Balázs Vásárhelyi

The rock mechanical behaviour of salt is a less-studied field of engineering geology. Its importance became clear when a surface stream flooded the subsurface salt mine of Praid (Transylvania, Romania). When experts were planning the response to the disaster after the accident in May 2025, they faced the problem that the mechanical properties of the host rock were hardly understood. This study tries to bridge this gap by using block samples from the salt mine. The rock mechanical parameters of the Praid rock salt were analysed using non-destructive tests such as ultrasonic pulse velocity tests. Cylindrical test specimens were prepared, and strength parameters were determined. Uniaxial compressive strength tests, Brazilian indirect tensile strength, and triaxial tests were performed. With the help of these measurements, it was possible to determine the failure criterion of rock salts. The composition and effect of the clay content of the salts on the mechanical properties were also analysed. Our goal was to provide data that could help plan further defence.

The Multiple Failure State (MFS) triaxial test enables several failure points to be obtained from a single specimen through successive loading under increasing confinement, offering substantial time and material savings over conventional tests. Since its introduction by Kovári and Tisa for 50 years ago, numerous modifications—such as the Continuous Failure State (CFS) and strain- or volume-controlled variants—have been developed to enhance test reliability. Despite its efficiency, the method’s applicability to brittle rocks remains debated due to cumulative damage and strain localization effects. This paper reviews fifty years of research on MFS testing, summarizing theoretical foundations, experimental progress, and comparisons with single-stage triaxial tests. The review identifies key parameters influencing test validity and provides recommendations for improving measurement accuracy and reproducibility in modern rock mechanics laboratories.

Limestone and sandstone are widely used in engineering, yet their behavior remains poorly constrained under combined thermal exposure and water saturation. We experimentally evaluate how temperature (22–750 °C) and saturation alter key physical and mechanical properties of these rocks using a consistent test matrix on 136 cores. Mass, density, and ultrasonic pulse velocity (UPV) were measured before and after heating; the strength was quantified by uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS). Both rocks retained capacity up to ~450 °C, with limestone stronger than sandstone. Above this threshold, the strength and UPV declined markedly, reflecting thermal cracking; limestone showed pronounced density loss at high temperature, consistent with mineral decomposition. Saturation reduced strength in both lithologies, with a larger drop in limestone, and changes in density and UPV tracked these trends. The conceptual advance is a side-by-side, cross-property comparison that links thermal and hydraulic states to coupled changes in mass, density, UPV, UCS, and BTS for two common sedimentary rocks. These results provide practical bounds for designing and assessing rock materials and rock-hosted infrastructure exposed to heat and moisture, from tunnels and foundations to fire and geothermal scenarios.

Residual strength is an important factor in rock mechanics and geotechnical engineering, referring to the remaining strength of rocks following considerable deformation or damage. This study investigates the post-failure behavior of granitic rock samples from Bátaapáti, Hungary, using multiple failure triaxial testing and residual strength models. Experimental results show that residual strength is significantly influenced by confining pressure, with values increasing as confinement rises. Key models, including the Hoek-Brown failure criterion, Geological Strength Index (GSI), Residual Strength Index (RSI), and Cohesion-Loss approaches, were evaluated for their ability to predict residual strength. Nonlinear regression of deviator stress versus confining pressure was found. Using this nonlinear model, the relationship between the residual GSI and the confining pressure could be determined. It was carried out that the increasing the confining pressure the residual GSI logarithmically increased. It means, the residual GSI is not a material constant.

The geometry of rock mass fractures is typically characterized through geological and geotechnical investigations. Detailed descriptions of granitic host rock can yield valuable data for constructing fracture network models. However, significant discrepancies often arise between data representing the mechanical and hydraulic properties of rocks. At the study site, fracture geometry data were gathered through surface and underground surveying, borehole logging, and underground mapping. Three-dimensional photogrammetry was utilized alongside traditional rock mass classification methods (Q-system, RMR, GSI) to derive key parameters of fracture networks, such as orientation, size, and intensity. This study focuses on Rock Quality Designation (RQD), a measure of fracture density derived from tunnel face mapping. Findings indicate that variations in fracture frequency are significantly affected by how fracture sets are defined and by the orientation distribution of fractures. Furthermore, using the D parameter (the 2D fractal dimension of fracture frequency) as a validation measure for RQD may lead to misleading interpretations if it aggregates fracture sets on the tunnel scale.

This paper presents the relationship between Rock Mass Quality (Q-system) and the Geological Strength Index (GSI) parameters. Equations are suggested based on field data and calculations of the empirical results of granitic rock masses (Hungary) and siltstones, sandstones and quartzite formations (Australia). Measured and calculated GSI values vs Q values are given for granitic rocks, showing a higher correlation than that of the sedimentary rocks of Australia. The different behaviors of rock masses explain the higher correlation between GSI vs Q and GSI chart vs GSI calculated for igneous rock bodies. Despite the differences in stress fields and the highly tectonised structural geological setting of the granitic rock mass, the isotropic nature of granitic rocks vs. anisotropy of sedimentary rock bodies is reflected in the correlation coefficients.

The accurate analysis of rock cores is of primary importance for designing in and on the rock mass environment. There are several methods for analyzing boreholes, but the most accepted and widely used method is the rock quality designation (RQD) value, which has been a core rating metric for six decades. The RQD value serves as: (1) an important input parameter for rock mass classifications such as RMR and Q; (2) a basis for calculating the Geological Strength Index (GSI) of boreholes; and (3) a key indicator in assessing rock mass quality, particularly in highly fractured or weak rock masses. The original RQD method has several drawbacks and shortcomings, which have led to numerous proposed amendments. This review paper aims to: (1) summarize alternative methods of calculating the RQD value; (2) analyze the sensitivity of different rock mass classifications to the accuracy of this value; and (3) present a systematic analysis of the practical implications of modified RQD methods, emphasizing advancements such as DFN modeling, seismic RQD techniques, and machine learning-based approaches. The findings provide a comprehensive framework for more robust and versatile assessments of rock mass quality.

Middle–Late Miocene clay layers, which occur in several places in Budapest (Hungary), contain varying amounts of sand, with predominance of sand in some cases. In this paper, the impact of this variability on the engineering properties of these clays is investigated, and comprehensive analysis is conducted on clay samples. The results of measurements are presented; in addition to the analysis of plastic soil (i.e., liquid limit, plasticity index), the grain size distribution was also investigated by performing standard geotechnical laboratory tests, including Atterberg limit tests and grain size analyses. Statistical analysis of the results was employed to define correlations between sand contents and both the liquid limit and the plasticity index. It was shown that both the plasticity index and the liquid limit decrease linearly with increasing sand content. This finding aligns with observations reported in the international literature. A general equation was derived to quantify this relationship, setting up a method for better estimation of the plastic properties of similar clay soils based on their sand content and a better understanding of the engineering geological behaviors of clay soils with varying sand content, which as a result have a practical implication for geotechnical engineers.

Tunneling in loose soil and urban areas presents numerous challenges. One effective solution is the use of Earth Pressure Balance Shields (EPBSs). Maintaining the correct balance of pressure at the tunnel face is critical, as applying too little pressure can cause a collapse, while excessive pressure may result in a blow-out. Therefore, a key aspect of using EPBSs in urban environments is determining the optimal pressure required to stabilize the tunnel face, taking into account the existing soil in the excavation chamber and controlling the screw conveyor’s rotation rate. This study focuses on a section of the second line of the Tabriz subway to evaluate the minimum pressure needed for tunnel face stability using empirical, analytical, and numerical approaches. The analytical methods involve evaluating the limit equilibrium of forces and considering soil buckling due to overburden, while the numerical methods employ 3D finite element analysis. Additionally, a sensitivity analysis of the parameters affecting the required pressure was conducted and compared across the three approaches. The results revealed that the formation of a pressure arch mitigates the full impact of overburden pressure on the tunnel face. For soil cohesion values below 20 kPa, the numerical results aligned well with the empirical and analytical findings. For a tunnel depth of 22.5 m and a water table 2 m below the surface, the estimated minimum pressure ranged from 150 to 180 kPa. Moreover, the analytical methods were deemed more suitable for determining the required support pressure at the tunnel face. These methods considered wedge and semi-circular mechanisms as the most probable failure modes. Also, for cohesive ground, the pressure from the finite element analysis was found to be almost always equal to or greater than the values obtained with the analytical solutions.