This article presents geotechnical reconnaissance data that characterize the formation of 122 new and 49 historical cover-collapse sinkholes within 1.13 km2 area in 12 months following the MW 6.4 earthquake that occurred on 29 December 2020, in Petrinja, Croatia. Data include a geological background, seismic sequence information, sinkhole geometric characteristics, rainfall data, and results of detailed geotechnical subsurface investigation. The sinkhole geometrical features were collected using aerial and satellite imagery, terrestrial lidar, and manual measurements. Soil properties and groundwater levels were obtained from four geotechnical boreholes, accompanied by in situ geotechnical characterization and standard penetration tests (SPTs). Soil parameters were obtained from consolidated undrained conventional triaxial compression, oedometer, soil water retention, and index tests performed on 31 soil samples. Clayey cover, 4–10 m thick, with sporadic gravel lenses overlying cavernous, intensely karstified carbonate rocks, characterizes the sinkhole area. Clays are mostly overconsolidated, with varying degrees of saturation ranging from very small to fully saturated. Seasonal and climate-induced variations in the groundwater table interact with artesian/subartesian karst aquifer, thus affecting the suction and the shear strength. Soil water retention curves (SWRCs) indicate that desaturation is possible for deeper groundwater tables, thus further affecting the effective stress, shear strength, and interparticle tensile forces. Finally, the observed vertical walls that accompanied sinkholes opening can occur in the overconsolidated cohesive cover clay layer with varying degree of saturation. The presented data provide essential geomechanical information necessary to understand the associated sinkhole failure mechanism. This article will help future investigators to perform detailed analyses and provide a background for complementing future sinkhole precursor research. Geotechnical, geological, seismic, and precipitation data generally indicate that the formation of cover-collapse sinkholes in the study area is a consequence of a specific local geological setting but is significantly expedited by earthquake-induced dynamic loading and complemented by multiple hydro-mechanical factors.

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Given the forthcoming need for the construction of a repository for low and intermediate radioactive level waste in the Republic of Croatia, this paper proposes a repository design which is, from a geotechnical point of view, simple, practical and safe, and significantly improved considering current conceptual designs. Existing low and intermediate radioactive level waste repositories are mostly vault-type, near-surface constructions with some kind of covering (top) system of protective layers. However, most of these repositories do not have a bottom protective system, apart from concrete flooring (base). The reasons for such designs include the presumed longevity of the waste packages (containers), which are mostly reinforced concrete and/or steel containers. Considering that the concrete is a material which will, under certain conditions, deteriorate (e.g. dissolution of the cement matrix), and so potentially release radionuclides to the environment, it is essential to design the repository in such a manner that all forms of early release of radionuclides are prevented. The improved conceptual design of low and intermediate radioactive level waste repository presented in this paper is intended to provide an improved containment of radionuclides from waste and ensure the long term safety of the repository. This paper is the first in a series which will cover the basic design of the repository, systems of protective layers and preliminary slope stability analyses.

This paper shows an overview of extensive geotechnical and geological investigation of soils around cover-collapse sinkholes that appeared in a constrained area around Mečenčani and Borojevići villages following the 2020–2021 Petrinja earthquake sequence. A total of 122 new and 49 pre-existing historical sinkholes were recorded, mapped, and classified during the geological and geotechnical reconnaissance fieldwork. Many sinkholes collapsed within an area of only 1.13 km2, a relatively rare phenomenon associated with earthquakes, thus motivating soil investigations to better understand associated failure mechanisms and underlying conditions. This paper shows an overview of triaxial test data in synergy with soil water retention curves of unsaturated soils detected in the area, along with results of standard physical soil tests. The soil in the area consists of a 4–15 m thick clayey cover with sporadic gravel lenses. Clays are mostly over-consolidated, with varying degrees of saturation ranging from very small to fully saturated. Underneath are intensely karstified Miocene carbonate rocks. Seasonal and climate-change-induced variations in the groundwater table interact with the artesian/subartesian karst aquifer, thus affecting the suction and the shear strength. In addition, soil water retention curves indicate that desaturation is possible for deeper groundwater table levels and can further affect effective stress, shear strength, and interparticle tensile forces. Collapsed sinkholes have predominately vertical walls, indicating brittle failure of a cohesive cover with varying degrees of saturation. Based on the specific geomechanical properties of soils, this paper offers several hypotheses of failure mechanisms based on the synergy of earthquake-induced dynamic loading and hydro-mechanical interactions of unsaturated soil layers and pore pressure dynamics between two interconnected aquifers.

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