Effect of short-term thawing on the mechanical properties of frozen glacial tills

       Glacial tills are ubiquitous in periglacial mountains and can be destabilized as the main source materials of glacial debris flows due to atmospheric warming. In general, these surface hillslope materials are internally mixed with debris, ice, fluids etc., where the constituent fluids may experience prolonged freeze-thaw cycles. Although many studies including laboratory tests, field investigations and numerical simulations have been conducted to examine the formation mechanism relating to glacial debris flows in a variety of circumstances, largely unknown mechanisms impel destabilization of loose, frozen, or non-frozen glacial tills on steep slopes. In the present study, a series of simple direct-shear tests were performed to further investigate the shear behavior and strength properties of glacial tills subjected to short-term thawing. The samples with differing water contents and dry densities were firstly frozen under the same period but sheared with varying thawing intervals. The results directly show that (1) the stress-strain curves of all tested samples depict strain-softening characteristic to some extents, but the difference between peak and critical resistance decreases with increase of thawing intervals; (2) the dry density can enhance the shear resistance but the initiation water content may result in the decrease of shear resistance for the relative denser samples; (3) the shear strength profiles manifest that the internal friction angle increases but the cohesion decreases with increase of thawing intervals. These laboratory results suggest that the frozen water content can have measurable effect on the strength properties of glacial tills in shear, and the phase transition process from ice to water may affect the water distribution as a consequence of thawing interval. It should be mentioned that the results preliminarily provide fundamental information regarding shear strength properties of glacial tills by considering short-term thawing effect, and further study will be needed to examine the shear behavior of glacial tills under other potential factors.