Quantifying “Transitional” Soil Behaviour

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

Fabric characterisation in transitional soils

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 (MIP) 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

Multiple contact compression tests on sand particles

Particle crushing has been recognised to be of key importance for many engineering applications. In soil mechanics, this phenomenon has become crucial in defining a complete framework able to describe the mechanical behaviour of sands. In this study, the effect of multiple discrete contacts on the breakage of a grain was investigated, 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 particle broke, which was through a number of distinct modes, by chipping, splitting or fragmenting which were observed with the use of high speed microscope camera. The Weibull criterion was applied to calculate the probability of surviving grain crushing and the fracture modes were observed for each configuration of the supporting particles. The data showed that in addition to the number of the contacts the nature of those contacts, controlled by the particle morphology and mineralogy, play a significant role in determining the strength of a particle. The sphericity affected the strength for the softer limestone 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

A micro-mechanical experimental study of highly/completely decomposed tuff granules

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

Coexpression of Helicobacter pylori's proteins CagA and HspB induces cell proliferation in AGS gastric epithelial cells, independently from the bacterial infection

Adenocarcinoma of the stomach is the second most common cause of cancer mortality in the world. The purpose of this study was to evaluate the potential role in carcinogenesis of two secreted Helicobacter pylori's proteins, CagA and HspB, both shown to increase the risk of gastric carcinoma in patients infected with H. pylori-positive strain. The effects of these two proteins on cell kinetics and the ability to selectively affect the expression of cell cycle-related proteins by transfection of a human gastric epithelial cell line (AGS) were analyzed. Using a genomic library of H. pylori, we isolated and cloned CagA and HspB. The effects of the overexpression of these proteins on cell growth were analyzed in AGS cells by immunoblots, proliferation assay, and flow cytometry. Coexpression of CagA and HspB in AGS cells in the first 48 h caused an increase of the level of E2F transcription factor, cyclin D3, and phosphorylated retinoblastoma protein, all involved in the G1-S checkpoint of the cell cycle Consistently, an increase of cell proliferation, corresponding to an augment of the fraction of the cells in the S-G 2-M phase of the cell cycle, was also demonstrated. Moreover, an increase of c-jun protein levels, but not of c-fos, was also found after coexpression of CagA and HspB. All these data suggest that CagA and HspB, independently from the bacterial infection, have a direct effect on the cell growth of the gastric cells acting on the G1-S checkpoint of the cell cycle