Search

In March 2018, a landslide in Hrvatska Kostajnica completely destroyed multiple households. The damage was extensive, and lives were endangered. The question remains: Can it happen again? To enhance the knowledge and understanding of the soil and rock behaviour before, during, and after this geo-hazard event, multi-level sensing technologies in landslide research were applied. Day after the event field mapping and unmanned aerial vehicle (UAV) data were collected with the inspection of available orthophoto and “geo” data. For the landslide, a new geological column was developed with mineralogical and geochemical analyses. The application of differential interferometric synthetic aperture radar (DInSAR) for detecting ground surface displacement was undertaken in order to determine pre-failure behaviour and to give indications about post-failure deformations. In 2020, electrical resistivity tomography (ERT) in the landslide body was undertaken to determine the depth of the landslide surface, and in 2021 ERT measurements in the vicinity of the landslide area were performed to obtain undisturbed material properties. Moreover, in 2021, detailed light detection and ranging (LIDAR) data were acquired for the area. All these different level data sets are being analyzed in order to develop a reliable landslide model as a first step towards answering the aforementioned question. Based on applied multi- level sensing technologies and acquired data, the landslide model is taking shape. However, further detailed research is still recommended.

The Kravarsko settlement area, in northern Croatia, has multiple landslides and damage to buildings and infrastructure caused by landslides. However, actual landslide investigation data for the wider Kravarsko area (pilot area PA1) is relatively sparse and no landslide inventory or typical landslide model exists. The aim of this research was to develop such a landslide inventory by integrating new approaches in geohazard research such as remote landslide mapping from high resolution digital elevation models (DEMs) and current and historical aerial images with existing and new geological data related to landslides. The conclusion is that detailed DEMs are more than adequate for the development of reliable landslide inventories but field checks are still necessary to account for the specific set of natural and man-made conditions found in the research area. The landslide inventory developed for Kravarsko has been field validated in a smaller validation area (VA1) and a typical simplified landslide model for PA1/VA1 was developed. Within the model, sliding is interpreted as complex with multiple generations of sliding and multiple sliding surfaces. Based on the analysis undertaken and the available field data, around 10-20% of urban structures are endangered in the Kravarsko area and anthropogenic activity was determined as an important landslide triggering factor for landslide activation or reactivation. Still the question remains of how to quantify the anthropogenic influence? The developed landslide inventory for PA1/VA1 could be used for local urban planning/development and endangerment assessment/evaluation.

The Croatian Geological Survey (HGI-CGS) is twinning with the world-leading geosciences research institutes, the Geological survey of Denmark and Greenland (GEUS) and the British Geological Survey (BGS-UKRI), ti significantly strengthen HGI-CGS's research, and, in a number of areas, transform it's capability. Within GeoTwinn catrography was utilized in different aspects.

Črešnjevec is one of the largest and long- active landslides in southern hilly zone of Medvednica Mt. that belongs to residential area of the City of Zagreb and where the problem with landslides occurs for decades. The first activity of Črešnjevec was recorded in 1960s and it continues until today. According to MIHALINEC et al. (2002) the landslide body consists of Pliocene – Pleistocene and Upper Miocene sediments with sliding surface at approximately 12 m of depth in zone of contact between two mentioned types of sediments. The aim of this paper is to represent a mineralogical composition of sediments from Črešnjevec landslide to see whether or not a mineralogical composition, especially clay minerals, has an influence on physical and mechanical properties of landslide sediments making them sensitive to sliding. Because of different environmental conditions during the Pleistocene and Upper Miocene periods (KOVAČIĆ & GRIZELJ, 2007 ; GRIZELJ et al., 2017), it was expected that the mineral composition would be also different. Furthermore, it was assumed that samples from shear zone would contain expandable clay minerals and high values of Atterberg limits and external specific surface area. Mineralogical and geotechnical analysis were conducted on 15 samples collected from Črešnjevec 35 m deep borehole drilled in 2016. during the project of Detail Engineering Geological Map of Podsljeme urbanized area M 1:5000 – Phase II (MIKLIN et al., 2016), in which three types of sediments were recognized: (a) heterogeneous Pleistocene sediments, (b) red sandy clays of presumed Upper Miocene- Pliocene age and (c) silts and sandy silts of Upper Miocene age. Atterberg limits were determined according to ASTM D 4318 – 10 (2010) standard proceeding. Residual shear strength parameters was obtained with standard direct shear test procedure. Specific surface area (SSA) was calculated from the Kozeny – Carman equation for hydraulic conductivity (URUMOVIĆ & URUMOVIĆ, 2016). Qualitative mineralogical analyses were performed by X-ray powder diffraction on random mounts of bulk samples and oriented mounts of < 2 μm fraction. Oriented mounts of the < 2 µm fraction were record after the following treatments: a) air drying, b) saturation with K+, c) solvation of K+ saturated mounts with ethylene-glycol, d) saturation with Mg2+, e) solvation of Mg2+ mounts with glycerol, f) heating to 400°C and g) heating to 550°C. Semiquantitative mineral composition of samples was obtained based on interpretation of bulk diffraction patterns and data processing in “RockJock” quantification program (EBERL, 2003). The main differences among all three sediment units are in carbonate and clay minerals content. Namely, heterogeneous Pleistocene sediments consist mostly of vermiculite, feldspars and quartz which are the main minerals in samples. Other mineral components in this unit are kaolinite, micas (illite and muscovite), goethite and amphiboles. Silts and sandy silts of Upper Miocene age consist mainly of quartz and micas in various quantitative ratios. They are in association with feldspars, carbonate minerals (dolomite, calcite), chlorite, smectite, kaolinite and goethite. Red sandy clays consist predominantly of quartz and micas in association with kaolinite, feldspar, goethite and small amount of vermiculite. Three of all samples match a shear zone with sliding surface. One of them is from units of heterogeneous Pleistocene sediments and other two from unit of red clays. They all have high clay content (38-76 %). The following mineral phases were detected in fraction < 2 µm: kaolinite, mica and two types of vermiculite (one with diffraction maximum on 18 Å and other with diffraction maximum on 14.5 Å, after solvation of Mg2+ mounts with glycerol). Furthermore, these samples have the highest values of liquid limits (41-86 %) and plasticity index (22-49 %) as well as SSA (60- 126 m2/g) and very low residual shear strength parameters (cr1=7 kNm-2, cr2= 6 kNm-2, φr1=13.5°, φr2=11.2°). According to obtained results it can be concluded that clay mineralogy, as a result of different sedimentation conditions, obviously had great influence on formation of sliding surface.

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.