## 1 Introduction  
The comprehensive design of large hydrotechnical structures such as dammed reservoirs requires detailed surveys of the project site [1, 2], including not only environmental and hydrological surveys but also geotechnical and engineering investigations, in order to determine the soil conditions at the future construction site. These investigations may be regulated by law, as exemplified by the Polish Regulation [3]. This regulation specifies the principles for establishing geotechnical foundation conditions for construction projects, classifies planned structures into appropriate geotechnical categories (based on factors such as structure size and soil complexity) and defines the standards [4, 5, 6, 7, 8, 9, 10] for conducting and interpreting laboratory tests. In addition to defining the overall geological structure of the entire reservoir area, the data that the geotechnical documentation must crucially provide for these structures include the identification of the subsoil beneath the dam (and the depth of the load-bearing soils) and the classification of the soils in the reservoir basin in terms of their permeability and suitability for the construction of the dam body embankment (for an earthen dam) or side levees, if any, to adjust the existing terrain morphology [11]. It is therefore essential to characterise the key parameters of the soils available, such as the plasticity index and degree, the filtration coefficient, the natural and optimum moisture content, the organic matter content, the angle of internal friction and the cohesion [12]. Comparable approaches are applied in the petrographic and petrophysical characterisation of natural stones intended for structural use, where understanding their mechanical behaviour is equally essential for ensuring durability and stability in engineering applications [13]. By properly identifying these soil conditions, it is possible to select the most advantageous design variant, ensuring the safety of the structure, i.e., a sufficiently deep dam foundation, the use of appropriately scaled additional structural elements (such as anti-filtration screens) and the use of soils with the best properties to reduce the internal erosion of the embankment, one of the most common causes of dam failures [14]. Recent advances in the estimation of geotechnical parameters, such as cohesion or Young's modulus, using regression analysis and soft computing techniques, significantly support the design of earth structures in complex geological settings [15; 16]. Significantly, having accurate data on the extent and type of deposits in the reservoir basin plays a major role in analysing the economic feasibility of a project. Being able to use available materials, with no need for purchase and transportation, substantially reduces the overall project cost [17]. Here, however, there is a risk that, if insufficient survey data are collected, the volume of soil deposits may be overestimated or that more contamination of the planned construction soil with other fractions may be discovered during excavation. Furthermore, if it is determined during the construction process that the dam body needs to be built at a greater depth than initially assumed, the volume of the dam body will become significantly higher, thus increasing the volume of materials that need to be obtained from deposits or external sources. Similar incidents, which always necessitate design modifications and substantially increase the overall project cost, can lead to delays or even the complete suspension of the construction work where no additional funding can be obtained. In this context, increasing attention is paid to the use of locally available natural stone materials and their classification according to standardised criteria to ensure both economic and structural efficiency. Studies addressing the conformity of such materials with European norms offer useful guidelines for their evaluation and application in engineering practice [18].