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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.

The last decade has seen an increasing amount of research on so called “transitional” soils that are characterised by incomplete convergence to unique normal compression lines and/or critical state lines in simple laboratory tests. This topic has often provoked reaction, perhaps because some have seen it as a challenge to critical state frameworks of soil behaviour. A particular issue is whether incomplete testing or other test defects might cause such an apparent behaviour. Confusion around the topic has not been helped by the wide range of degrees of convergence seen for different materials and differences seen between convergence in compression and shearing. This paper proposes a unifying means of plotting laboratory test data from such soils that will hopefully provide a rational framework for such discussions, since it makes explicit the degree of convergence towards unique volumetric states for different forms of loading. Data are examined for three “transitional” soils. The results show that bringing about convergence for these soils would require strains beyond those that may easily be applied and that the lack of convergence cannot solely be an artefact of test defects. Plastic volumetric strain was found to cause much faster convergence than plastic shear strain.

In this paper, an experimental micromechanical study is presented investigating the contact mechanics and tribological behaviour of highly/completely decomposed tuff granules. The parent material was taken from two locations—named the top and bottom—from a recent landslide in Hong Kong, and in this study the tested granules were obtained from the parent material after drying and sieving processes. Basic material characterisation was conducted quantifying the particle shape, the surface roughness and the strength of a set of grains. A set of twenty-nine monotonic inter-particle shearing tests were conducted on pairs of granules taken from the top and bottom of the landslide. It was found that the granules had very high friction angles at their contacts, in general greater in comparison with other materials reported in the literature. The slightly greater inter-particle friction for the granules taken from the top of the landslide might be because of their higher roughness in comparison with the ones from the bottom. Additional experiments were conducted to investigate the normal and tangential load–displacement response of the granules subjected to cyclic loading. A good curve fitting for the normal load–displacement response could be obtained by using very low apparent Young’s moduli in the Hertzian model. In general, the decomposed tuff granules showed significant plastic response during the first normal load cycle, and this plastic behaviour continued for the subsequent third and fourth cycles. In the cyclic inter-particle shearing tests, the nonlinearity and hysteresis increased for larger cyclic displacements, but the effect of the number of shearing cycles on the energy loss was generally small. Finally, a limited discussion is presented on the applicability of a theoretical model on the tangential load–displacement behaviour of the granules.

This study reports on the inter-particle coefficient of friction and tangential stiffness at the contacts of completely decomposed volcanic (CDV) granules taken from a landslide in Hong Kong. Micromechanical sliding experiments were conducted using a new generation inter-particle loading apparatus on pairs of granules of about 1.18–2.36 mm in size in a quasi-dry state to explore their tangential load – displacement behaviour. For this purpose, small shearing paths were applied in monotonic mode under nominal normal loads in the range of 0.5–1.0 N. The results demonstrated that the inter-particle coefficient of friction was much higher and the tangential stiffness was much lower than reported values in the literature for quartz sand grain contacts. Repeating shearing tests following the same shearing path showed a notable change of the tangential force – displacement relationship from the first to the second cycle, while subsequent cycles had a relatively small effect. Scanning electron microscope (SEM) images revealed some damage to the surface of the granules on the meso-scale, but no effect on the structure of the grains at the micro-scale was observed. It is believed that the soft nature of the grains and their high surface roughness may have contributed to these trends.

The frictional characteristics of granular materials are of major interest in research and practice in geotechnical and petroleum engineering. In this study, micromechanical sliding experiments were conducted at the contacts of crushed limestone grains in a range of vertical forces from 0.5 to 5.0 N capturing the frictional response during a steady state sliding. This was obtained after the completion of small shearing paths of about 100–300 µm. The results indicated that the dynamic coefficient of friction was slightly lower than that of reported values in the literature on quartz grain contacts. These differences might be, partly, due to the relatively smooth surfaces of the grains of the study. However there were not observed notable differences on the frictional response between surfaces tested in a fairly dry state and surfaces immersed in oil.

Particle crushing has been recognised as being of key importance for many engineering applications. In soil mechanics, this phenomenon has become crucial for defining a complete framework capable of describing the mechanical behaviour of sands. In this study, the effect of multiple discrete contacts on the breakage of grain particles was investigated by crushing coarse grains of a quartz sand and a crushed limestone sand between a number of support particles, thereby varying the number of contacts, i.e., the coordination number. The stress at failure was calculated when the particles broke, which occurred through a number of distinct modes, namely, chipping, splitting or fragmenting, which were observed with the use of a high-speed microscopic camera. The Weibull criterion was applied to calculate the probability of surviving grain crushing, and the fracture modes were observed for each configuration of supporting particles. The data showed that in addition to the number of contacts, the nature of those contacts, controlled by particle morphology and mineralogy, also plays a significant role in determining the strength of a particle. The sphericity affected the strength of the softer limestone sand, while the local roundness at the contacts was important for the harder quartz sand. Catastrophic explosive failure was more often observed in particles with harder contacts, while softer contacts tended to mould relative to their neighbouring particles inducing a more frequent ductile mode of crushing.

The existence of a unique normal compression, NCL, and critical state line, CSL, for clean sands and clays in the state plane, v-lnp’, is generally assumed. Recently, some soils of either complex grading or mixed mineralogy did not exhibit this convergent type of behaviour in the state plane at typical stress and strain levels. This investigation proposes different normalisation techniques for non-convergent soil mixtures consisting of fractally-graded single mineral sands, either quartz or calcium carbonate, with very large amounts of non-plastic fines. For the mixtures, the typical normalisation techniques do not work because a straight NCL, parallel to the CSL, could be defined only in the region of very low specific volumes and extremely high stress levels. Similarly, a normalisation based on an equivalent pressure p’e,CSL taken on a unique straight CSL could not be applied because of the existence of curved and multiple CSLs.

A “transitional” mode of soil behaviour implies that dense and loose samples do not converge towards the same volumes within the strains and stresses applied by simple oedometer and triaxial tests. As this behaviour involves soils with different gradings and mineralogies (e.g. gap graded, well graded and/or mixed mineralogies), identifying the factors responsible is difficult. Nevertheless, it has been previously speculated that strong forms of fabric that are difficult to break down as strains and stresses are applied, might be the common cause. This paper aims at investigating some elements of fabric at the microscale of transitional soils. A gap graded and two well graded mixtures with large amounts of non-plastic fines were investigated by oedometer and triaxial tests. As it would be difficult to identify experimentally many commonly used elements of fabric in these soils, e.g. the contact network, mercury intrusion porosimetry was used as a first step to characterise the evolution of pore size distributions (PSDs) of dense and loose samples undergoing the same stress paths, using the PSDs as a proxy of fabric. Multi-directional bender element testing was performed to confirm the isotropy of the elastic stiffness, from which it might be inferred that the fabric is also isotropic. Statistical parameters of the PSDs were calculated, the changes of which were related to the evolution of macroscale void ratios. The robust fabrics causing lack of convergence were characterised by a complex evolution of the PSDs, the initial differences of which could not be erased during conventional testing. This work also provided a simple method to examine the fabric of particularly well graded or gap graded materials, for which other techniques, such as CT or SEM, could not reveal the multi-scale nature of the fabric.

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 non-conforming (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.