Testing a new shear loading apparatus for in-situ studies of weak snow layers

Here we show preliminary results of shear tests carried out with a new shear test appara- tus designed and constructed by Politecnico di Torino, DISEG (Italy). The ultimate objective of the work is to develop an instrument for in situ measurements of the mechanical properties of weak snow layers. At the present, it is a portable force-controlled apparatus with adjustable shear-loading rate and normal pressure for specimen dimensions 0.16 m × 0.16 m × 0.08 m. The first series of cold laboratory experiments were focused on calibration, experimental methodology, and, in particular, on sintering (conducted at CEN / Meteo-France). As an illustration of the instrument's performance, here we show a set of tests dedicated to sintering effects on snow interface strengthening with time. The protocol of experiments could be briefly described as follows: two snow blocks were placed on top of one another and immediately subjected to rapid horizontal loading at a constant rate of about 19 N s −1 (i.e., effec- tively cutting the sintering time to zero); then the same samples were left for four hours and re-tested; finally, the same procedure was repeated after 16 more hours. For each test, shear displacements and force were recorded with high-frequency gauges. Interfacial strength evolved rapidly under con- stant normal pressure, air temperature and relative humidity (about 0.1 kPa, −10 °C, and 70 %, re- spectively) and corresponded to an increase of failure load by an order of magnitude (about 0.3 kPa h −1 ; that is comparable to field measurements by Birkeland et al., 2006). Following further cold labora- tory testing with various types of snow and, possibly, some technical modifications, we intend to con- tinue the development of the instrument for its future usage in the field at slab avalanche release zones.