The thesis deals with laboratory testing of unsaturated soils, with an emphasis on investigation of soil suction phenomenon. Soil suction is referred to as the free energy state of soil water. In geotechnics suction is measured using a variety of methods, with the chief interest in the values of matrix suction. At the Slovenian National Building and Civil Engineering Institute suction was measured using Bishop-Wesley doublewall triaxial cell apparatus and dew point potentiometer, where the measurements were carried out in order to determine soil water characteristic curve in the phase of drying samples. Research was performed on two different types of materials: very stiff sandy silt and on samples of silty clay. Very stiff silt is very rare at construction sites in Slovenia and such areas therefore receive extensive geotechnical research. Especially worrying is the change in strength properties in relation to saturation, since they are prone to crushing or collapse when wet or overloaded. Selected samples were first subject to basic soil classification tests: sieve analysis, measurements of humidity and bulk density. In addition to the measurement of suction in the drying phase, suction measurements were also carried out at the stage soil moistening. Soil water characteristic curve was based on the use of different mathematical models to describe retention curves. The resulting curves were compared to the trial earths and the models used in order to determine which model fits best the measured values. The results showed good fit of retention curves to measured values. Our results also corresponded to the measured values of suction in other research papers and scholarly articles.
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