Observations and Micromechanical Modeling of the Behavior of Snow/Ice Lenses Under Load in Order to Understand Avalanche Nucleation

The microstructural evolution of snow under a temperature gradient has been of interest for many years since this can lead to persistent weak layers, which is a possible cause of avalanches. Ice crusts can form on top or within a snowpack from a variety of meteorological conditions, including significant melt/freeze or freezing rain events. Once buried, they can persist throughout the entire winter season and act as an ideal sliding surface for dangerous slab avalanches in seasonal mountain snowpacks. These phenomena are important because the number of fatalities from avalanches in the U.S. has increased annually since the 1970s. Avalanches can also have substantial economic impacts due to road closures, the costs of rescue, and building damage. To understand avalanche nucleation, snow and layered snow-ice specimens will be deformed in a cold room while imaging with micro computed tomography. Macroscopic deformation experiments on larger samples at both different rates and different temperatures will also be conducted while imaging with a high-speed video camera. The final deformed microstructures in both cases are imaged at high resolution using a scanning electron microscope, which provides information on both the effects of crystal orientation on deformation while clearly delineating one ice crystal orientation from another. Based on the experimental observations, a multiscale computational model is being built to understand crack initiation, crack propagation, and deformation mechanisms. Three types of experiments are performed and are accompanied by micromechanical modeling. The first experiment performs dynamic loading in an X-ray micro CT of non-homogeneous snow samples in which there are differences in the layering in terms of grain size, type, and bulk density to resolve how these differences influence both the local and global deformation. Second, a X-ray micro CT is used to examine snow specimens containing artificially-created ice lenses oriented either at 90o or 45o to the loading direction. In addition to observing the microstructure as a whole and determining a wide variety of microstructural parameters that can be used to characterize the snow quantitatively, the evolution of individual ice crystals to observe bond formation and bond-breaking in detail are examined. Also, more macroscopic deformation experiments on 10 cm cubes containing an ice lens at either 45 or 90 degrees to the loading direction that are being strained at both different strain rates and different temperatures on a servo-hydraulic testing machine located in a cold room are performed. These specimens are being imaged using a high-speed video camera during loading. The observations of snow under load and subsequent high resolution SEM imaging are being used as input to build robust multiscale computational models that describe the microstructural evolution of the snowpack under load and the microscale deformation and failure mechanisms. The resulting model, which will describe the deformation as function of loading rate and temperature, and include the ductile-to-brittle transition of ice, will be transformative for our understanding of the deformation of snow and for avalanche prediction. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.