Quantifying Changes in Weak Layer Microstructure Associated with Loading Events

Researchers and practitioners have long utilized a variety of penetrometers to investigate the snowpack. Identifying definitive relationships between penetrometer-derived microstructural information and stability has been challenging. The purpose of this study is two-fold: 1. We propose a simple field test that can be implemented by the scientific community to establish relationships between load and penetrometer-derived microstructural strength, 2. Utilizing the SnowMicroPen (SMP) data, we quantify changes in weak layer residual strength and structural dimensions associated with a loading event. Our dataset is from Moonlight Basin, Montana and includes three modified loaded-column tests, each paired with 5 SMP profiles. Depth hoar comprised the targeted weak layer. Results indicate that loading events cause the residual bond strength and bond frequency in large-grained weak layers to decrease significantly. Much like a compression test at a micro-scale, the force required for the SMP to rupture individual bonds as well as the micro-strength decrease significantly when the slab stress is increased by artificially adding blocks of snow. A decrease in observed bond frequency within the weak layer (or an increase in the distance between bonds) also occurs after a loading event, probably because some bonds within the weak layer have already failed or are so close to failing that the penetrometer cannot detect their rupture. Artificial removal of slab stress resulted in greater rupture forces and distances between bonds, likely due to elastic rebound. This indicates that long after a natural loading event has occurred, elastic deformation still exists within the weak layer.