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| In the heart of clay |
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Clays and water: Dangerous liaisons |
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Coveted for their therapeutic properties or maligned for the rampant damage they are capable of causing to both structures and human life, clays remain close to us in so many ways and facets. For the geotechnical engineer, they conjure enigma and remain synonymous with risk and uncertainty. A material that must be described, analysed and dissected down to its finest details to lend optimism for eventually containing its potential danger, and moreover a material for which the term "mastery" would be a hazardous exaggeration.
Research conducted on clay behaviour has been based, among other things, on observations at the particle scale. At present, the most advanced and best-adapted tool for visualising the microscopic structure of these soils is the environmental scanning electron microscope, developed during the 1990's. With this microscope, relative humidity around the observed sample can indeed be "mastered", which eliminates the need to dry samples prior to observation, thus avoiding the risk of modifying their microstructure. But most of all, this technology opens a new field of investigation by making it possible to observe clays at the microscopic scale as the atmosphere around the material becomes either wet (in which case, the clay swells) or dry (the clay contracts). In short, this minor breakthrough has provided a glimmer of hope to better understand the behaviour of such materials.
This page presents a few of the observations gleaned using this device, which is now available to LCPC researchers working at the Paris site.
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Microscopic organisation at various scales within a clay sample |
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At a low level of magnification, an aggregate-type organisation with sizeable voids between the aggregates can be observed. This mode of organisation, which is characteristic of clays, has been identified at all scales along with a notable influence on material behaviour.
At higher magnification powers, the most elementary clay particles may be distinguished.
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Gradual magnifications in a highly plastic clay (with a plasticity index on the order of 60), composed of 60% smectite and 40% illite. The sample was compacted with a limited water content and at low density.
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Effects of wetting within a clayey sand |
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Photographs have been taken sequentially during a wetting-drying cycle in a clayey sand. Some clay packets may be distinguished that "bond" onto a sand grain a few hundred microns in size.
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During the wetting process, water penetrates into the clayey aggregates and, once these aggregates have become saturated, fills the external voids. Water actually appears in the form of a black film that develops as humidity increases and then withdraws during drying
The relative movements of clay packets caused by variations in the hydric state also merit attention. This phenomenon, observed at the microscopic scale and in this instance in small magnitudes, can be expressed as well at a larger scale and constitutes the cause of disorders in structures (residences, civil engineering structures, roads, etc.) located in clayey zones.
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