Continuum numerical modeling of dynamic crack propagation has been a great challenge over the past decade. This is particularly the case for anticracks in porous materials, as reported in sedimentary rocks, deep earthquakes, landslides, and snow avalanches, as material inter-penetration further complicates the problem. Here, on the basis of a new elastoplasticity model for porous cohesive materials and a large strain hybrid Eulerian–Lagrangian numerical method, we accurately reproduced the onset and propagation dynamics of anticracks observed in snow fracture experiments. The key ingredient consists of a modified strain-softening plastic flow rule that captures the complexity of porous materials under mixed-mode loading accounting for the interplay between cohesion loss and volumetric collapse. Our unified model represents a significant step forward as it simulates solid-fluid phase transitions in geomaterials which is of paramount importance to mitigate and forecast gravitational hazards.
In snow, acoustic emissions originate from the breaking of bonds between snow crystals and the formation of cracks. Previous research has shown that acoustic signals emanate from a natural snowpack. The relation between these signals and the stability of the snowpack has thus far remained elusive. Studies on other hazardous gravitational processes suggest that damage accumulation precedes major failure. If increased cracking activity could be detected in snow this might be used for avalanche prediction. We report on the development of a seismic sensor array to continuously monitor acoustic emissions in an avalanche start zone. During three winters, over 1400 sensor days of continuous acoustic data were collected. With the aid of automatic cameras and a microphone the main types of background noise were identified. Seismic signals generated by avalanches were also identified. Spectrograms from seismic signals generated by avalanches exhibit a unique triangular shape unlike any source of background noise, suggesting that automatic detection and classification of events is possible. Furthermore, discriminating between loose-snow and snow-slab avalanches is possible. Thus far we have not identified precursor events for natural dry-snow slab avalanche release. Detailed investigation of one dry-snow slab avalanche showed that signals observed prior to the release originated from background noise or small loose-snow avalanches.
Abstract. Dynamic crack propagation in snow is of key importance
for avalanche release. Nevertheless, it has received very little
experimental attention. With the introduction of the propagation saw test
(PST) in the mid-2000s, a number of studies have used particle tracking
analysis of high-speed video recordings of PST experiments to study crack
propagation processes in snow. However, due to methodological limitations,
these studies have provided limited insight into dynamical processes such as the
evolution of crack speed within a PST or the touchdown distance, i.e. the
length from the crack tip to the trailing point where the slab comes to rest
on the crushed weak layer. To study such dynamical effects, we recorded PST
experiments using a portable high-speed camera with a horizontal resolution
of 1280 pixels at rates of up to 20 000 frames s−1. We then used digital
image correlation (DIC) to derive high-resolution displacement and strain
fields in the slab, weak layer and substrate. The high frame rates enabled
us to calculate time derivatives to obtain velocity and acceleration fields.
We demonstrate the versatility and accuracy of the DIC method by showing
measurements from three PST experiments, resulting in slab fracture, crack
arrest and full propagation. We also present a methodology to determine
relevant characteristics of crack propagation, namely the crack speed
(20–30 m s−1), its temporal evolution along the column and touchdown
distance (2.7 m) within a PST, and the specific fracture energy of the weak
layer (0.3–1.7 J m−2). To estimate the effective elastic modulus of
the slab and weak layer as well as the weak layer specific fracture energy,
we used a recently proposed mechanical model. A comparison to already-established methods showed good agreement. Furthermore, our methodology
provides insight into the three different propagation results found with the
PST and reveals intricate dynamics that are otherwise not accessible.
Abstract. The necessity of characterizing snow through objective,
physically motivated parameters has led to new model formulations and
new measurement techniques. Consequently, essential structural parameters such as density
and specific surface area (for basic characterization) or
mechanical parameters such as the critical crack length (for avalanche
stability characterization) gradually replace the semiempirical
indices acquired from traditional stratigraphy. These advances come
along with new demands and potentials for validation. To this end, we
conducted the RHOSSA field campaign, in reference to density (ρ) and specific surface area (SSA), at the Weissfluhjoch research site
in the Swiss Alps to provide a multi-instrument, multi-resolution
dataset of density, SSA and critical crack length
over the complete winter season of 2015–2016. In this paper, we present
the design of the campaign and a basic analysis of the measurements
alongside predictions from the model SNOWPACK. To bridge between
traditional and new methods, the campaign comprises traditional
profiles, density cutter, IceCube, SnowMicroPen (SMP),
micro-computed-tomography, propagation saw tests and compression
tests. To bridge between different temporal resolutions, the
traditional weekly to biweekly (every 2 weeks, used in this sense throughout the paper) snow pits were complemented by daily
SMP measurements. From the latter, we derived a
recalibration of the statistical retrieval of density and SSA for SMP
version 4 that yields an unprecedented spatiotemporal picture of the seasonal
evolution of density and SSA in a snowpack. Finally, we provide an
intercomparison of measured and modeled estimates of density and SSA
for four characteristic layers over the entire season to demonstrate the
potential of high-temporal-resolution monitoring for snowpack model validation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.