Tendencje do zmian struktury lessów podczas ich obciążania

Witold Cezariusz Kowalski, Ewa Hoffman



Theoretical considerations show that effective stresses on contact surfaces between grains, particles and their aggregates of loess mineral skeleton are many times greater than conventionally calculated normal stress on the upper surface of the loess as well as any other cohesive soil (Figs 1 and 2). Initial increase of these stresses results in desintegration of aggregates into particular grains and particular grains and particles after resistance of aggregates has been exceeded. Further increase of these stresses brings closer grains and particles of the mineral skeleton, leading, therefore, to their aggregation. The last stage in increase of these stresses, connected with exceeding strength of particular and particles, leads to their crumbling and, thus, increase of percentage content of smaller fractions in grain-size distribution. The essential influence on the loess structure changes has the value of its load as well as the moment of its water saturation. The changes in the aggregate and grain-size distributions point the changes of a loess structure quantitatively. The change tendencies of the aggregate and grain-size distributions under the influence of a load are illustrated on the figures 4 and 5 (in air - dry state), 7 and 8 (after water saturation under load 0,0125 MPa), 6 and 9 (after water saturation under load 0,3 MPa). The studies make possible to draw the following general conclusions: the described loess structure changes can explain the absence of so called typical loesses among fossil, silty soils, older than the Baltic glaciation; the observed structures of cohesive soils, that are or were loaded with a suitable thick overburden, ought not to be discussed as original sedimentary structures; the more exact engineering-geological forecast of the foundation changes under the bigger buildings ought to take into considerations also the possibility of the cohesive soil structure changes; the aggregate and grain-size distributions appear to be the less constant features as it is generally assumed.

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