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Verification of mineralogical composition of Kostanjek landslide sediments using a visible and near-infrared (VIS-NIR) spectrometer

One of key point in the investigation and interpretation of sliding and swelling phenomena in nature materials is the role of mineralogical composition on physical and mechanical properties, especially regarding clay minerals. Therefore detailed investigations of the mineralogical composition of Kostanjek landslide sediments were conducted. The Kostanjek landslide is the largest landslide in MedvednicaMt. hilly zone placed in the residential area of the City of Zagreb. It was activated in 1963 due to massive blasting in the Bizek Quarry and excavation in a marl quarry at the foot of the hill. Unlike to all other landslides in Medvednica Mt. hilly zone which are mostly small shallow landslides, the Kostanjek landslide is an atypical, deep-seated large translational landslide with multiple (re)activations of landslide bodies (Ortolan, 1996 ; Mihalić Arbanas et al., 2013). Except sliding, in the area of the Kostanjek landslide there are evidences of subsidence and bulging of ground surface that might be associated with the processes of swelling. This phenomenon is usually noticed after the periods of heavy rain or dry summer. Kostanjek landslide is also one of the pilot areas in the frame of the Croatian – Japanese SATREPS FY2008 joint research project on “Risk Identification and Land – Use Planning for Disaster Mitigation of Landslides and Floods in Croatia” (Krkač et al., 2011). According to historical data, the mineralogical analyses of Kostanjek landslide sediments were performed by Balen (1975) and Slovenec (1989) but only on the samples from the marl quarry. In the frame of scientific activities of the Croatian – Japanese project, one 100 m deep borehole was drilled in the middle part of the Kostanjek landslide in 2012. The core inspection revealed four lithological units as follows: (1) engineering soil of Quaternary age (0 – 10 m), (2) Upper Pannonian massive marls (10 – 45 m), (3) Lower Pannonian marls intercalated with limestone (45 – 60 m) and (4) Sarmatian thin laminated marls (60 – 100 m). For the purpose of mineralogical analyses a total of 17 samples were collected, several from each unit. The samples was analysed by X-ray powder diffraction method on random and oriented mounts after certain treatments. Based on interpretation of X-ray diffraction patterns a quantitative mineral composition (w%) of whole rock was obtained and a semi quantitative content of clay minerals were also determined, among which the most abundant were smectite clays (Martinčević et al., 2012). It is well known that clay minerals have a main role in the processes of swelling, especially those from smectite group (Grim, 1968). To verify the results obtained by x – ray diffraction the method of visible and near – infrared (VIS-NIR) spectroscopy was used. The operating principle of the method is based on sending light onto the sample and then measuring the light which is reflected back from the sample. Frequencies which are absorbed appear as a reduced signal of reflected radiation. The measured wavelength region ranges from 380 – 2500 nm which mean that covers a visible (400-700 nm) and near- infrared (700-2500 nm) regions. The spectral patterns include overtones and combination bands that occur from chemical bonds within soil minerals (Viscarra Rossel et al., 2009 ; Stenberg et al., 2010). Analysis were performed in laboratory, on the above mentioned core samples using a portable Terraspec Explorer spectrometer. The interpretation of results was based only on qualitative description of spectral patterns (Fig. 1). A special attention was given to the samples from the first two units (0-20 m) which are placed in unsaturated zone. Before the scanning, samples were dried on the air and grinded to the fraction less than 2 mm. As a result of the samples drying the effect of natural moisture should be negligible. The most common minerals in the clay fraction are smectite, kaolinite and muscovite. Smectite has two very strong water bends near 1400 and 1900 nm from molecular water and another at 2200 nm. Kaolinite is an aluminium silicate with strong reduced signal near 1400 nm due to OH stretch, and near the 2200 nm due to the combination of OH stretch and Al-OH bend. Samples from first, second and fourth unit have in spectral pattern the large reduced signal near 1400 and 1900 which were associated with molecular water in clay minerals. In spectral pattern of samples from third unit this signal is smaller than in other samples. Based on these results, it can be concluded that, for some reasons, the clay minerals incorporated less water molecules in their crystal lattice. The reduced signals near 2200 nm are constant through the spectral patterns of all samples. According to the other reduced signals a presence of iron oxides and oxyhydroxides (near 900 nm) and calcite component (2330 nm) was identified. As expected, the identification of soil mineral composition by VIS- NIR spectroscopy was in general with good agreement to the results obtained by X-ray diffraction.

VIS-NIR spectroscopy; mineralogical composition; sliding; swelling; Kostanjek landslide

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