Rock strength assessment in tectonically deformed calcareous rocks integrating Equotip, ultrasound velocity, and geo-structural fracture analysis

In complex geological environments, the analysis of drill cores to determine rock strength can be challenging due to the wide variability in the degree of fracturing, leading to subjectivity in the collection of representative samples for uniaxial compressive strength testing. This study evaluates non-destructive techniques on calcareous rocks with different tectonic deformations, including Equotip hardness, ultrasound P-wave velocity, thin section analysis, and calcimetry, integrated with photogrammetric fracture analysis. The investigated carbonate rock samples are sourced from drill cores derived from the Umbria-Marche fold and thrust belt (northern Apennines, Italy), including a gently dipping limb of an anticline, a hinge zone of an anticline, and a fault zone associated with a thrust. Fracture intensity, quantified by the P21 parameter using photogrammetric techniques on pre-loading rock samples, is assessed alongside macroscopic identification of discontinuities (such as stylolites, veins, and joints) using marker colours to monitor failures during uniaxial compression testing. Empirical correlations depicted by single and multi-linear relationships indicate a strong dependence between the mechanical and physical properties of limestones. Both Equotip and P-wave velocity are influenced by fracture intensity, but P-wave velocity varies significantly with discontinuity orientation, especially at 45°–90°. To refine uniaxial compressive strength predictions and mitigate multicollinearity, statistical approaches, including linear and multilinear regression, Principal Component Analysis and Gaussian Process Regression, were tested. Findings improve the reliability of non-destructive techniques for assessing rock strength in structurally complex settings, with implications for geotechnical applications.