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Organic matter in clays often controls their geotechnical behaviour because of its influence on the strength and strain properties in bulk. It is integrated in the clay particle matrix and serves as a weak ductile component to an extent that depends on the degree of decomposition, which is a function of the moisture conditions, temperature and microfauna. Nematodes and arthropods, bacteria and fungi feed on it and make the soil porous, allowing infiltration of air and water. In the final stage of decomposi- tion, the rest product is humus. The function of organic constituents in clay depends on the atomic and molecular bonds within them and between them and the clay minerals. Under moderate deviatoric bulk stress conditions, strain occurs in the molecular scale involving slip when energy barriers, determined by the bond strength, are overcome. Shearing makes the slips accumulate and causes successive changes of the energy bond spectrum, implying that low-energy barriers are used while high-energy barriers become dominant. For low and moderate safety factors, this leads to a blueshift of the spectrum and to stiffening and brittleness. The mineral phase has a certain barrier spectrum, while the organics have their own, implying rapid overcoming of the lower-energy barriers. This results in large shear strain and higher strain rate and lower shear strength and ‘quickness’. Organic clay undergoes creep strain that is similar to that of smectite clay and controls long-term settlement of buildings and the stability of natural and excavated slopes.
clay, creep, energy barrier, humus, illite, microstructure, sensitivity, shear stress/strain, smectite
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