Experimental analysis of snow micropenetrometer (SMP) cone penetration in homogeneous snow layers

Abstract

The cone penetration test (CPT) is widely used to determine in situ soil and snow characteristics and stratigraphy. For avalanche forecasting, knowledge about snow stratigraphy is of crucial importance. Portable electric cone penetrometers have therefore been developed with the goal of obtaining rapid and accurate measurements of snow stratigraphy. The most widely used electric penetrometer is the snow micropenetrometer (SMP), a constant-speed small-diameter cone penetrometer. The SMP was specifically designed to study snow and is not a penetrometer in the geotechnical sense, as the diameter of the SMP cone (5 mm) is comparable to the typical size of snow grains (0.1 to 1 mm). Previous numerical and experimental studies of the CPT in granular materials have highlighted the importance of material compaction around the cone. Nevertheless, given the high porosity of snow, compaction of failed elements around the SMP cone is generally neglected when interpreting SMP force signals. To verify this assumption, microcomputed tomography and particle image velocimetry were used to investigate the deformation of snow during SMP cone penetration. Results from laboratory experiments with uniform snow show that a compaction zone around the SMP tip develops during penetration. The size of the compaction zone was on average twice as large as the actual size of the cone, increasing with increasing snow density. Furthermore, an average penetration depth of about 40 mm was required for the compaction zone to develop fully. This critical penetration depth roughly decreased with increasing snow density. These results show that the compaction zone around the SMP tip is far from negligible and has to be accounted for when interpreting SMP force measurements.