Weak/soft rocks and rock masses (e.g., fault rocks, turbidites, complex geological units, Block-In-Matrix – BIM rocks, salts and sulphates) represent a challenge in several geoengineering contexts, due to their low strength, high heterogeneity, high proneness to drastic weathering or fracturing processes, and to the fact that they can develop time-dependent and water-interaction-dependent deformations (e.g., creep, swelling, squeezing).
Brittle rocks, on the other hand, require specific attention for the description and quantification of their complicated fracture behavior (e.g., dominant fracture mode, microcrack initiation and crack coalescence). This can be investigated through multiple laboratory techniques, including ultrasonic waves, X-ray tomography, 2-D and 3-D digital image correlation, and acoustic emissions to identify the initiation and progression of micro and macro cracks that form in the rock prior to failure.
Contributions will afford these topics across multiple scales, from/across Angström to basin scales, proposing applications for the stability of natural slopes and seacliffs and for the mitigation of geological risks in engineering projects. Among other applications, session will explore in detail applications related to the energy transition, including carbon capture and storage, subsurface energy storage, geothermal energy, non-carbon gas exploitation (e.g. helium and white hydrogen), wind energy, hydroelectric energy, solar energy and battery storage for smoothing of Intermittent Renewable Energy Sources (IRES).
The session collects contributions that integrate geological observations, investigation surveys, laboratory data and modeling of soft and brittle rocks and rock masses to offer a fruitful discussion about the THMC behaviour of these materials and to explore and foster the contribution of petrophysics and geomechanics in the improvement of sustainable energy and material resources in the transition to low-carbon energy and net zero.
Orals: Thu, 7 May, 08:30–10:15 | Room -2.31
The onset of an earthquake is associated with a nucleation phase, which is necessary to create the conditions for the subsequent dynamic rupture propagation. Theoretical models and laboratory experiments have been proposed to compensate for the lack of direct observations of earthquake nucleation, remaining poorly understood and described through conceptual models. A central and unresolved question is whether and how the nucleation and breakout phases influence the subsequent dynamic rupture propagation and arrest and ultimately determine the final earthquake size. Recent seismological evidence points towards weak determinism between nucleation and final earthquake size.
Here we present the Mechanics of Earthquakes and Extended Ruptures Apparatus (MEERA) - a horizontal multiaxial apparatus designed to nucleate dynamic instabilities on an experimental fault. The extended size of the fault (30 x 5 cm) enables the simulation of rupture propagation under a controlled environment. This provides an opportunity to study physical controls on final rupture size. Surface and along-fault deformation before and during dynamic instabilities are monitored with the help of digital image correlation and fiber optic sensing. In addition, an array of 12 high frequency (10 MHz) acoustic emission sensors record elastic waves radiated from dynamic instabilities. The aim of experiments on MEERA, part of the wider ERC-FORESEEING project, will be to bring observations on natural earthquake data to the scale of laboratory fractures and to understand whether the onset of acoustic emissions signals follows a similar trend with magnitude as observed for small natural earthquakes. We will present preliminary results of experiments showing the emergence of a critical nucleation length for dynamic rupture propagation during experiments and will discuss the implications of these findings for larger scale natural earthquakes, in the context of Earthquake Early Warning applications.
How to cite: Kiss, A., Spagnuolo, E., Cornelio, C., Aretusini, S., Longobardi, V., Cocco, M., Taddeucci, J., and Colombelli, S.: Observations of laboratory earthquake rupture: implications for earthquake early warning, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17668, https://doi.org/10.5194/egusphere-egu26-17668, 2026.
Predicting the geometry of natural fracture networks in the subsurface is a challenging endeavor. In this study, we present a model in which burial-related stress curves are combined with the theory of sub-critical crack growth to investigate the timing of propagation of fractures that constitute the background network, i.e. not genetically related to faults and/or folds.
To predict principal stress orientations and magnitudes as a function of crustal depth, we relate the vertical stress to the density of the overburden, and the horizontal stresses are estimated by calculating the Poisson effect caused by this overburden. In addition, we assume a tectonic stress in the direction of the maximum horizontal stress. We correct the resulting values for the effects of pore fluid pressure.
Based on the orientation and magnitude of the principal stresses at various depths, we calculate the normal and shear stresses on planes ideally oriented for opening and shear fractures (perpendicular to σ3 and at 30 degrees with σ1, parallel to σ2 respectively). Following linear elastic fracture mechanics, the normal and shear stresses are used to compute the stress intensity at fracture tips and estimate the related fracture propagation rate adopting sub-critical crack growth theory.
This simplified model gives insight into the relationship between depth and i) magnitudes of horizontal and vertical stresses, ii) permutations of principal stresses and associated changes of stress regimes and iii) the magnitude of fracture stresses driving fracturing.
We test our model in the Lower Cretaceous limestones of the Geneva Basin, a naturally fractured formation targeted for geothermal exploitation. The burial curve of this formation is marked by two distinct burial phases. The first is in the Late Cretaceous with maximum burial depths of +-500 m. After this, the carbonate rocks have been exhumed to the surface in the Paleogene, followed by deep burial (up to 4000 m) in the foreland of the emerging Alpes in the Miocene. Our model predicts that sub-critical fracture growth only occurred during the latest burial phase, in a reverse and strike-slip regime.
The results are compared with analogue outcrops of the Lower Cretaceous carbonate rocks. Multiple generations of calcite veins from different mountain ranges surrounding the Geneva Basin (Jura, Vuache, Bornes Massif) are sampled for absolute dating with the U/Pb geochronology. The obtained ages confirm a change in stress regime from reverse to strike-slip in the Oligocene to Miocene times
How to cite: Hupkes, J., Bruna, P.-O., Bertotti, G., Nuriel, P., Guillong, M., and Caudroit, J.: When do background fractures form? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18501, https://doi.org/10.5194/egusphere-egu26-18501, 2026.
The roughness of natural fault surfaces means that faults make contact at discrete, high-stress bumps and other geometrical highs, producing extreme spatial heterogeneity in normal stress on the sliding interface. This heterogeneity plays a crucial role in nucleating and arresting earthquake rupture. However, heterogeneity resulting from contact of rough surfaces has not been systematically tested in laboratory experiments, which have previously been restricted to nominally flat surfaces. We investigate how the spacing of macroscopic contact regions controls slip stability by shearing cement blocks designed and manufactured to have prescribed contact spacing, as well as replicas of a natural fault surface. Arrays of hemispherical bumps were manufactured with initial spacings of 22-235 mm. Experiments were conducted in direct shear at normal loads ranging from 1 to 10 kN, resulting in local contact stresses of ~20-120 MPa.
Across 27 experiments, sliding ranged from stable creep to unstable stick-slip behavior. Instability is controlled by the minimum spacing between adjacent contact regions (λc), which evolves during wear. Faults remain stable when λc is less than the critical nucleation length Lc predicted by fracture mechanics (tens to ~180 mm for our measured G, Dc, σ, and Δf). When λc exceeds Lc, stick-slip initiates regardless of overall friction coefficient or surface type (regular hemisphere arrays, random bumps, or natural fault replicas). Increased contact normal stress also promotes instability by reducing Lc. These findings are corroborated by a case study of a single experiment, in which λc increased abruptly upon the loss of a few individual contacts, resulting in the immediate transition from stable to unstable sliding that occurred precisely as λc crossed Lc, independent of changes in contact radius.
Our results demonstrate that the spacing of high-stress contact patches may significantly influence slip stability on faults. Because this spacing length scale can be directly observed on real fault surfaces, it provides a physically grounded predictor of where rupture can nucleate or arrest across scales from hand samples to fault segments.
How to cite: Baumgarte, J., Yakuden, L., and Kirkpatrick, J.: Contact Stress Distribution and Slip Stability on Experimental Faults, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-313, https://doi.org/10.5194/egusphere-egu26-313, 2026.
Reliable characterization of crystalline geothermal reservoirs requires linking rock microstructure to effective physical properties across scales, from pore to reservoir level. Digital rock physics (DRP) provides a promising framework by combining three-dimensional imaging with numerical simulation. However, established DRP workflows for sedimentary rocks are often insufficient when applied to crystalline lithologies. Low porosity, complex mineral intergrowths, fine inclusions, and alteration textures complicate phase identification and introduce biases in predicted elastic, permeability, and thermal properties, limiting established DRP workflows for low-porosity crystalline rocks. This study presents a geologically driven workflow for a granitoid rock sample from the Frontier Observatory for Research in Geothermal Energy (FORGE) site in Utah, USA. High-resolution X-ray computed tomography (XRCT) of cylindrical core plugs (10 mm diameter, 40 mm length) at 6.9 µm voxel resolution provides the basis for digital pore-scale analysis. Multiphase segmentation, i.e., assigning gray-scale intensities in XRCT volume to specific mineral phases, was performed by integrating grayscale-based thresholding techniques with geological constraints derived from thin-section petrography and scanning electron microscopy (SEM). This integrated workflow reduces misclassification caused by overlapping gray-scale intensities, partial-volume effects at phase boundaries, and unresolved microporosity. The resulting segmentation distinguishes pore space, quartz, feldspar, ferromagnesian minerals (amphibole, biotite), titanite, and accessory phases (zircon, opaque oxides, apatite). Initial digital twin analysis shows results that deviate from laboratory measurements for porosity and the determined P- and S-wave velocities. We suspect that assigning completely intact single-crystal properties to the segmented phases may be incorrect, as the microstructure provides clear information about mechanical stresses, e.g., undulatory extinction or mineral alignment. Additionally, the analyzed subvolume (4003 with an edge length of 2.76 mm) does not yet constitute a representative volume element (RVE) relative to the coarse feldspar grain size (1-3 mm). This results in the following challenges for a DRP workflow in relation to crystalline rocks compared to established sedimentary rocks: (1) XRCT scans of larger field of views to encounter the larger minerals within the granitoid sample, (2) assigning reduced elastic properties to the individual segmented mineral phases due to microcracks and fluid inclusions, (3) Preserving high-resolution imaging to resolve the small volumes of porosity (~1.2 %). We present a refined DRP workflow that addresses these challenges through multi-scale imaging strategies and microstructure-informed elastic property assignments, validated against laboratory measurements on FORGE crystalline samples.
How to cite: Keutchafo Kouamo, N.-A., Balcewicz, M., Beiers, L. M., Renner, J., and Saenger, E. H.: A digital rock physics workflow for crystalline reservoirs: Developing digital twins through a geologically driven workflow, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17521, https://doi.org/10.5194/egusphere-egu26-17521, 2026.
The effects of high temperature on the tensile strength and physical properties of rocks were investigated using furnace heating and simulated fire treatments. Four rock types—basalt, granite, limestone, and sandstone—were examined under dry, wet, and saturated conditions (0%, 50–60%, and 100% water content, respectively). Tensile strength was measured before and after heating using the Brazilian test, while changes in porosity, thermal conductivity, mass, and P- and S-wave velocities were also evaluated. Thermal measurements indicate that both heating methods reached maximum temperatures of approximately 600 °C and produced comparable effects on rock properties. Initial water content had a negligible influence on post-treatment tensile strength and physical properties. In contrast, rock lithology strongly controlled the degree of thermal damage. Basalt, characterized by high initial tensile strength, exhibited minor reductions in tensile strength and wave velocities, whereas sandstone showed greater degradation. Granite and limestone exhibited pronounced reductions in P- and S-wave velocities. Rocks with higher thermal conductivity, such as sandstone, experienced larger decreases in thermal conductivity after heating, while basalt showed the smallest change. Conversely, basalt exhibited the greatest increases in porosity and mass loss. Overall, rock lithology and initial mechanical strength are the primary factors governing rock degradation under high-temperature exposure.
How to cite: Nguyen, X.-X., Blahůt, J., Racek, O., Rastjoo, G., Polezhaev, A., and Loche, M.: Assessing Changes in Rock Properties and Tensile Strength due to High Temperature from Laboratory Simulation Studies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18000, https://doi.org/10.5194/egusphere-egu26-18000, 2026.
Compacting granular systems composed of brittle materials not only deform but can also fracture. In these systems, stress transmits across grain contacts and forms force chain networks. A fracture can occur when the heterogeneously distributed stress becomes locally high and critical. Here, we use photoelasticity to visualize the evolution of stress distributions within a uniaxially loaded, homogeneous 2D array of discs made of soda-lime glass, a transparent, isotropic non-crystalline material. We run experiments in water-saturated and nominally dry conditions, through loading, holding and unloading periods. We optically and acoustically (via acoustic emissions) monitor the evolution of stress field, bulk deformation, and crack propagation. Photoelasticity data are analyzed by image recognition and processed to map stress distributions.
Preliminary results show five characteristics that set the fracturing of granular matter apart from continuum solids. First, we can extract the stress transmissions, which, despite the macroscopic homogeneity, show force chains and an inhomogeneous stress field. Second, these stress concentrations lead to a local excess of strength and disc fractures. The birefringence patterns in individual discs are altered by fractures but still carry load. During unloading, the fractures can slip or frictionally lock and the stress acting on them don’t fully relax. Third, unloading and reloading cause cracking before reaching the previous target. Fourth, cracking continues during holding periods in a time-dependent manner; perhaps subcritical crack growth redistributes stresses and thus leads to cascades or spurts of acoustic emission events. Finally, homogeneously highly stressed subdomains of discs develop that confine grains and thus suppress localization and fracturing.
How to cite: Nakagawa, S., Voigtländer, A., Zhang, Y., and Gilbert, B.: Stress evolution within a granular system undergoing subcritical failure: insights from photo-elastic imaging of a 2D glass disc pack, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15924, https://doi.org/10.5194/egusphere-egu26-15924, 2026.
Accurately acquiring elastic and anisotropic parameters is critical for hydrocarbon prediction. Studies have shown that pore and fracture aspect ratios significantly influence the elastic and anisotropic properties of rocks. However, obtaining accurate pore aspect ratio data is extremely difficult and costly, and conventional logging data typically lack this information. Consequently, pore and fracture aspect ratios are generally assumed to be constant based on experience, which does not accurately reflect the actual geological conditions of the formation. To address this limitation, this study proposes a nonlinear petrophysical inversion method based on the Tetragonula Carbonaria Optimization Algorithm (TGCOA), an algorithm inspired by the nest-building and temperature-regulating behavior of tetragonula carbonaria, notable for its structural simplicity and fast convergence. First, a complex fractured-vuggy petrophysical model and inversion objective function are developed by integrating the Xu-White dual-pore model with the Eshelby-Cheng model. Then, constrained by measured acoustic logging data, the TGCOA global optimization algorithm is employed to perform nonlinear petrophysical inversion, solving for the pore and fracture aspect ratios. Finally, these estimated ratios are used as inputs for the petrophysical model to calculate the elastic and anisotropic parameters of rocks. This method comprehensively utilizes various well-logging data to obtain more accurate elastic and anisotropic parameters. Application of the proposed approach to field data in eastern China demonstrates its high computational efficiency and accuracy.
How to cite: Xiong, Z., Yin, X., Ma, Z., and Xiang, W.: Petrophysical inversion of pore and fracture aspect ratios in complex fractured-vuggy reservoirs using TGCOA algorithm, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12392, https://doi.org/10.5194/egusphere-egu26-12392, 2026.
Swelling in anhydritic claystones remains a major tunnelling risk. Although this phenomenon has been widely studied, knowledge gaps persist regarding its swelling behaviour. One of them being the effect of porosity on the volumetric strains that develop during the anhydrite to gypsum transformation. To address this gap, a series of laboratory tests was carried out on artificial specimens made from highly compacted mixtures of anhydrite and kaolin powders. The initial porosity was varied between 0.22 and 0.35, and volumetric strain development was monitored during gypsum formation. The experiments show that transformation-induced strains decrease with increasing initial porosity. The observations further suggest two distinct mechanisms: in more porous specimens, gypsum precipitation occurs largely within the existing pore space, reducing porosity and limiting bulk expansion; in more highly compacted specimens, gypsum growth forces matrix expansion, leading to larger macroscopic swelling. These results are applicable to porous media where crystallisation may occur within pores. Overall, the experimental campaign provides observations and a dataset that can support the development and calibration of coupled chemo-hydro-mechanical models for anhydrite swelling, enabling more realistic predictions of strain development due to gypsum growth in tunnelling applications.
How to cite: Nousiou, A. and Pimentel, E.: Effect of porosity on anhydrite swelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12906, https://doi.org/10.5194/egusphere-egu26-12906, 2026.
The characterisation of tuffaceous soft rocks represents a substantial challenge for slope stability evaluation. The geomechanical behaviour of these materials depends heavily on the microheterogeneity of pyroclastic rocks and varies considerably based on the degree of alteration and water-rock interaction processes.
Tuffaceous lithologies outcrop in a wide variety of areas worldwide, often forming landslide-prone vertical cliffs, both in coastal and continental settings. These instability processes are controlled by progressive rock failure (PRF) mechanisms that govern the enucleation and propagation of fractures, the evolution of which can lead the slope to a state of instability. PRF process can be numerically analysed by adopting hybrid stress-strain numerical solutions able to capture the transition from continuum to discontinuum behaviour. However, these tools require micromechanical input parameters governing the fracture mechanics (e.g., fracture toughness/energy), which are often difficult to calibrate numerically.
This study focuses on the tuffaceous cliffs of Ventotene Island (Italy), highly susceptible to rock-falls and topples and exposed to sea-wave actions, combining laboratory testing and numerical modelling. In the site the mechanisms of progressive fracturing in this type of soft rock and its relationship with environmental forcings are deepened through the design and implementation of the “Ventotene Field Laboratory”. This natural field laboratory, part of EPOS Field-Scale Laboratories, allows the integration of field observation, in situ monitoring data and numerical investigations.
To characterise the mechanical behaviour of the Ventotene tuffs, uniaxial compressive strength (UCS), indirect tensile (Brazilian) and fracture toughness (FT) tests were performed at Aachen Rock Mechanics Laboratory on representative rock samples under both dry and saturated conditions. The laboratory results highlight a strong influence of water content on the mechanical properties of the tuff, with a marked reduction in strength and stiffness under wet conditions. In addition, the thermal properties of the material were also investigated to support thermo-mechanical analyses.
The laboratory test results were used to provide (micro)mechanical input parameters to a FDEM slope numerical model using the Irazu software (Geomechanica Inc.). Overall, the results show that numerical calibration is essential to obtain a tuned parametrisation of tuffaceous soft rocks and to bridge the gap between laboratory-scale measurements and field-scale responses.
The laboratory tests were numerically simulated, and the calibrated parameters have been transferred to a numerical domain representative of the Ventotene sea cliffs. This latter model served to perform accurate slope stability analysis of coastal cliffs, by combining the action of different (marine and environmental) controlling factors.
The calibrated micromechanical parameters also provide a robust basis for future modelling FDEM studies since calibrations of this nature have rarely been conducted on tuffaceous lithologies.
How to cite: Montagnese, M., Feliziani, F., Marmoni, G. M., Grechi, G., Lemaire, E., Hamdi, P., and Martino, S.: FDEM numerical calibration of mechanical properties of tuffaceous rocks for slope stability analysis , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14093, https://doi.org/10.5194/egusphere-egu26-14093, 2026.
The Calita landslide (Northern Apennines, Italy) is a large, complex phenomenon extending over approximately 0.9 km² and characterized by a roto-translational rockslide, in a flysch-dominated area, that evolves downslope into an earth slide–earthflow covering a length of approximately 2.5 km. The earthflow sector involves clay-rich soils derived from a chaotic and strongly tectonized melange partially mixed with the progressively degradated rocky head scarp. Landslides of this type are highly sensitive to hydro-mechanical perturbations, including pore water pressure increases and undrained–drained mechanisms induced by static loading. Moreover, the litho-structural setting controls deep filtration pathways, potentially promoting localized pressurization beneath the sliding surface.
This contribution presents ongoing work aimed at developing a geotechnical model of the Calita landslide and identifying its predisposing and triggering factors. Geological surveys, field instrumentation, laboratory tests have been integrated to characterize the hydro-mechanical behaviour of the landslide body. Direct shear, oedometer, triaxial and water retention tests were performed, allowing derivation of strength parameters, permeability, and pore pressure response under saturated and partially saturated conditions. Mineralogical analyses revealed the presence of gypsum and pyrite along the landslide’s shear surfaces, indicating possible chemo-mechanical weakening mechanisms and enhanced fluid–rock interaction.
GNSS, inclinometer, and piezometric monitoring delineated the spatial variability of displacement and hydraulic pressures, with piezometers recording artesian conditions in some portions of the earth slide-earthflow. DEM of differences were produced on high-resolution Digital Terrain Models obtained during the monitoring years, from 1973 to 2024, allowing to appreciate the areas where displacements occurred and the related mobilized volumes. Finally, numerical analyses were carried out using both finite element (hydro-mechanical) and limit equilibrium approaches to evaluate slope stability under different hydraulic regimes. The results provide a consistent geotechnical framework for future scenario analyses and mitigation planning.
How to cite: Fraccica, A., Bonasera, M., D'Angiò, D., Di Paola, G., Maggi, M., Chiessi, V., Monti, G. M., Pellegrini, F., De Simone, N., and Spagni, R.: Experimental and numerical study of a landslide in a complex geostructural context: the case of Calita (Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21876, https://doi.org/10.5194/egusphere-egu26-21876, 2026.
Posters on site: Wed, 6 May, 14:00–15:45 | Hall X2
Nephrite jade is a monomineralic rock composed of fine-grained interwoven amphibole fibres of the tremolite-actinolite series. This rock has a great cultural importance for many countries due to its combination of beauty, toughness, and high fracture resistance. In New Zealand, nephrite jade (pounamu) is culturally significant to Māori, with a documented history of use in the manufacture of tools, weapons, jewellery, and talismans. Despite this historical importance, there is a lack of systematic quantitative data on the physical and mechanical properties that contribute to its workability and durability.
Traditionally nephrite jade’s suitability for carving has been relying on carvers’ expertise and empirical knowledge. Drawing on the collective experience of Māori artisans, a preliminary survey of pounamu jade carvers (n=20) ranked the quality of a collection of specimens, showing positive correlation between perception of material quality and measured anisotropy in compressional (P-) wave velocity. Current research aims to evaluate the intrinsic origin of this anisotropy and it’s link to mechanical properties through combined microstructural and mechanical analysis.
Electron backscatter diffraction (EBSD) maps of three samples characterised microstructural patterns associated with perceived quality. A sample subjectively rated by carvers as high-quality is noted for homogeneous microstructure with a relatively small grain size (Equivalent diameter on average <8 μm), a weaker crystallographic preferred orientation (CPO) (M-index <0.04), and a lower density of pre-existing microcracks. In contrast, samples identified as poor quality exhibit a stronger CPO (M-indexes 0.14 and 0.09, J-indexes 5.36 and 3.66), particularly within larger grains, greater grain size variability (Equivalent diameter from first μm to 120 μm), and a generally coarser grain size.
Fracture toughness measurements (K1C) were conducted on 3×4×25 mm samples using a universal testing machine equipped with a four-point bending setup, following ISO 6872. Results of these measurements correlate with microstructural observations above. A sample with smaller, more homogeneous grain sizes demonstrates higher fracture toughness. This relationship is consistent with previously described toughening effect related to crack deflection. In nephrite jade, the fracture path is interpreted to be deviated or deflected around the fibres, thereby increasing the effective fracture surface energy. Thus, the coarser microstructures observed in lower-quality samples can contribute to a reduction in this toughening effect, leading to lower fracture resistance. A higher density of pre-existing microcracks observed in coarser-grained samples also leads to lower fracture toughness
Thus, the empirical assessments of nephrite jade quality by carvers correlate with quantifiable microstructural parameters, where a fine-grained, homogeneous fabric with weak CPO promotes crack-deflection toughening and better fracture resistance.
How to cite: Seliutina, N., Palmer, M., Li, K. C., Prior, D. J., Cox, S. C., Mortimer, N., Ford, A., and Kuo, L.-W.: Microstructural controls on the quality and fracture toughness of New Zealand nephrite jade (pounamu), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2630, https://doi.org/10.5194/egusphere-egu26-2630, 2026.
As society works to reduce carbon emissions and phase out fossil fuels, new and sustainable energy sources are increasingly sought. Hydrogen is typically viewed as an energy carrier, but naturally occurring geogenic hydrogen has emerged as a potential primary energy source. Its appeal, namely zero carbon footprint and continuous generation through subsurface processes, is tempered by major uncertainties. Although the mechanisms that produce natural hydrogen are reasonably well understood, successful exploration cases remain rare and its migration pathways and interactions within the subsurface are poorly constrained.
This project investigates hydrogen migration from a likely ophiolitic source along an active, highly segmented strike‑slip fault system in the Neogene Lavanttal Basin (Austria). We combine short‑term and long‑term soil‑gas measurements with subsurface information from the recently completed Koralm railway tunnel and vintage 2D seismic data. The integrated dataset suggests a possible link between elevated near‑surface hydrogen concentrations and structural features such as subsurface faults and surface lineaments. If confirmed, these results would improve our understanding of hydrogen migration in faulted crust and support more reliable site selection for future natural hydrogen exploration and production.
How to cite: Bezwoda, N., Schöpfer, M., Grasemann, B., and Tari, G.: Fault‑Controlled Migration of Geogenic Hydrogen in the Lavanttal Basin (Austria), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5645, https://doi.org/10.5194/egusphere-egu26-5645, 2026.
Natural hydrogen is emerging as a promising sustainable energy source with a negligible carbon footprint. Among the most striking surface indicators of subsurface hydrogen production are “fairy circles”, distinctive, sub‑circular features marked by anomalous vegetation patterns and subtle topographic depressions, often with depth‑to‑diameter ratios as low as 1:100. While previous numerical studies have examined soil‑gas hydrogen anomalies associated with active fairy circles, the mechanism responsible for the observed surface subsidence has remained unclear.
Here, a geomechanical model grounded in soil‑mechanics principles is developed to explain the formation of these depressions. Using coupled simulations of two‑phase flow and volumetric deformation driven by changes in effective stress, the model reproduces surface expressions consistent with those observed in natural hydrogen‑emitting fairy circles. These results provide a physically plausible mechanism for the development of fairy‑circle topography and offer a framework for interpreting surface indicators of subsurface hydrogen generation.
How to cite: Schöpfer, M., Detournay, C., and Tari, G.: The formation of hydrogen-emitting “fairy circle”, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7492, https://doi.org/10.5194/egusphere-egu26-7492, 2026.
The mechanical response of polymineralic rocks is the result of the combination of the individual strengths of all the mineralogical phases that are included in it. But which percentage of a softer mineral is sufficient to soften the mechanical strength of the entire rock? And, on the other hand, which percentage of a stronger material is enough to increase the strength of a soft rock? And do these percentages depend on the microstructure?
Gypsum-anhydrite rocks have the requisites to answer these questions, due to the high mechanical contrast between anhydrite and gypsum mineralogical phases and to the high heterogeneity of microstructures that are usually present even in a single rock mass because of the low temperatures of the phase transition between gypsum and anhydrite.
For these reasons, the present study investigated the strength and creep response under uniaxial compression of Triassic sulphates from the Italian Western Alps. The samples considered may be clustered in three main groups, depending on the occurring microstructural organization of gypsum and anhydrite mineralogical phases: i) pure gypsum, ii) gypsum with relicts of anhydrite at the nuclei of the crystals and iii) anhydrite with gypsum bordering the rims among the crystals.
Results of mechanical tests showed that even a low percentage of anhydrite present at the nuclei of the gypsum crystals strongly controls the mechanical response, causing an increase in the uniaxial strength from 20-25 MPa to 50-70 MPa. On the other hand, the presence of small quantities of gypsum at the rim of anhydrite crystals implies a decrease of mechanical strength of up to 50% with respect to the values expected for pure anhydrite.
Unlike the results about strength, creep strain rate data in gypsum showed a high predictability, suggesting that time-dependent deformation is mainly controlled by mechanisms occurring at the rim of crystals (e.g., pressure solution).
How to cite: Caselle, C., Paschetto, A., Baud, P., Bonetto, S., and Mosca, P.: Mechanical control of soft mineralogical phases on the global strength of the rock: the case of anhydrite and gypsum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9311, https://doi.org/10.5194/egusphere-egu26-9311, 2026.
Flysch rock masses pose significant challenges in geotechnical design due to their pronounced heterogeneity, anisotropy, and susceptibility to disintegration, particularly in deep urban excavations. These characteristics often result in unpredictable slope behavior, requiring a careful combination of geotechnical modelling and practical experience.
This study presents a case of an excavation approximately 11 m high, constructed for a building in the Pujanke area of Split, Croatia, situated in typical Dalmatian flysch. The original design proposed a wide excavation with relatively gentle slopes, based on conservative geotechnical parameters previously validated in similar conditions through the authors’ prior studies. During construction, the designed geometry was not fully respected, and significantly steeper slopes were implemented, leading to localized slope failure. This situation provided a rare opportunity to observe classical disintegration mechanisms, layer interactions, and the influence of flysch heterogeneity on excavation stability, complementing insights from previous research.
Following the collapse, a remediation project was successfully implemented. However, due to subsequent modifications of the excavation geometry and construction conditions, additional design iterations were required on the same slope. These successive redesigns illustrate the core of rework in AEC design, where changes in fundamental assumptions such as geometry, boundary conditions, and construction phasing necessitate repeated reinterpretation of the same geotechnical problem.
A back-analysis of slope stability demonstrated a strong correspondence between previously proposed design parameters and the actual behaviour of the rock mass, confirming their appropriateness and highlighting the critical importance of strict adherence to design assumptions during execution. The study further discusses various technical solutions and their robustness against potential deviations from planned conditions, including minor slope modifications and reinforcement measures.
The results contribute to a better understanding of the behaviour of flysch rock masses in deep excavations and provide practical guidance for safer and more resilient geotechnical design in urban areas with heterogeneous soft rocks, enabling a more stable continuity of design assumptions and a reduction in rework in AEC design.
How to cite: Vlastelica, G. and Salvezani, D.: Geotechnical Design and Rework in Flysch Excavations: A Case Study from Split, Croatia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13985, https://doi.org/10.5194/egusphere-egu26-13985, 2026.
In geological reservoirs, pore fluid chemistry significantly impacts rock strength through mineral dissolution, precipitation, and surface charge modifications. Understanding these interactions is crucial for geological applications such as CO₂ storage and geothermal energy applications, where fluid chemistry controls reservoir integrity, exploitability, and aging.
With increasing depth, porous rocks transition from localized to ductile deformation regimes, with the latter characterized by compactant behavior that drastically reduces permeability and affects reservoir exploitability. The role of fluid chemistry in controlling this transition remains poorly understood and varies with fluid composition, mineralogy, and stress conditions.
We performed triaxial deformation experiments on Tavel limestone (84% calcite, 7.5 % quartz and 6% phyllosilicates,14% porosity) at effective confining pressures of 20 MPa (localised regime) and 100 MPa (ductile regime) under constant strain rate (10⁻⁶ s⁻¹). Samples were tested dry and saturated with: deionized water, 0.01 M HCl solution, CO₂-water solution, CaCO₃-saturated solution, 0.1 M MgCl₂, 6 M NaCl and 0.1 M NaOH solutions. During deformation, we continuously monitored spectral electrical conductivity (0.1 Hz–1 MHz), permeability, and P-S wave velocities. Pore fluid chemistry variations were analyzed using ICP-OES, and post-mortem sample were characterized at SEM.
Results reveal water weakening in the localised regime, while in the ductile regime water or fluid chemistry only marginally affect rock strength. These findings contrast sharply with previous results on sandstone under identical conditions (Lazari et al., 2025), where chemical effects were negligible in the localized regime but caused 30-35% weakening during ductile deformation.
In the localized regime, the presence of Mg+2 or CaCO3 leads to a slight increase of peak stress, while the presence of HCl creates dissolution patterns on the sample, though without altering the mechanical properties of the rock in the observed timescale.
Increased pore connectivity is evidenced by increasing electrical conductivity with deformation, while calcite dissolution is testified by increased Ca+2 concentration in the fluid after deformation.
Our results have critical implications for reservoir management: (1) carbonate integrity in shallow reservoirs is more sensitive to formation water chemistry than siliciclastic rocks; (2) CO₂ injection requires careful assessment and evaluation of long-term processes; and (3) rock-specific understanding of chemical-mechanical coupling is essential—behaviors cannot be extrapolated across lithologies. These findings underscore the importance of accounting for specific rock-fluid interactions in geological reservoir management.
How to cite: Lazari, F., Meyer, G., Pluymakers, A., and Violay, M.: Effects of pore fluid chemistry on localised and ductile deformation of porous rocks., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19502, https://doi.org/10.5194/egusphere-egu26-19502, 2026.
Underground storage facilities are receiving increasing interest for a variety of geo-engineering applications in the realm of renewable and sustainable energy: geothermal systems, carbon capture and storage, nuclear waste repository and hydrogen storage. Particularly for the hydrogen storage case, this gaseous fluid in injected and withdrew cyclically, causing variations of the effective stress field of the reservoir and caprock.
Based on the Terzaghi effective stress law, changing the effective stress can be achieved by either changing the principal stress or the pore pressure. The first option, called cyclic loading and unloading, has been used extensively to study the effects of cyclic conditions on the hydro-mechanical properties of different rock types. However, the actual phenomenon occurring in a reservoir rock is the cyclic variation of pore pressure, or cyclic pressurization. The cyclic flow of pressurized fluids may mobilize particles, which can clog fluid pathways, and trigger chemical reactions such as dissolution. This leads to an alteration of the microstructure of the rock matrix differently from that caused by the cyclic loading and unloading case.
Although cyclic pressurization experiments cannot be run on every rock at the laboratory scale due to poor hydraulic properties, we chose a highly porous and permeable rock, Bentheim sandstone, which guarantee us a pore pressure equilibrium throughout a rock sample during this type of experiment. Apart from its hydraulic properties, Bentheim sandstone is regarded as a conventional georeservoir rock even at great depth, due to its mineral composition, homogeneity, micro- and macrostructure. Therefore, it has been extensively tested for a variety of applications to understand its physical and mechanical properties under changing environmental conditions.
As part of the TEN.efzn project, we performed a series of laboratory experiments on both intact and fractured rock samples, carried out in a servo-controlled triaxial apparatus, capable of simulating in-situ pressure and temperature conditions at relevant depths. By combining mechanical and hydraulic data with acoustic emission and ultrasonic velocity data, we observe that cyclic pressurization leads to higher sample compaction compared to cyclic loading and that the presence of a fracture zone leads to higher changes of the hydro-mechanical properties.
Our results suggest that the values of specific properties obtained during cyclic loading experiments underestimate the real values of reservoir rocks under cyclic fluid injection and withdrawal.
How to cite: Fazio, M., Gottlieb, M., and Sauter, M.: Experimental study on the variation of hydro-mechanical properties of reservoir rocks under cyclic loading and cyclic pressurization , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19936, https://doi.org/10.5194/egusphere-egu26-19936, 2026.
Mode I fracture toughness, KIc, is a key measure of rock strength. In volcanology, KIcis particularly relevant for dike propagation, as it quantifies the critical stress intensity factor required for fracture propagation under tensile stress. KIc has been extensively studied for rock types and construction materials relevant to civil engineering, mining, and hydrocarbon-related applications. However, there is a paucity of data for volcanic rock. In this study, we present KIcvalues for basalt samples of varying porosity and texture from four different lava flows on Mt. Etna (Italy) and investigate the influence of key microstructural parameters on KIc.
We conducted 12 mode I fracture toughness experiments under dry, ambient pressure conditions on Cracked Chevron-notched Brazilian Disc specimens and determined KIc using a standardised method. Additionally, we characterised rock physical properties including porosity, elastic wave velocities, and permeability, and analysed thin sections to determine mineralogical composition and rock texture. We compared the physical and microstructural properties of the four lavas and then assessed those properties regarding any correlations with KIc.
The fracture toughnessmeasurements were successful for 10 of the 12 specimens, yielding KIcvalues of 0.6–1.3 MPa·m1/2. Average connected porosity varied between 9 and 17%. P-wave velocities varied from 3.1 to 3.8 km/s, while permeability varied from 6.7·10-17 to 6.3·10-12 m2.
Our fracture toughness data are consistent with experimental data from the literature, fitting the general trend of high KIc typically corresponding to low porosity. However, within our small data set of rather heterogenous porosity characteristics and rock textures, we observe no strict inverse correlation of KIc,and porosity, since the porosity range of our samples is only moderate and other microstructural factors (e.g. pore size and shape) can dominate fracture behaviour in individual cases. We observe no systematic relationship between elastic wave velocities and KIc.
How to cite: Papanagnou, L., Furst, S., Noël, C., Heap, M. J., Violay, M., and Urlaub, M.: The influence of porosity and microstructure on the fracture toughness of basalts from Mt. Etna: laboratory measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21233, https://doi.org/10.5194/egusphere-egu26-21233, 2026.
Chemical weathering induced by reactive fluid circulation critically affects the mechanical properties of sedimentary rocks involved in subsurface energy applications such as geothermal systems and underground energy storage. This work investigates chemo-mechanical degradation in bonded granular geomaterials through a multiscale approach combining discrete and continuum modeling.
At the grain scale, Discrete Element Method (DEM) simulations are used to study dissolution-driven debonding under oedometric conditions. The evolution of the lateral earth pressure coefficient k0 used as a proxy for stress state, is analyzed as a function of cementation degree, confining pressure, initial stress anisotropy, and loading history. Progressive dissolution leads to convergence toward an attractor stress state, with k0 stabilizing between 0.3 and 0.4 independently of initial conditions. This behavior results from the collapse of cement-stabilized force chains and chemical softening of grains.
At the continuum scale, a phase-field fracture model coupled with damage-enhanced reactive diffusion is developed, informed by micromechanically derived degradation laws from DEM simulations. The model reveals that higher initial cementation delays brittle fracture initiation, while increased acidity may induce a chemical ductilization effect that counter-intuitively postpones fracture due to localized softening ahead of crack tips. The competing effects of chemical softening and degradation of fracture toughness are quantitatively characterized.
How to cite: Rattez, H., Sac-Morane, A., Wu, F., Hu, M., and Veveakis, M.: From grain-scale dissolution to reactive fracture: A multiscale geomechanical study of chemo-mechanical couplings in reservoir rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21311, https://doi.org/10.5194/egusphere-egu26-21311, 2026.
Digital image correlation (DIC) is a powerful tool in lab-scale rock mechanics, yet its application to cylindrical samples is compromised by perspective-induced distortions. These optical effects lead to significant inaccuracies in measuring deformations, especially impacting the reliability of Poisson's ratio calculations. To address this, we developed a specialised preprocessing workflow to rectify raw images before correlation.
The proposed method uses a custom Python script that performs image denoising, camera calibration, and lens distortion correction and an unwrapping algorithm that projects the cylindrical surface onto a 2D plane, effectively "flattening" the sample geometry. This allows standard 2D DIC software, such as NCORR, to process the data without the geometric bias inherent in radial perspectives.
To validate the workflow, results were benchmarked against a 3D-DIC system and physical sensors. Preliminary data shows that our rectification process significantly improves displacement accuracy on lateral surfaces, providing a low-cost yet precise alternative to complex 3D setups. This enhancement is crucial for characterising displacement over the full sample surface where traditional strain gauges are limited. Future work will focus on refining pixel-level interpolation to further minimise noise in high-strain zones.
How to cite: Chalé, A., bu, F., jaboyedoff, M., and derron, M.: Framework for mersuring deformation of cylindrical sample in 2D DIC , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12588, https://doi.org/10.5194/egusphere-egu26-12588, 2026.
Posters virtual: Mon, 4 May, 14:00–18:00 | vPoster spot 1a
EGU26-10911 | Posters virtual | VPS29
Mineralogical Drivers of Ground Failure in Neogene Sediments: a Case Study from Northwest BulgariaMon, 04 May, 14:00–14:03 (CEST) vPoster spot 1a
The stability of critical infrastructure in Northwest Bulgaria (Western Moesian Platform) could be compromised by ground instability within Neogene sediments that cover the region. This is evidenced by the collapse of the I-1 national road near Dimovo town in 2006, which involved vertical displacements of 3–4 meters. The purpose of this study is to identify the underlying geological drivers of this failure and to evaluate the specific hazard in the area resulting from the interaction between the sediments and the local environmental conditions. We hypothesize that the instability is not merely a result of conventional failure mechanisms but is governed by an anomalous mineralogical composition, specifically by the presence of aragonite and gypsum layers, which could create a dual hazard.
To elucidate geological drivers, we employed a methodology that integrates field mapping and sampling with laboratory analyses. Samples from the Neogene sediments in the area of 2006 damage underwent mineralogical analyses using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) to determine phase composition and morphology. These analyses were coupled with X-ray fluorescence (XRF) for chemical profiling and standard geotechnical testing to determine grain size distribution, Atterberg limits, and Activity index according to Skempton’s classification.
The analysis reveals a heterogeneous sediment succession with the presence of inorganic clays with high plasticity. The XRD and SEM results identified a mineralogical anomaly where the high concentrations of metastable, acicular aragonite coexist with active swelling phyllosilicates (smectite/illite). Furthermore, various amounts of gypsum were detected in some of the samples, indicating an evaporitic paleoenvironment. Geotechnically, these materials exhibit extreme reactivity. Liquid limits range from 34.85% to 67.88%, and plasticity indices reach up to 47.39%. The Activity index peaks at 2.00, categorizing the sediments as "highly active" and prone to volume change driven by moisture variations.
The study concludes that ground failure is a direct consequence of a synergistic hydro-chemo-mechanical mechanism driven by the sediments' mineralogy. The specific aragonite fabric allows rapid water infiltration, triggering the hydration of smectites that could lead to loss of shear strength. Simultaneously, gypsum dissolution could create secondary porosity, reduce effective stress, and release sulfate ions, which could pose a potential chemical hazard to concrete foundations through sulfate attack. Furthermore, the high silt content facilitates internal erosion and possible piping through fracture networks, which could explain the sudden loss of support and large vertical displacements observed in the 2006 case. These findings imply that standard geotechnical data alone are insufficient for risk assessment in this region. Effective mitigation strategies must integrate mineralogical analysis to address both the physical swelling and the chemical durability risks.
Acknowledgements: This research was funded by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, Project No BG-RRP-2.004-0008-C01.
How to cite: Dotseva, Z., Vangelov, D., and Stanimirova, T.: Mineralogical Drivers of Ground Failure in Neogene Sediments: a Case Study from Northwest Bulgaria, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10911, https://doi.org/10.5194/egusphere-egu26-10911, 2026.
