Characterisation of mineral composition and strength parameters of varved clays

Varved clays from the valley of Junikowski Stream (Poznan area, Poland) are investigated in this paper. Such clays are characterised by an anisotropic structure, in a form alternate occurrence of thick summer (light) layers and thin winter (dark) layers. Soils forming the light and dark layers have different granulometric compositions. Light layers are built mainly of clays with silt and clays with silt and sand (siCl, sasiCl) while dark ones are composed mainly of clays (Cl). The light and dark layers were tested separately in a direct shear apparatus. The test results revealed differences in strength parameters for soils forming the light and dark layers. The X-ray diffraction analysis was also used to determine the mineral composition of the clay fraction. It is interesting to note that the set of identified clay minerals does not differ very much in both layers. The dominant mineral is illite with presence of kaolinite, smectite and chlorite. A detailed analysis of northern rock pebbles from the terminal moraine of the Poznan glaciation phase and from the outwash plain, which could have been a sourcing area for varved clays, confirmed the largest content of acid igneous rocks in them. Decomposition of the igneous rocks resulted in the dominant presence of illite in the mineral composition of the studied clays.