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The constant threat from landslides in the northeastern part of Istria, Croatia, calls for the need to apply accurate and reliable methods in landslide hazard assessment in order to prevent landslide damage and to set an early warning system if necessary. Furthermore, landslide susceptibility and hazard assessment enable optimal area management and regional urban planning. The study area is in the northeastern and central part of the Istrian Peninsula, well known as an area of frequent, small and shallow slope instability phenomena. Landslide susceptibility assessment in the area around the city of Buzet was performed using a deterministic landslide susceptibility model in the LS-RAPID software. LS-RAPID was developed to analyze stability at one single location, but the performed analysis has shown that LS-RAPID can be used as a powerful tool in landslide susceptibility and hazard assessment on regional scale. The objective of this paper is to establish the influence of the runout potential on the enlargement of the landslide-susceptible zones, due to expansion of the failure area around the initial failure zone. Performed analysis of rainfall return periods shows the frequency of landslide occurrence and provides the possible correlation with the time component of landslide hazard in the area. [ABSTRACT FROM AUTHOR]

Slaking and weathering of weak rocks result in slope instability. Siltstones from flysch rock masses are highly susceptible to weathering, which causes rapid changes in the geotechnical properties and durability. This study investigated siltstone samples of different weathering grades from flysch rock masses from the Istria Peninsula, Croatia, and determined the effects of weathering on their engineering properties. Laboratory testing of siltstone samples of different weathering grades was conducted to determine the specific gravity, grain size distribution, Atterberg limits, and uniaxial compressive strength. The standardized slake durability index is not sufficient to classify the durability of weak rock masses such as siltstones. Therefore, the durability of siltstone samples of different weathering grades was quantified from the fragment size distribution after each of five slaking cycles. The tested samples were classified based on the disintegration ratio, and the modified disintegration ratio was used to determine potential long-term degradation of the tested samples. The results indicated that weathering has a significant influence on the plasticity, uniaxial compressive strength, and durability characteristics and thus affects the landslides and erosion processes in siltstones in the flysch zone of the Istria Peninsula. [ABSTRACT FROM AUTHOR]

The Krbavčići landslide occurred in January 1979 near the town of Buzet, Croatia, after a long period of heavy rainfall. It is located in Northern Istria in the area built of flysch rock mass where numerous mass movements in the past and recent history have been recorded. A flysch rock mass is highly susceptible to weathering, which leads to material disintegration, changes in geotechnical properties, and shear strength decrease, finally resulting in instability processes in flysch slopes. This paper describes existing information about the Krbavčići landslide occurrence, laboratory testing of siltstone samples from a flysch rock mass, and numerical slope stability analyses of a possible landslide reactivation caused by possible long rainy periods and further weathering of the flysch rock mass. Slope stability analysis using the Rocscience, Slide software, as well as landslide numerical simulations using the LS-Rapid simulation software were performed on the basis of the digital elevation model (DEM) and laboratory test results of siltstones with different weathering grades. A DEM of the Krbavčići landslide was obtained on the basis of the unmanned aerial vehicle (UAV) survey conducted in March 2016. The residual shear strength of siltstones to predict a reactivation of landslides is of highest importance and was determined by ring shear and direct shear tests on siltstone samples with different weathering grades. The results of the numerical simulations show that an increase of the groundwater level in the landslide body in combination with the further weathering of the flysch rock material at the sliding surface would have the main influence on a possible landslide reactivation and the further development of the landslide displacement. [ABSTRACT FROM AUTHOR]