the weak layer, as well as the layer above the weak layer, were found to affect fracture propagation. Stiffer slabs were associated with large avalanches as well as whumpfs and remotely triggered avalanches. Furthermore, a correlation analysis of snowpack variables with the size and width of the investigated slab avalanches further accentuated the importance of these slab properties with regards to fracture propagation.
Stability tests are widely used to identify and qualify potential failure layers for slab avalanches. As an addition to stability test scores, researchers at the University of Calgary have been systematically classifying fractures in compression and rutschblock tests since the winter of 1996–1997. The classification system comprises five categories: Progressive Compression (PC), Resistant Planar (RP), Sudden Planar (SP), Sudden Collapse (SC) and non-planar Break (B). Some 2654 fractures were classified in compression tests performed at study slopes in the Columbia Mountains of British Columbia, Canada. Specific snowpack characteristics, including hardness difference and difference in crystal size across the failure layer, associated with the different fracture characters were identified. Data from skier-tested slopes show that fracture characterization can improve the interpretation of compression test results. Sudden fractures (Sudden Collapse and Sudden Planar) are more often the failure layer of slab avalanches than other fractures.
A micropenetrometer has been developed that produces snow grain bond ruptures at the microstructural level and provides a unique signal for different snow types. A micromechanical theory of penetration has been developed and used to recover microstructural and micromechanical parameters for different snow types from the penetration force–distance signal. These parameters are the microstructural element dimension, the mean grain size, the critical microstructural deflection at rupture and the microstructural coefficient of elasticity. Additional derived mechanical properties include the compression strength and elastic modulus of microstructural elements and continuum scale volumes of snow. Analysis of the force–distance signal from a Monte Carlo simulation of micropenetration indicates that microstructural and micromechanical parameters may be recovered with a measurement accuracy of better than 5% when spatial and force resolutions are high and the penetrometer tip area is of similar size to the structure dimension.
Stability prediction from SnowMicroPen (SMP) profiles would support avalanche forecasting operations, since objective stability information could be gathered more quickly than with standard tests, thereby allowing sampling at higher resolution and over larger spatial scales. Previous studies have related the snow properties derived from the SMP to observed snow properties at Rutschblock (RB) and compression test failure planes. The goals of this study are to show to what extent snowpack stability for artificial triggering, based on RB, can be derived from SMP measurements and how multiple measurements at the RB scale improve the results. Measurements at 36 different sites were used for the development of a classification scheme. Each site included a RB test, a manual profile, and 6 to 10 adjacent SMP measurements, for a total of 262 SMP profiles. A recently improved SMP theory was applied to estimate the micro-structural and mechanical properties of manually defined weak layers and slab layers. SMP signal quality control and different noise treatment methods were taken into consideration in the analysis. The best and most robust predictor of RB stability was the weak layer micro-scale strength. In combination with the SMP-estimated mean density of the slab layer, the total accuracy of predicting RB stability classes was 85% over the entire dataset, and 88% when signals with obvious signal dampening (11% of the dataset) were removed. The total accuracy increased when multiple SMP measurements at the RB scale were used to calculate the mean weak layer strength, when compared to using just one SMP measurement at a site. The analysis was robust to trends and offsets in the absolute SMP force, which was a frequent signal error. However, it was sensitive to dampened or disturbed SMP force micro variance. The sensitivity analysis also showed that the best predictor of instability, the weak layer micro-scale strength, was robust to the choice of SMP signal noise removal method.
Snowpack measurements and stability tests are, next to recent avalanche activity and weather history, currently the basis for snowpack stability assessment in most avalanche warning operations. The SnowMicroPen (SMP), a high-resolution penetrometer for snow, measures penetration resistance force or snow hardness. In order to be useful for an avalanche warning service, stability information needs to be provided and must be derivable from the SMP signal. SMP profiles (25 on slopes, 14 on flat sites) were taken together with manual snow profiles and stability tests, such as Rutschblock and compression tests. The data are from three winter seasons of the years 2001–2002 to 2003–2004 in the Swiss Alps. According to their stability test score and failure interface properties the manual profiles were classified as stable or unstable. Based on the manual observations the failure interfaces were identified in the SMP profiles and possible indicators of instability were derived from the SMP signals at these interfaces. The distinct indicators of instability were the failure layer micro-structural length and hardness, the difference in structural length across the failure interface and the failure layer macro-elastic modulus. The cross-validated accuracy of classification into stable or unstable failure interfaces gained from SMP parameters was comparable to the classification accuracy from manual profile parameters (about 65%). It remains to be tested if stability information can be derived from a SMP measurement without knowing the location of the failure interface found by a stability test. If this can be done successfully and reliably, avalanche warning operations could definitely benefit from the instrument.
A Swiss–Canadian data set of over 400 snow profiles from skier-triggered slopes and slopes that were skied but not triggered was contrasted to derive statistically relevant differences that can be used in snow profile interpretation. The critical weakness was identified either by the adjacent avalanche, rutschblock test or compression test. A failure layer and failure interface was identified in each profile. For discriminating between stable and unstable profiles, univariate analysis revealed the importance of the rutschblock score, failure layer hardness, failure layer grain size as well as difference in grain size and hardness across the failure interface. These same variables were used in two multivariate classification methods, which suggest that approximately 65% of profiles can be correctly classified with or without the stability test score.
Snow profile interpretation is part of the every day work of any avalanche forecasting service. Snowpack information is crucial for assessing such things as the probability of a skier triggering an avalanche in between storms or the probability and size of natural avalanches during and after storms. However, profile evaluation is considered an art. There are few standard procedures and most forecasters have their own method. Some expert systems have been developed to interpret snow profiles but their knowledge base is not documented. The avalanche forecasting service in Switzerland has to analyse about 110 snow profiles recorded by its observers twice a month. This task is presently quite time-consuming, and the results are not fully consistent. Therefore, part of the decision-making process of some experienced forecasters at the Swiss Federal Institute of Snow and Avalanche Research was explored. Based on this broad experience, each parameter observed in a snow profile with a stability test has been described in view of stability evaluation. A tentative snowpack stability rating scheme is proposed for dry snow conditions in transitional or intermountain climate zones. The principal criteria are: rutschblock score, hardness, presence and type of weak layers, grain type and size. It should help the forecasters in the future to more consistently interpret snow profiles.
Forecasting for slab avalanches benefits from precise measurements of
snow stratigraphy. Snow penetrometers offer the possibility of providing
detailed information about snowpack structure; however, their use has
yet to be adopted by avalanche forecasting operations in Canada. A
manually driven, variable rate force-resistance penetrometer is tested
for its ability to measure snowpack information suitable for avalanche
forecasting and for spatial variability studies on snowpack properties.
Subsequent to modifications, weak layers of 5 mm thick are reliably
detected from the penetrometer signals. Rate effects are investigated
and found to be insignificant for push velocities between 0.5 to 100 cm
s-1 for dry snow. An analysis of snow deformation below the penetrometer
tip is presented using particle image velocimetry and two zones
associated with particle deflection are identified. The compacted zone
is a region of densified snow that is pushed ahead of the penetrometer
tip; the deformation zone is a broader zone surrounding the compacted
zone, where deformation is in compression and in shear. Initial
formation of the compacted zone is responsible for pronounced force
spikes in the penetrometer signal. A layer tracing algorithm for tracing
weak layers, crusts and interfaces across transects or grids of
penetrometer profiles is presented. This algorithm uses Wiener spiking
deconvolution to detect a portion of the signal manually identified as a
layer in one profile across to an adjacent profile. Layer tracing is
found to be most effective for tracing crusts and prominent weak layers,
although weak layers close to crusts were not well traced. A framework
for extending this method for detecting weak layers with no prior
knowledge of weak layer existence is also presented. A study relating
the fracture character of layers identified in compression tests is
presented. A multivariate model is presented that distinguishes between
sudden and other fracture characters 80% of the time. Transects of
penetrometer profiles are presented over several alpine terrain features
commonly associated with spatial variability of snowpack properties.
Physical processes relating to the variability of certain snowpack
properties revealed in the transects is discussed. The importance of
characteristic signatures for training avalanche practitioners to
recognise potentially unstable terrain is also discussed.
In recent years, most avalanche fatalities have been due to dry-snow slab avalanches triggered by the victims themselves during recreational activities, at least in countries where recreational skiing takes place, as in Western Europe and North America. Simple analysis suggests, and previous measurements have shown, that the skier's dynamic impact is relevant for triggering, decreasing with increasing depth within the snow cover and depending on the layering of the snow cover. The stress distribution below a skier in the snow cover is not known, but is important in view of the critical area required for fracture propagation preceding dry-snow slab avalanche release. The skier's zone of influence was measured with load cells buried within the snow cover in a level study plot for different depths. The size of the area of influence is of the order of a few 0.1 m2, but also depends on the weak-layer strength. Stresses in the snow cover below skiers who walk behind each other are not cumulative. The effect on the influence size is in accordance with the independent estimate for the critical size (0.1-1 m) for fracture propagation in the case of rapid loading. It supports the hypothesis that skiers induce a brittle fracture within the weak layer or at a weak interface between layers, leading under certain conditions to slab release, without necessarily hitting a pre-existing flaw.
Avalanches released by deep weak layers are known to be difficult to forecast. This study considers the predictive merit of weather and snowpack data for avalanches that released throughout two winters on layers of faceted crystals. These layers formed above rain crusts in November 1996 and in November 1997 in the Columbia Mountains of western Canada. This study focuses on the first winter, during which the facet layer released an estimated 500,000 tonnes of snow in 700 dry slab avalanches.The facet–crust combinations were the result of a cold air mass cooling a layer of dry snow on top of a rain-wetted layer. The resulting temperature gradient in the dry snow formed a relatively weak layer of facets on top of a hard crust.By early January 1997, the faceted layer from the first winter was buried 1–2.5 m below the surface in many starting zones in the North Columbia Mountains. Most avalanches occurred during or within 2 days of loading by snowfall or wind transported snow. Increases in air temperature over 4–5 days correlated with increased avalanche activity. The effects of warming and cooling on slab stability are discussed, but for thick slabs, current theories do not explain observations of decreased stability. We argue that the fractures that release natural deep slab avalanches may be initiated where the slab is locally thin.Based on rank correlations, the highest ranked predictors of natural avalanches include previous avalanche activity, accumulated snowfall over several days, changes in air temperature over 4–5 days, snowpack properties including a shear frame stability index, and the difference in hardness between the facet layer and the crust.
Dry-snow slab avalanches involve the release of a cohesive slab over an extended plane of weakness. In most fatal avalanches, the triggering of the initial failure occurred by localized rapid near-surface loading by people — followed by fracture propagation. Whereas a limit-equilibrium (LE) approach to snow slope failure only takes into account slab depth, slab density and weak layer strength, it omits properties such as the stiffness of adjacent layers and the fracture propagation process. Nevertheless, LE has been applied with some success to the frequency of skier triggering, suggesting that it is relevant to failure initiation. Since field studies have shown that, for a given slab thickness, stiffer slabs are less likely to be triggered, slab properties influence failure initiation, fracture propagation or both. A highly simplified finite element (FE) model of static skier loading was used to assess the effect of slab and substratum properties on skier-induced stresses in the weak layer. Compared to a uniform slab, the skier-induced stress at the depth of the weak layer varied by a factor of 2 due to layering. In particular, the simplified FE model suggests that while stiffer layers in the slab will reduce the skier-induced stress in the weak layer, stiff layers just below the weak layer can increase the shear stress. These results were incorporated into a modified stability index and compared to stability test results. However, by taking into account snowpack layering the correlation between the modified stability index and stability test results did not improve. While our simulations suggest that less stress penetrates through stiffer slabs and thus fracture initiation is less likely, other studies show that, once initiated, fractures under stiffer slabs have high propagation propensity.
Deep slab avalanche hazard forecasting and mitigation: the South Face at Big Sky ski area
- Savage
Telluride, CO. Savage, S., 2007. Deep slab avalanche hazard forecasting and mitigation: the South Face at Big Sky ski area. Proceedings of the 2006 International Snow Science Workshop, pp. 483–490.
Estimating slope stability from spatial snow cover observations
- S Bellaire
- J Schweizer
Bellaire, S., Schweizer, J., 2009. Estimating slope stability from spatial snow cover observations. Cold Regions Science and Technology: (this issue).
Observation guidelines and recording standards for weather, snowpack and avalanches
- Canadian Avalanche Association