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This paper describes a laboratory investigation of the response of coarse-grained soil particles at their contacts. The tests were carried out in a newly developed micromechanical inter-particle loading apparatus capable of imposing and measuring loads and deflections at the contacts of nonconforming (i.e. non flat-to-flat) surfaces. The apparatus was designed to investigate the behaviour at contacts over a range of load levels from the initial contact stiffness to failure under sliding. The paper presents tests on a variety of particle contacts, investigating the effects of test conditions as well as particle properties and quantifying the particle roughness using an interferometer microscope. The initial tangential load-deflection behaviour is shown to be highly non-linear and predominantly plastic. The stiffness depends on the normal load applied and the particle type, but does not degrade with small numbers of cycles. Shearing led to a significant decrease in the amplitude of the surface roughness of the particles, mostly through the removal of asperities, which shows that the tangential stiffness might not be significantly affected by the amplitude of surface roughness.

3Particle crushing has been recognised to be of key importance for many engineering 4 applications. In soil mechanics, this phenomenon has become crucial in defining a complete 5 framework able to describe the mechanical behaviour of sands. In this study, the effect of 6 multiple discrete contacts on the breakage of a grain was investigated, crushing coarse grains 7 of a quartz sand and a crushed limestone sand between a number of support particles, thereby 8 varying the number of contacts, i.e. the coordination number. The stress at failure was 9 calculated when the particle broke, which was through a number of distinct modes, by 10 chipping, splitting or fragmenting which were observed with the use of high speed 11 microscope camera. The Weibull criterion was applied to calculate the probability of 12 surviving grain crushing and the fracture modes were observed for each configuration of the 13 supporting particles. The data showed that in addition to the number of the contacts the nature 14 of those contacts, controlled by the particle morphology and mineralogy, play a significant 15 role in determining the strength of a particle. The sphericity affected the strength for the 16 softer limestone while the local roundness at the contacts was important for the harder quartz 17 sand. Catastrophic explosive failure was more often observed in particles with harder 18 contacts while softer contacts tended to mould relative to their neighbouring particles 19 inducing a more frequent ductile mode of crushing. 20 21 22

The fronts of two rock glaciers located in South Tyrol (Italian Alps) failed on 13 August 2014, initiating debris flows in their downslope channels. A multimethod approach including climate, meteorological, and ground temperature data analysis, aerial image correlation, as well as geotechnical testing and modeling, led to the reconstruction of the two events. An integrated investigation of static predisposing factors, slowly changing preparatory factors, and potential triggering events shed light on the most likely reasons for such failures. Our results suggest that the occurrence of front destabilization at the two rock glaciers can only partly be explained by the occurrence of heavy rainfall events. Indeed, antecedent hydrological and thermal ground conditions were characterized by a saturated active layer favored by a snow‐rich winter and extensive precipitation in late spring and summer. Also, the rising trend of air temperature during spring and summer months since 1950s might explain the concurrent marked displacement of the two rock glaciers. Indeed, geotechnical investigations have provided strong indications that one of the investigated rock glacier fronts was at a marginally stable state prior to 2014. As rainfall events more intense than the one that occurred in August 2014 were previously recorded in the same area without resulting failures at the studied rock glaciers, we propose that both predisposing and preparatory destabilizing factors have played a key role in the 2014 rock glacier front failures.

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