SLAB AVALANCHE CROWN SURFACE FRACTOGRAPHY: OBSERVATIONS AND APPLICATIONS

Dave Gauthier

Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Canada

ABSTRACT: Fractography is the study of the morphological fracture surface features in artificial and natural materials, including bone and rock. Several patterns of distinctive macroscopic features, such as linear and curved ridges or valleys, often form during tensional fracture. These are often different at difference stages of fracture. ‘Plumose’ or feather-shaped structures are the most common in natural materials, and form away from the fracture origin after steady-state propagation is achieved. The sense of symmetry in the plumes indicates the propagation direction. The plumes converge at the fracture origin, in a region often showing other distinctive surface features marking the initial propagation conditions. In 1969, Sommerfeld reported the occurrence of these morphological features on the crown surfaces of a number of slab avalanches. This represents the only direct evidence to confirm that the crown is a tensional fracture surface in a typical avalanche. Furthermore, he identified the origin and propagation direction of the crown fractures at several avalanches, and found each to have propagated from the surface downwards, with no lateral component. In this paper I summarize the general principles of fractography, and discuss some possible applications to slab avalanche mechanics, including the notion that we may differentiate weak layer collapse from shear fracture based on crown surface features. In addition, I include a number of photographs showing the distinctive ‘plumose’ features on crowns of recent avalanches.

1. INTRODUCTION Large-scale morphological features on avalanche fracture surfaces appear to be as common in snow as in other natural materials, and can provide very unique and direct field evidence of the initiation and propagation of fractures that lead to avalanche release, and possibly similar failures in other materials.

The ‘crown’ is the slope-normal, upper perimeter fracture surface of a downed avalanche, and forms by mode-I slope-parallel tension due to the down slope component of the weight of the slab following weak layer failure (Perla, 1975). Lateral (‘flank’) and downslope (‘stauchwall’) failures complete the separation of the slab. It is widely accepted that the crown fracture occurs in tension, although Sommerfeld (1969) presents the only direct evidence found in the literature. He noted the occurrence of recognizable, repeated morphological features on the crown surfaces of a number of avalanches, and related them to similar features known to form only due to tension fracture in synthetic glass and other engineered materials. Furthermore, he identified the origin and

propagation direction of the crown fractures at several avalanches based on the morphological features; he observed each to have propagated from the surface downwards, with no lateral component. In this paper I report on new observations of ‘plumose’ morphological features on crown surfaces, which appear identical to those observed on tensional rock joints (e.g. Woodworth, 1896; Hodgson, 1961; Bahat, 1991; Figure 1). The occurrences of these confirm the mode-I nature of the crown fractures, and may allow for the identification of fracture origin and propagation direction. I discuss several applications of these

Corresponding author address: Dave Gauthier, Geological Engineering, Queen’s University, Kingston, ON Canada K7L 1N6; tel: 613-893-4920; e-mail: gauthier@geol.queensu.ca

Figure 1. Schematic of typical plumose fracture morphology on a hypothetical tension fracture surface (white) in a plate-like body, e.g. a sedimentary bed or snow slab. Recognizable features are in indicated.

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observations for both slab avalanche mechanics and for other mechanically similar slope failures, e.g. large earthflows and lateral spreads in sensitive glaciomarine clay (Quinn et al., 2011).

Bahat (1991) provides an excellent review of the science of ‘fractography’, or the study of the morphological fracture surface features in artificial and natural materials, including rocks. Figure 1 shows a hypothetical fracture surface parallel to the short dimension of a tabular or plate-like specimen, e.g. a snow slab or sedimentary bed in rock, where fractographic features tend to be best recognized (Bahat, 1991). ‘Plumose’ structures are the most common, and form away from the fracture origin after steady-state propagation is achieved (e.g. Bahat, 1991; Kulander and Dean,

1985). A planar and smooth ‘mirror’ zone often occurs at the fracture origin (Figure 1), which may represent subcritical crack propagation or a pre-existing flaw (Woodworth, 1896; Hodgson, 1961). The mirror zone may be surrounded by a wavy or rough ‘mist’ zone, which is transitional to a ridged or barbed ‘hackle’ zone. In the hackle zone, the ridges and barbs are linear or curved topographic features or steps on the main fracture surface. Remote from the fracture origin the curvature of the barbs is typically symmetrical about a layer-parallel central axis, forming the distinctive plumose pattern (Figure 1). The barbs are convex toward the propagation direction, with divergence in propagation direction representing a fracture origin point (Woodworth, 1896; Hodgson, 1961). Other recognizable features include subtle, curved

ridges oriented perpendicular to the plumes, and coarse-hackled fringe zones at boundaries or near the surface of individual layers. Some may mark hesitation in the fracture or even interfering stress waves (see Bahat, 1991). Sommerfeld (1969) focused on the mirror and mist zones near the fracture origin in snow slabs; here I focus on observations of the plumose pattern remote from and near the fracture origin along slab avalanche crown surfaces.

2. OBSERVATIONS As a first example, fracture morphology on an avalanche crown surface is obvious in photographs taken at the site of a fatal avalanche that occurred on 13 January 1997 (Figure 2a,b; Jamieson et al, 2010). A search of the collection of snow avalanche photographs at the University of Calgary and elsewhere revealed numerous instances of similar morphological features on crown surfaces of many different avalanches. In several cases, the propagation direction is identifiable based on plume curvature and symmetry. Sommerfeld (1969) also presented several examples, and unrecognized examples may be found in many other published sources, including Seligman (1936, figure 262). Together, these examples suggest that while not ubiquitous, recognizable fractographical features may occur commonly along avalanche crowns. Unfortunately, our experience with photography at avalanche sites suggests that capturing any features of the crown or other fractures using basic photographic techniques is often difficult or impossible due to the constraints of time, weather, lighting, etc.; as

Figure 2. (a) Annotated photograph of the 13 January 1997 avalanche, showing the initial small avalanche thought to have triggered the larger one; assumed weak layer fracture propagation directions; areas of rock exposed at snow surface and assumed stress concentrators and crown fracture nucleation sites; observed direction of crown segment fracture propagation based on large scale plumose markings. Photo: Bruce Jamieson; (b) example from right side of avalanche showing well-developed plumose markings in several places, including a central symmetry axis near the centre of the slab. Slab thickness indicated, and researcher standing on bed surface for scale. Photo: Phil Hein

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such, most photos that we found were unsuited for detailed analysis or publication. One particularly good example is shown in figure 2b.

The wide view (figure 2a) shows the location of a smaller, thinner slab avalanche that released just prior to the main slide (see Jamieson et al., 2010 for more details on this avalanche). The smaller slide was likely triggered by the passage of a skier. The disturbance associated with it appears to have caused a fracture to propagate in a weak layer deeper in the snowpack, releasing the larger avalanche. The slab had average thickness of approximately 1.2 m, but varied between 0.3 and 2.6 m thick. In several places along or adjacent to the crown rocks protruded through the snowpack to the surface. Density, stiffness, hardness, etc., measurements are not available for this slab, although I expect that it varied from very dense and stiff in thick areas to relatively soft and loose in the thinnest locations around rocks. Plumose structures are clearly visible along part of the crown surface in Figure 2b, indicating propagation from left to right through one of the thicker segments of the slab. The plume axis is near the centre of the slab, indicating that at this location there was no slope normal component of propagation. A plumose pattern is also visible on crown segment on the opposite side of the avalanche (fig. 2b); unfortunately, that photograph was not suitable for publication. Based on these observations, it appears that the crown had propagated toward the flanks of the avalanche in at least one segment on each side, and in both cases away from rocks buried just beneath the snow surface or protruding through it.

Figure 3 shows an area of bed surface and the intersection of the crown and flank fractures of a small avalanche that released on 9 March 2010 near Blue River, BC. It was a relatively thin and soft slab compared to the one in Figure 2. The slab was 0.55 m thick with an average measured density of 128 kg/m3, which corresponds to a porosity of 86.1% and a void ratio of 6.19. Clearly visible are several morphological features on the fracture faces. Ridge- or rib-like features oriented perpendicular to the slope and the slab are found along the crown face in figure 3; however, these lack any systematic sense of curvature. On the flank surface in figure 4 a well developed plumose structure formed near the intersection with the crown fracture and graded down slope into the ridge morphology similar to the crown, transitioning into the ribbed morphology downslope.

Figure 3. Photo showing crown (left) and flank (right) fractures of an avalanche that occurred on 9 March 2010 near Blue River, BC. The slab is approximately 0.55 m thick. Both Plumose and ridge/rib features are visible, including on the flank. This suggests that it occurred with mode-I displacements. Photo: ASARC.

Figure 4a shows a closer view of similar ridge morphology on a different crown surface, viewed from an oblique angle. This avalanche occurred on a small ‘pillow’ of snow (Figure 4b) on 10 February 2009. The slab was 0.5 m thick with an average measured density of 124 kg/m3, with 86.5% porosity. This is an example of fracture features forming in a case with no crown-flank distinction. This is often observed where bed surface curvature is very high and the down slope tension is radial. The fracture morphology is similar to the typical twist- or fringe-hackle, with the depth or offset in the step increasing toward the slab boundary.

3. DISCUSSION AND APPLICATIONS One example of the possible applications of fractography in slab avalanches comes from figure 2. In this case, we can partially reconstruct the release mechanisms in a way not possible without knowledge of the propagation direction of at least some segments of the crown. At both locations that we observed plumose features, their symmetry and sense of curvature suggest that the crown was propagating laterally (no slope-normal component) and away from pre-existing discontinuities in the slab, i.e. protruding rocks. These locations are expected to represent stress concentrations in the slab because of the discontinuity, so it is not surprising that the crown fracture includes those locations; however, a common assumption is that the propagating crown

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fracture seeks or propagates toward stress concentrations in the slab (e.g. Jamieson and Johnston, 1992). Assuming that the propagation directions were consistent within the segments having plumose structures, our observations suggest that the crown fractures nucleated at the protruding rocks, and propagated away from them. Furthermore, this avalanche was likely triggered from low on the slope by the initial, smaller slide, in which case the weak layer fracture would have propagated upslope in most locations, and therefore mostly opposite to the lower parts of the crown. That means that the crown fracture may not necessarily be coupled to the weak layer fracture in cases of confined avalanche release (i.e. limited in extent by rocks or other terrain features), as may be assumed for unconfined slopes (e.g. Jamieson and Johnston, 1992). With more detailed fractographical observations at this location, it may have been possible to identify many other propagation direction indicators. That could have lead to a more comprehensive understanding of how and where the perimeter fractures occurred and how they related to the weak layer fracture, although the relative timing or sequence of these may not be resolvable in the absence of video or photographic evidence captured during the release of the avalanche. More fractography fieldwork is required at fresh avalanches and in seasonal snow in order to understand better how and why particular fracture

features form, and to verify their relationship to fracture propagation.

While there is widespread agreement on the fracture sequence proposed by Perla (1975), the details of the failure in the weak layer are unclear. McClung (1979, 1981) was the first to provide a mode-II fracture mechanics model for weak layer failure, while more recently Heierli (2005), Heierli and Zaiser (2006, 2008), Heierli et al (2008, 2011) suggested that the fracture is mostly a weak layer collapse phenomenon called an anti-crack (Fletcher and Pollard, 1981). Neither are verified for natural slopes nor universally accepted, although the anti-crack models were successful in predicting the outcome of experiment (e.g. Heierli, 2005; Heierli et al, 2008), and are physically plausible given the very high collapsibility of a typical weak snowpack layer. The anti-crack models predicts that the fracture through the thickness of the slab occurs from the surface downward, due to slab bending associated with the collapsing weak layer (e.g. Heierli and Zaiser, 2006), whereas the shear fracture models predict an upward propagating slab fracture due to high strain occurring near the tip of the propagating fracture (e.g. McClung, 1981). Both analytical solutions are based in two dimensions, ignoring the cross-slope direction. In nature, fracture nucleation is probably complicated by snowpack or terrain spatial variability, stress concentrations, etc., although Sommerfeld (1969) did report direct evidence from fracture surface morphology for downward-directed crown fracture propagation. Those observations support the anti-crack propagation model at least in those cases. Of course, more field evidence is required, and given the relatively common occurrence of fractographical features on crown fractures a systematic and targeted campaign at fresh avalanches could yield more observations of the slope-normal component of crown fracture propagation – particularly near its origin – and shed new light on the fracture mechanics and slab kinematics during slab avalanche release.

Slab avalanche mechanics may be similar to that of other rare but often-catastrophic mass wasting events (e.g. Heierli et al, 2008; Quinn et al, 2011), meaning that any insights gained through the study of avalanche fractography may be applicable to other hazards. For example, Quinn et al (2011) recently approached the problem of large landslides in sensitive clay in eastern Canada using traditional shear fracture mechanics, following the example of McClung (1979, 1981) for

Figure 4. Photos of (a) the crown of a small avalanche that occurred on 10 February 2009, showing (b) a highly curved bed surface and no distinct flank fractures. Slab thickness was 0.5 m. Twist-type hackles are recognizable on the crown, with the scale of the step increasing toward the free surface. Photo: ASARC

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slab avalanches in snow. For sensitive clay the concept of progressive failure, rather than retrogression by series of circular slips, is controversial, although neither model is well validated. While Quinn et al.’s (2011) approach applied mode-II brittle failure following strain-softening, no direct evidence prohibits anti-cracks in sensitive clays; these are known to occur in sandstones (e.g. Sternlof et al, 2005) and probably in snow, particularly in cases of propagation on near-horizontal slopes (e.g. Heierli et al, 2008, 2011). Notwithstanding the mode of a basal weak layer failure progressive loss of shear support would generate slope-parallel tension upslope and eventually lead to failure in clay deposits, just as it does in snow slabs (Quinn et al, 2011). Nowhere in the retrogressive models would this occur, as the failure area expands by internal shear failure in the material forming repeated circular slips, only arresting once toe-support develops near the scarp. This means that any evidence for slope-normal mode-I fracture at large earthflows or flowslides in sensitive clays would support the idea of progressive failure. One recent landslide near Gatineau, QC had vertical scarp surfaces, suggesting that slope-parallel tension failure caused them. At that site and at the site of a 2008 landslide we broke in bending several block-shaped hand specimens of desiccated sensitive clay. In each case well-developed, small-scale mirror-mist-hackle and plumose patterns were observed on the fresh fracture surfaces (figure 5). While not conclusive, these observations suggest that recognizable morphologic features may form on fractures in sensitive clay, although it is unknown what role natural moisture and stiffness, etc., may play in their formation. Again, more fieldwork is required.

Unfortunately, the rarity and unpredictable nature of large landslides in sensitive clays make direct evidence difficult to gather. If the mechanics of large landslides are indeed similar to dry snow slab avalanches, then they may be used as a natural proxy, having the advantage of being much more predictable and even artificially triggerable compared to large landslides.

8. CONCLUSIONS Although Sommerfeld (1969) presented several examples of near-origin fracture morphologies, the more remote plumose structures often observed on tensional joint planes in rocks have not been recognized as such in snow. One clear example of a plumose structure on a slab avalanche crown fracture surface is included in this paper; it shows

that the fracture occurred under mode-I opening conditions and propagated away from stress concentrations in the slab near protruding rocks. We also noted that these structures are not ubiquitous, but appear commonly enough to be of use in kinematic or mechanical analysis of the fractures and failures that lead to slab avalanche release. We expect that a focused field campaign of observations at fresh slab avalanches, including detailed photographic or quantitative techniques (e.g. terrestrial LiDAR), could provide excellent direct evidence to validate existing theories or generate new ones for slab avalanche release and snow fracture.

Given the distinctive similarities between seasonal snow and other highly porous geotechnical materials such as sensitive glaciomarine clay, fractography could also be applied to landslide problems in those materials. We observed small scale plumose and other structures on hand-fractured specimens of sensitive clay, suggesting that it may be possible to observe larger-scale features indicative of tension fracture during the release of large landslides. Indeed, recent research suggests that the fracture mechanics in clay slopes may be similar to those leading to avalanche release (e.g. Quinn et al., 2011), which means the snow case could represent an important and readily observable proxy for sensitive clay landslides. We suggest that future field research regarding slab avalanches and sensitive clay landslides consider the potential applications of fractography to solve otherwise intractable problems.

9. ACKNOWLEDGEMENTS The Railway Ground Hazards Research Program (RGRHP), a collaborative effort funded by NSERC, CN Rail, CP Rail, and Transport Canada, supported this research. Bruce Jamieson provided photos.

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