engleski

Clay minerals and landslides on the slopes of Medvednica Mt. in Zagreb city area (Croatia)

The landslides problem is pronounced on the slopes of Medvednica Mt. in Zagreb city area. Therefore, during the 2005-2007 period Zagreb city government engaged Croatian Geological Survey to make detailed engineering geological map M 1:5.000 of this area. The landslides problem occurrs in the Miocene and the Pleistocene deposits so those sediments were the main object of investigation. Litological composition of the Miocene deposits varies from marls, limestones, silts, sands and sandstone to rarely represented coarser-grained sediments such as conglomerates and breccias. The Pleistocene sediments which discontinuously cover the older deposits consist of poorly sorted clays, silts, sands and gravels. The aim of this study was to determine relationship between clay mineralogy and occurence of landslides. Apart from standard engineering-geological investigations (SPP tests, pocket penetrometar, wane shire tests, liquid limit and shrinkage limit, grain size distribution, uniaxial compressive strength test), mineralogical investigations of sediments (XRD, DTA, SEM and chemical analysis) were included. It was found that the largest number of landslides are related with Pontian silty-marl deposits (7.1-5.6 Ma) and Pleistocene silty-clay deposits (1.8 Ma – 10.000 years). These deposits contain continuous layers with clay minerals having high to very high and extremely high plasticity. In the Pontian sediments the most common clay minerals are smectite and illite / muscovite, and in some samples small quantities kaolinite and chlorite appear [1] [2]. Vermiculite and illite / muscovite are the most common clay minerals in the Pleistocene deposits. Kaolinite is present in almost all Pleistocene samples [1]. Smectite occurs in very small quantities only in certain samples, while chlorite is present mainly in the sandy and the silty fraction of these deposits. In the wider Zagreb city area landslides are activated mainly during the first spring months, which is associated with content of water in soil, high groundwater levels and increased yield sources. It is known that increasing the amount of water in soil leads to swelling of clay minerals such as vermiculite and smectite. The presence of swelling clay minerals in the Pontian and the Pleistocene depositescausee cause the instability of slopes in the wider Zagreb city area. Differences in levels of slope instability are related to the different amount of expandable clay minerals in sediments and differences in water content in soil that directly influence the behavior of expandable clay minerals. Other factors which have important influence on instability of slopes are tectonics, shear strength, increase of the incline slope and antropogenic influence. [1] Tibljaš, D. & Prohić, E. (2007) unpublished data [2] Grizelj, A. (2008) PhD thesis, University of Zagreb, Croatia. 192. Clay minerals, landslides, Zagreb city area., Miocene, Pleistocene

Clay minerals; landslides; Zagreb city area. Miocene; Pleistocene

nije evidentirano