## 4 Discussion
Considering the complex subsoil structure of the reservoir basin, a borehole may have encountered a local anomaly, inaccurately reflecting the overall deposit structure [46]. By contrast, excavations, where soil is obtained from a hole measuring up to approx. 3 \(\times\) 3 m, involve only a minimal risk of encountering a localised anomaly. Moreover, they allow the percentage of the rock fraction and cobbles in a given layer to be determined with greater accuracy. This means that excavations should be deemed more effective as investigations for assessing the volume of soil deposits that lie relatively close to the surface [47]. Large-scale excavations could, however, be difficult to organise at the project planning stage, given that, at this point, the properties within the reservoir basin are usually not yet owned by the developer and are sometimes still being used by people, for instance, for agricultural purposes. Using heavy equipment and organising large-scale excavations in such an area could give rise to social unrest and generate extra costs related to potential compensations. Nevertheless, given the significance of identifying the volume of deposits available for the construction of earthen dams, it would be advisable to consider more frequently combining both subsoil investigation technologies: boreholes as the foundation of geological surveys and excavations as a supplementary method. It should also be noted that, in engineering geology, the selection of borehole locations is a key element of subsurface investigation planning, directly influencing the quality of the geological model and the risk of misinterpretation. Two contrasting approaches--random placement and representative placement--differ in both purpose and implications. Random selection of locations, used mainly in statistical studies, helps to avoid spatial bias but may fail to capture zones of critical engineering importance (e.g., dam axes, fault zones, karst depressions). In contrast, the representative approach involves deliberate placement of investigation points based on available knowledge of the geological structure, landform, geophysical survey results, and the planned locations of structural loads. In engineering practice, a mixed strategy is generally preferred: a grid of primary boreholes is laid out in a regular pattern (e.g., parallel to the construction axis or along geophysical survey profiles), while additional borehole locations are determined adaptively--in areas of anomalies, facies changes, or suspected voids. Such a strategy increases the representativeness of the results while reducing the risk of systematic spatial errors [48]. According to USACE guidelines (2004), the distribution of boreholes should be adjusted to the complexity of the geological conditions and the level of project risk, with a denser drilling grid recommended in areas of potential discontinuities or known tectonic structures. Particular attention should be paid to areas with heterogeneous subsurface conditions, where point data may not accurately reflect the actual distribution of layers and voids [11]. Despite the availability of modern subsurface investigation methods, a significant number of hydrotechnical projects still experience construction issues due to underestimated geological conditions. A review of the literature and project data highlights several recurring causes of this situation: (1) budget constraints, (2) time and procedural constraints, (3) fragmentation of responsibilities, and (4) technological and interpretative challenges. In engineering practice, expenditures on site investigations are often underestimated relative to actual needs--frequently amounting to <0.5-1% of the total project value, even though institutions such as the World Bank recommend a minimum level of 3-5%, particularly in challenging settings (e.g., karst or tectonically active areas). Underfunding results in overly sparse drilling grids, the omission of geophysical surveys, and oversimplifications in geological modelling [49].