Damage and progressive failure processes must be considered to understand the time-dependent hydro-mechanical behaviour of fault damage zones and principal slip zones, and their interplay (e.g. earthquakes vs aseismic creep), volcanic systems and slopes (e.g. slow rock slope deformation vs catastrophic rock slides), as well as the response of rock masses to stress perturbations induced by artificial excavations (tunnels, mines) and loading. At the same time, damage processes control the brittle behaviour of the upper crust and are strongly influenced by intrinsic rock properties (strength, fabric, porosity, anisotropy), geological structures and their inherited damage, as well as by the evolving pressure-temperature with increasing depth and by fluid pressure, transport properties and chemistry.
In this session we will bring together researchers from different communities interested in a better understanding of rock deformation and failure processes and consequence, as well as other related rock mechanics topics. We welcome innovative and novel contributions on experimental studies (both in the laboratory and in situ), continuum / micromechanical analytical and numerical modelling, and applications to fault zones, reservoirs, slope instability and landscape evolution, and engineering applications.
Hydrothermal alteration exerts a first-order control on the mechanical behaviour of rocks in deep mining environments by modifying mineralogical composition, grain bonding, and internal structures. In porphyry copper systems, quartz–sericite, potassic and chloritic alteration produces strong contrasts between mechanically competent and weak mineral phases, influencing damage accumulation and failure under cyclic stress conditions. However, experimental constraints on how alteration intensity governs mechanical degradation and acoustic response during cyclic loading remain limited. We investigate the mechanical and acoustic behaviour of rocks exhibiting variable degrees of alteration from a porphyry copper deposit. A total of 57 specimens, classified according to their alteration intensity, were subjected to single-cycle and multi-cycle compression tests under unconfined and confined conditions (σ₃ = 15 MPa). Acoustic emission (AE) monitoring was performed continuously to track microcrack activity and damage evolution during loading. Analysis of the unloading modulus throughout cycles reveals a progressive stiffness degradation that correlates with internal damage accumulation. In general, ‘perfect elasticity’, where loading and unloading gradients converge, is only observed at low stress levels, typically between 20% and 40% of the peak strength. These results contrast with previous studies on more homogeneous rocks, where a broader elastic range were reported. Our findings indicate that beyond 40% threshold, the divergence between loading and unloading moduli increases sharply as a function of cycle accumulation. Samples enriched in softer mineral phases (sericite-rich) exhibit distinct acoustic signatures that reflect a more distributive damage mechanism, whereas quartz- and K-feldspar–dominated rocks, characterized by higher mineral hardness, show a greater damage and microcrack accumulation. This is quantitatively supported by the Felicity Effect analysis: under unconfined conditions, rock dominated by harder mineral phases exhibit lower Felicity Ratio (FR) values, indicating significant pre-peak damage. However, the introduction of a 15 MPa confining pressure leads to a homogenization of the FR across all alteration intensities, as the external stress suppresses micro-cracking regardless of the initial mineralogical heterogeneity. Ongoing analysis explores relationships between alteration degree, mineralogical composition, cyclic damage thresholds, and post-test fracture patterns. By integrating mechanical measurements and acoustic emission data, this work aims to clarify how hydrothermal alteration governs damage accumulation and failure processes in heterogeneous rocks subjected to cyclic stressing, with implications for deep mining stability and induced seismicity.
How to cite: Valdés, F., Clunes, M., Roquer, T., Cortez, J., Garrido, M., Browning, J., and Orellana, L. F.: Alteration-related damage thresholds in cyclically loaded rocks from deep mining environments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15305, https://doi.org/10.5194/egusphere-egu26-15305, 2026.
In the upper crust, rock pore spaces may be occupied by fluids of diverse chemical compositions. Pore spaces can be naturally filled with water, carbon dioxide, oil or gas, or artificially saturated with reactive fluid for geo-engineering purposes, including geothermal energy, wastewater disposal, carbon dioxide or hydrogen storage. The presence of water and other fluids modifies the mechanical strength of porous rocks in both the brittle (i.e., localised) and ductile (i.e., distributed) regimes. According to micromechanical models, the strength of porous rock in the brittle regime is controlled by both frictional parameters and fracture toughness of the material, while inelastic compaction by cataclastic pore collapse is governed exclusively by fracture toughness. Experimental studies indicate that the presence of fluid affects the fracture toughness and static friction of limestones and sandstones. Accordingly, for a given rock type, fluid-induced weakening of the rock strength should be explained entirely by a decrease in fracture toughness and/or frictional parameters.
This interpretation is supported by measurements of the mode-I fracture toughness (KIc) and static friction (µs) of sandstones and limestones, under both dry and water-saturated conditions, which allow for the estimation of the uniaxial compressive strength and quantification of the degree of water-weakening. In this context, we investigate the influence of fluids and fluid composition on the mode-I fracture toughness and frictional strength of Adamswiller sandstone. This sandstone was selected because its mechanical behaviour is well-documented in the literature, and because both fluid presence and fluid composition have been shown to affect its response under uniaxial and triaxial compression. We tested a range of fluid-saturated conditions, including dry, deionised water, 6 mol NaCl solution, 0.1 mol HCl solution and 0.1 mol NaOH solution. For KIc, most of the weakening occurs between dry and fluid-saturated conditions, with additional reductions observed for acidic and basic solutions, with the greatest under basic conditions. For a saline solution, the extent of weakening relative to water-saturated conditions is unclear. In contrast, the measured static and peak friction coefficients are unaffected by either the fluid presence or the fluid composition. Incorporating the measured toughness and frictional strength into micromechanical models (wing crack model and pore collapse model) successfully reproduces fluid-weakening under uniaxial and triaxial conditions. The models capture the effective pressure dependence of fluid-weakening in both the brittle and ductile regimes, reproducing the observed strength variation associated with different fluid compositions. This experimental dataset provides new insight that constrains the micromechanical mechanisms governing porous rock deformation in natural and anthropogenic fluid-saturated environments, with direct implications for the safe exploitation of geo-reservoirs.
How to cite: Noël, C., Baud, P., Lazari, F., Shahin, G., and Violay, M.: The effect of fluid chemistry on sandstone’s fracture toughness and frictional strength: Implications for brittle and ductile strength, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5261, https://doi.org/10.5194/egusphere-egu26-5261, 2026.
Fault zones in clay-rich formations play a critical role in controlling deformation, fluid flow, and stability in both natural and engineered subsurface systems, including earthquake rupture, underground storage, and radioactive waste disposal. However, the coupled hydro-mechanical behavior of natural fault rocks remains poorly constrained due to the difficulty of obtaining representative samples and performing fully coupled laboratory experiments. Here, we present results from petrophysical and fully hydro-mechanically coupled triaxial compression tests on preserved natural fault material from scaly clay sections of the Main Fault intersecting the Opalinus Clay formation at the Mont Terri Underground Research Laboratory, Switzerland.
The hydraulic properties of scaly clay Opalinus Clay were measured using flow-through experiments on back-saturated specimens. Permeability coefficients determined sub-parallel to the orientation of bedding and tectonic shears are up to three orders of magnitude larger than those of the intact rock. The shear experiments were conducted under undrained conditions at different effective confining stresses, allowing direct observation of stress–strain behavior, pore pressure evolution, and effective stress paths up to large axial strains. In contrast to intact Opalinus Clay, the faulted scaly clay exhibits continuous strain hardening without a distinct peak stress or post-peak weakening. Deformation is distributed and accommodated by the reactivation of multiple pre-existing tectonic micro-shear. The shear strength analysis within a Mohr–Coulomb framework reveals that the scaly clay fabric has effectively zero cohesion and a shear strength that is lower than even the residual strength of intact Opalinus Clay. Microstructural observations confirm that deformation proceeds through distributed sliding along an anastomosing network of polished micro-shears surrounding undeformed microlithons.
These results demonstrate that inherited fault-zone fabric exerts a first-order control on both mechanical strength and hydro-mechanical coupling in clay-rich faults. Incorporating fabric-and stress-dependent behavior as well as critical-state deformation into constitutive models is therefore essential for realistic predictions of fault reactivation, pore pressure evolution, and long-term stability of low-permeability clay formations.
How to cite: Winhausen, L., Ziegler, M., and Amann, F.: The hydro-mechanical coupling, reduction of effective strength, and critical state shearing of faults: Evidence from laboratory testing on natural fault rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13046, https://doi.org/10.5194/egusphere-egu26-13046, 2026.
El Teniente, located in central Chile, is the largest underground copper mine in the world, with ore extraction reaching depths of up to one thousand meters below the surface. The primary ore body is hosted in volcanic rocks of basic composition, extensively intersected by a stockwork of closely spaced veins. At present, the combination of the regional stress regime, localized stress concentrations around excavations, and the heterogeneous nature of the veined rock mass poses significant challenges to operational safety, particularly when considering an extension of the mine’s productive life. Discontinuities, ranging from large-scale faults to small-scale veins, are closely linked to failure processes in mine excavations. In highly homogeneous, intact rock, spalling failure in tunnels is typically initiated at about 50% of the rock’s uniaxial compressive strength (UCS). This relationship is not consistently observed in veined rocks, where standard UCS tests often fail to account for the mechanical influence of discontinuities during deformation. In this study, an experimental setup that integrates four strain gauges, acoustic emission (AE) monitoring, and digital image correlation (DIC) during uniaxial compressive strength (UCS) testing was developed. The method was applied to 20 mm-diameter cylindrical rock specimens from El Teniente to investigate the role of sulfide-rich veins in rock deformation. Samples were selected to minimize the occurrence of multiple veins, containing instead a single primary vein (< 4 mm thick) oriented between 0° and 90° relative to the axial loading direction. Particular emphasis was placed on strain partitioning, with DIC employed to obtain full-field strain measurements, enabling the quantification of strain differences between the rock matrix and the veins. Experimental results indicated that veins accommodated greater strains than the surrounding rock matrix during both the elastic and plastic regimes. All the samples shown a rotation of the local stress tensor on the vein when reaching the onset of dilatancy. Veins oriented between 0 to 40° yielded before the bulk material, with yield onset occurring at 50-60% of the UCS. These findings suggest that precursory shear strain within favorably oriented veins, evidenced by the onset of AE activity, dilatancy and DIC, may play a critical role in initiating rockmass failure at excavation boundaries.
How to cite: Robbiano, F., Violay, M., Orellana, L. F., Guggisberg, A., and Heinkel, E.: The role of veinlets in the unconfined behavior of El Teniente Mine rock samples: Implications for mining-induced rockmass failure., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19556, https://doi.org/10.5194/egusphere-egu26-19556, 2026.
How to cite: Chandler, M., Li, X., Cartwright-Taylor, A., Butler, I., Freitas, D., Woldemichael, B., Liptak, A., Atwood, R., Main, I., Mangriotis, M.-D., Curtis, A., Fusseis, F., and Chapman, M.: Grain-scale 4D visualisation of strain partitioning during brittle creep in sandstone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3514, https://doi.org/10.5194/egusphere-egu26-3514, 2026.
Understanding how water influences slow fracture growth in rocks remains a major gap in our ability to predict time-dependent failure. In particular, it is still unclear how moisture-related weakening push a subcritically stressed rock from stable deformation into sudden collapse.
In this study, we investigate how hygroscopic weakening—caused by liquid water entering a notch—affects the creep behavior of a clastic rock loaded below its short-term strength. Using a sandstone beam (400 mm×90 mm×90 mm) in an inverted three-point bending setup, we first load the sample to about 67% of its failure strength more than 5 days, then introduce a controlled water drip directly into the notch.
We track the fracture response using digital image correlation, ultrasonic transmission, acoustic emission, and crack-opening measurements. The results show two distinct stages after water arrives:
- a rapid increase in crack opening and loss of stiffness, consistent with moisture-driven softening; and
- a slower but sustained rise in microcracking activity, leading to accelerated creep and, in some cases, catastrophic failure.
In contrast, identical dry beams remain stable over several days, confirming that water—not load alone—initiates the transition to instability.
These findings demonstrate that even small amounts of liquid water can sharply alter the long-term mechanical stability of brittle stressed rocks. This work highlights a potential pathway through which more frequent or intense wetting events could increase the likelihood of sudden rock failure in natural and engineered settings.
How to cite: Wu, R., Kang, H., Gao, F., Peng, X., Dong, S., Zhao, C., Li, B., Leith, K., Lei, Q., and Selvadurai, P.: Hygroscopic weakening accelerates the transition to catastrophic failure during brittle creep in clastic rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-317, https://doi.org/10.5194/egusphere-egu26-317, 2026.
Volcanic rock can be subjected to high and fluctuating pressures and stresses associated with volcanic activity and geothermal production. When subject to a high constant stress in the brittle regime (shallow depths), strain and porosity increase as a function of time, eventually leading to macroscopic failure—a process called brittle creep. In the ductile regime (deep depths), rare experiments have shown that strain accumulates and porosity decreases at a constant stress. During this process—called compaction creep—the rates of strain accumulation and porosity reduction decrease as a function of time. Here, we performed triaxial constant strain rate experiments and triaxial compaction creep experiments on three samples of tuff sampled from boreholes drilled into Krafla volcano (Iceland). The tuffs differ in terms of their source depth (~395, ~505, and ~690 m), macroscopic texture (grain size and distribution), and mineral content (different quantities of clay minerals/chlorite). The connected porosities of the tuffs, however, are very similar (0.29–0.35). We first performed X-ray computed tomography on each tuff in order to provide a quantitative description of their microstructure (grain size and distribution, and pore size, distribution, shape, and orientation). Triaxial constant strain rate experiments were then performed at different effective pressures to map out the yield cap for each tuff. Finally, triaxial compaction creep experiments were performed at effective pressures corresponding to the same position on the yield cap for each tuff. The constant differential stress used in these experiments was selected as the same proportion between the onset of inelastic compaction and the inflection point in the stress-strain curve from the constant strain rate experiment performed at the same effective pressure. All three tuffs accumulated strain and lost porosity as a function of time under a constant stress, although the rates of strain accumulation and porosity reduction, and therefore the maximum strain and porosity loss achieved at the end of the experiment, were different. For example, the porosity loss at the end of the experiments (after 100 hours) for the three tuffs was 0.014, 0.015, and 0.023. Because the connected porosity of the three tuffs is the same, differences in their compaction creep behaviour can be explained by differences in their microstructure and mineral content. The time-dependent compaction of porous volcanic rocks, demonstrated here for tuffs, has implications for volcano stability and geothermal production.
How to cite: Heap, M., Bayramov, K., Baud, P., and Mortensen, A.: Time-dependent compaction creep in tuffs from Krafla volcano (Iceland), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5089, https://doi.org/10.5194/egusphere-egu26-5089, 2026.
Salt caverns are formed by solution mining and may be used for energy storage purposes after the production phase. During the operational lifetime, and in particular during abandonment, the spatial and temporal distribution of stress changes and deformation around caverns leads to convergence of salt around the caverns. In turn, this may lead to surface subsidence and potentially affect cavern integrity. Deformation around salt caverns is governed by different creep mechanisms, encompassing transient and steady-state creep stages. Steady-state creep is governed by a combination of dislocation and diffusion creep mechanisms. Deformation due to dislocation creep is dominant at relatively high stresses, whereas grain size-sensitive diffusion creep, particularly pressure solution, contribute significantly at low stresses. When modelling rock salt behaviour, these mechanisms need to be properly accounted for. Recently, studies have suggested that a threshold differential stress may exist below which pressure solution does not take place, which needs to be accounted for. In addition, dynamic recrystallisation may take place through grain boundary migration, driven by differences in energy stored in neighbouring grains due to dislocation creep strain. The process of grain boundary migration reduces the (work hardening) energy in the system as old grains are consumed by new ones, causing weakening. Furthermore, incorporation of transient creep is generally based on the description given in the Munson and Dawson model.
In this study, we aim to simulate different cavern operation phases, such as leaching, production, and abandonment, to analyse the effect of transient creep, pressure solution creep and its threshold stress on the stress and deformation evolution around the cavern. The coupled effects of these complex creep characteristics on cavern behaviour have not yet been studied in detail. Such, more extensive coupling, are needed to better align laboratory- and field-based observations of salt mechanical behaviour, and apply it to large-scale numerical models. The commercial mechanical simulator FLAC (Fast Lagrangian analysis of continua) has been used to develop a 2D model for a single cavern system, which can be used to examine cavern convergence, subsidence and cavern integrity. An empirical model is used to define the threshold strain limit for dynamic recrystallisation by grain boundary migration, analogous to the Munson-Dawson strain limits for transient creep.
The results show a significant effect of pressure solution creep on stress and deformation behaviour around the cavern. In the production phase, the transient creep does not show any significant effect on cavern behaviour; however, it could be important under varying loading conditions. The extent and magnitude of convergence and subsidence are dependent on the rate of pressure solution creep and its threshold stress. A preliminary analysis of the onset of the dynamic recrystallisation around the cavern suggests that DRX may be active in the lower regions of the cavern.
How to cite: Jain, G., Wassing, B., Hangx, S., Ter Heege, J., and de Bresser, H.: Impact of creep mechanisms on stress and deformation behaviour around salt caverns, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17519, https://doi.org/10.5194/egusphere-egu26-17519, 2026.
Rock fracturing driven by temperature fluctuations, rainfall, and freeze–thaw cycles governs both long-term landscape evolution and the onset of rock-slope instabilities. However, the thermo-mechanical stress field that develops within the first decimeters of exposed rock—and its role in sub-critical crack growth—remains poorly constrained, largely because it cannot be directly observed at relevant spatial and temporal scales in-situ.
We present a new dense ultrasonic monitoring experiment designed to image near-surface stress, rigidity, and damage in an unstable limestone cliff. The system consists of more than 50 permanently installed ultrasonic transducers deployed over a 4 m² area on a 50-m-high limestone pillar located in the foothills of Larzac, southern France. Half of the sensors operate as emitters and half as receivers, allowing repeated, highly redundant measurements of travel times and waveforms across hundreds of ray paths. Using acousto-elasticity, temporal changes in ultrasonic velocity provide a quantitative proxy for stress and crack evolution, while waveform decorrelation enable tracking of micro-damage and scattering.
The high spatial density of the array enables 2-D and potentially 3-D tomographic imaging of stiffness and damage within the rock surface layer, resolving gradients that are invisible to sparse instrumentation or bulk resonance methods. First results reveal pronounced diurnal velocity variations that correlate with surface temperature and solar radiation, indicating strong thermo-elastic control on near-surface stress and fracture opening.
This new monitoring approach opens the door to direct, time-lapse imaging of climate-driven damage in rock slopes, providing a critical link between environmental forcing, sub-critical cracking, and the progressive weakening that precedes rockfall and cliff collapse.
This work was funded by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project.
How to cite: Rolland, A., Rousseau, R., Bontemps, N., Starke, J., Moreau, L., Baillet, L., and Larose, E.: Dense ultrasonic imaging of thermo-mechanical stress changes in a limestone cliff, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17417, https://doi.org/10.5194/egusphere-egu26-17417, 2026.
The behaviour of slowly moving rock slope instabilities in alpine regions is governed by the interaction between inherited structural discontinuities and externally imposed environmental forcing, yet the mechanisms linking these controls across scales remain poorly constrained due to the lack of continuous subsurface observations. Rock slope toppling represents a typical form of such structure-controlled deformation. Here, we present a comprehensive investigation of a structurally complex toppling rock slope in pre-Variscan metamorphic units in the Bedretto Valley (Swiss Alps). This landslide is intersected by the unlined Bedretto Tunnel over a length of approximately 450 m. The presence of the tunnel and the associated facilities of the Bedretto Underground Laboratory provides a unique opportunity to resolve spatially variable deformation.
To understand the controls on the motion of this landslide, we installed a multi-parameter monitoring network, which integrates surface and subsurface measurements. It combines meteorological stations covering the toppling crown and toe, sectional groundwater pressure monitoring, and a 2-km-long distributed fiber optic sensing (DFOS) cable anchored along the Bedretto Tunnel. This configuration provides a unique internal view of deformation within the rockmass, enabling continuous, decimeter-scale observations of microstrain and temperature beneath up to 1500 m of overburden.
The measurements reveal two distinct deformation responses of the landslide. First, reversible, centimeter-scale strain oscillations correlate with surface temperature fluctuations but exhibit anomalously high amplitudes and penetration depths, which cannot be explained solely by conductive heat transfer. This points to a non-local thermoelastic response, whereby far-field thermal stresses are generated and anisotropically transmitted through the fracture network within the rockmass. Superimposed on this cyclic thermoelastic background, the data reveal discrete, irreversible strain steps near critical fracture zones. These steps temporally coincide with major seasonal hydrologic events including sustained snowmelt and intense rainfall when a two-layer bucket model predicts corresponding peaks in groundwater storage and pressure transients. This correlation provides direct evidence for hydro-mechanically driven, progressive damage within the fracture network of the slope.
Multivariate decomposition of the deformation time series isolates not only the dominant thermo-hydraulically driven cyclic signal, but also residual components characterized by spatially variable, and locally opposing, monotonic strain trends. These opposing trends are partially explained by the mechanical and geometrical heterogeneity of the fracture network and reveal the accumulation of progressive inelastic deformation. Beyond direct thermal or hydraulic forcing, such components suggest a creep-like weakening mechanism of the rockmass under quasi-static gravitational stress.
These findings reveal the dynamics of a coupled thermo-hydro-mechanical system, in which seasonal forcing drives both reversible deformation and irreversible damage. The study thus highlights the critical role of discontinuities in controlling slope behavior, showing how transient hydrology and thermal cycling progressively degrade rockmass strength along pre-existing fractures and joints, ultimately weakening large rock slope failures.
How to cite: Yuan, M., Hirschberg, J., Oestreicher, N., de Palézieux, L., and Aaron, J.: Resolving Thermo-Hydro-Mechanical Coupling and Progressive Damage in a Metastable Toppling Rock Slope using Integrated Fiber-Optic Monitoring, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13221, https://doi.org/10.5194/egusphere-egu26-13221, 2026.
Understanding the mechanical behavior of porous carbonate rocks is critical for improving reservoir management and developing sustainable energy solutions. Carbonate rocks are formed through complex sedimentary processes and diagenesis, leading to significant microstructural variability at multiple scales, which influences their mechanical properties. This complexity necessitates advanced experimental tools to accurately describe their behavior under various stress conditions.
Recent studies have demonstrated that strain is accommodated heterogeneously in porous sedimentary rocks, such as sandstones and limestones. Precisely, formation of deformation bands has been observed under various loading conditions in porous limestones which can significantly affect the capacity, i.e., porosity, and thus permeability, of carbonate reservoirs. The inherent multi-scale nature of carbonate microstructure and deformation bands – from the grain to the reservoir scale – leads to a lack of comprehensive and high-quality data on the relationship between deformation modes and microstructure, despite significant advancements in the field. We propose to carry out extensive experimental investigations on a material at multiple scales and various loading conditions.
This work explores empirically the relationship between the initial porosity distribution of the heterogeneous Saint-Maximin limestone and the deformation modes observed from the micrometer to the centimeter scale. At the standard laboratory centimeter scale, it was shown that the band pattern was controlled by the porosity heterogeneity at the centimeter scale, and initiated preferably in the zones of lower porosity, showing first order control of porosity at this scale. The abundance of SML experimental data and its heterogeneity were key advantages for exploring strain accommodation at lower scales. By conducting a series of in situ tests on smaller, 8 mm in diameter samples, we aimed to elucidate the role of porosity heterogeneity in the onset and propagation of deformation bands, thus enhancing our understanding of the mechanical processes governing carbonate rocks. Ultimately, the results could contribute to improved modeling of multiscale geosystems.
How to cite: Dore-Ossipyan, C., Quacquarelli, A., Sulem, J., Bornert, M., Dimanov, A., and King, A.: Multi-scale control of initial porosity distribution on deformation processes in a heterogeneous porous carbonate rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-338, https://doi.org/10.5194/egusphere-egu26-338, 2026.
Understanding damage accumulation under cyclic loading is critical for assessing the stability of deep excavations and heterogeneous crustal rocks subjected to repeated stress perturbations. Subvolcanic and volcanic lithologies typical of porphyry copper systems, and analogous to shallow volcanic crust, exhibit strong mineralogical and structural heterogeneity due to intrusive processes, veining, and hydrothermal alteration, challenging models derived from homogeneous rocks. We present results from uniaxial and triaxial cyclic loading experiments on five lithologies from the El Teniente porphyry copper deposit (tonalite, diorite, porphyritic dacite, veined andesite, and hydrothermal breccia), conducted under confining pressures up to 25 MPa and coupled with continuous acoustic emission (AE) monitoring. Cycles of increasing stress amplitude were used to quantify stiffness degradation and acoustic memory through the Felicity Ratio (FR). Elastic moduli were derived from unloading branches, allowing direct comparison with elastic reversibility frameworks proposed for crystalline rocks. Homogeneous to moderately heterogeneous lithologies exhibit gradual stiffness loss and limited departure from elastic reversibility, whereas strongly heterogeneous rocks display pronounced stiffness fluctuations, early deviation from elastic behaviour, and broad FR dispersion, indicating intermittent strain localization and partial loss of elastic memory. Increasing confinement reduces mechanical and acoustic scatter, highlighting the stabilizing role of lateral stress. Ongoing work integrates photogrammetry-based quantification of grain-size distributions, vein density, vein thickness variability, and alteration intensity. These micro- and mesoscale descriptors are used to explore correlations with mechanical degradation rates and acoustic reactivation patterns observed during cyclic loading. This combined mechanical–microstructural approach aims to clarify how lithological heterogeneity governs the style, rate, and intermittency of cyclic damage in subvolcanic crust, with implications for deep mining stability and stress cycling in volcanic systems.
How to cite: Clunes, M., Valdés, F., Roquer, T., Cortez, J., Garrido, M., Browning, J., González, R., and Orellana, L. F.: Multiscale controls on cyclic damage and elastic memory in heterogeneous rocks from a porphyry copper system, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15252, https://doi.org/10.5194/egusphere-egu26-15252, 2026.
Temperature fluctuations can influence the internal stress field of a rock mass, especially in its surficial layer. Inherent properties (mineral composition, porosity, and fracturing) and external forcing (air temperature, humidity, and solar radiation) control the heat flux and temperature within the rock. Long-term thermal forcing, particularly when combined with wetting-drying cycles, can exacerbate rock deterioration and weathering, leading to progressive changes in mechanical properties, as shown by laboratory experiments.
In this study, granodiorite samples from the Požáry field laboratory (Central Czechia) were subjected to thermal cycling in a controlled environment of a climate chamber, with repeated and increasing cycles reaching 80°C, a temperature that was most probably never reached after the rock´s formation. During the cycling, repeated UPV measurements were made (P and S waves) to observe the changes in their velocity and the inferred dynamic elastic moduli.
The results showed slow but progressive decrease in the P and S wave velocities, suggesting rock damage after only a few cycles. This indicates possible increased rock wear in case of an expected future temperature rise.
How to cite: Blahůt, J., Lokajíček, T., Polezhaev, A., Racek, O., and Loche, M.: Changes in rock dynamic elastic moduli after thermal cycling in a controlled environment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5766, https://doi.org/10.5194/egusphere-egu26-5766, 2026.
Large-magnitude earthquakes in the continental crust predominantly occur near the brittle–ductile transition zone, where the deformation behavior of rocks plays an important role in earthquake nucleation and energy release.Rocks deforming under high-temperature and high-pressure conditions within the brittle–ductile transition zone may exhibit mechanical responses controlled not only by temperature and stress level, but also by pre-existing microstructural features; in particular, perthitic feldspar, a widespread feldspar solid solution in the crust, commonly contains exsolution-related lamellar structures that may introduce orientation-dependent deformation behavior.Despite its common occurrence in mid-crustal rocks, the influence of pre-existing lamellar fabric orientation on the deformation behavior of perthitic feldspar, especially under brittle–ductile transition conditions, remains poorly constrained by experiments.Based on this background, we conducted high-temperature and high-pressure deformation experiments using a Griggs-type solid-medium apparatus to systematically investigate the deformation behavior of perthitic feldspar with different pre-existing lamellar fabric orientations.Samples were prepared with lamellar orientations at angles of 0°, 45°, and 90° relative to the maximum principal stress, and deformed at a confining pressure of 1 GPa, over a temperature range of 600–1050 °C, at strain rates ranging from 5 × 10⁻⁵ to 2 × 10⁻⁶ s⁻¹. Microstructures of the samples before and after deformation were characterized using scanning electron microscopy and electron backscatter diffraction, and the mechanical responses and microstructural features were compared among samples with different fabric orientations.The mechanical results show significant differences in peak strength among the three lamellar fabric orientations, with sample strength decreasing in the order of 45°, 0°, and 90° at the same temperature.All samples entered a plastic deformation regime above 800 °C (σd<Pc).Microstructural observations reveal that at low temperatures (<900 °C), pervasive brittle cracks crosscut both feldspar phases and are accompanied by localized ductile shear zones; at intermediate temperatures (900–950 °C), cracks are mainly confined within albite grains and are commonly oriented perpendicular to grain boundaries; at high temperatures (>950 °C), samples exhibit bulk plastic flow with a marked reduction in cracking.Notably, samples with a 45° lamellar orientation experienced pronounced bulk fragmentation at 1000 °C and 1050 °C.EBSD results show that K-feldspar does not develop significant changes in crystallographic preferred orientation during deformation, whereas albite exhibits progressively heterogeneous orientation patterns with increasing temperature, consistent with plastic deformation associated with subgrain rotation recrystallization.Together, the mechanical and microstructural results demonstrate that pre-existing lamellar fabric orientation exerts a significant influence on the deformation behavior of perthitic feldspar under brittle–ductile transition conditions, providing experimental constraints on strength anisotropy in feldspar-rich rocks.
How to cite: yaqi, C., jiaxiang, D., and yongsheng, Z.: Deformation behavior of perthitic feldspar under brittle–ductile transition conditions: effects of pre-existing lamellar fabric, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6075, https://doi.org/10.5194/egusphere-egu26-6075, 2026.
Frost heave and thaw settlement are among the most widespread and destructive geotechnical hazards in cold regions, posing serious threats to the safety and long-term performance of infrastructure. The initiation and evolution of these hazards are highly dependent on the mechanical properties of frozen soils, such as compressive strength, cohesion, internal friction angle, and deformation modulus. These properties are jointly controlled by temperature, ice content, water content, and freeze–thaw cycling, resulting in strong nonlinearity, temporal variability, and spatial heterogeneity. As a result, conventional laboratory testing and empirical approaches often suffer from high cost, low efficiency, and limited applicability in parameter determination and prediction. In recent years, machine learning techniques have been increasingly applied to predict soil mechanical parameters due to their ability to handle multi-source data and capture complex nonlinear relationships. However, the strong temperature sensitivity of frozen soil behavior makes it difficult to achieve high prediction accuracy by solely establishing mappings between temperature–moisture–structural characteristics and mechanical responses. This challenge highlights the necessity of data-driven modeling frameworks that explicitly consider stress states and thermomechanical coupling effects. In this study, a machine learning–based framework was developed to predict the strength characteristics of frozen clay. A total of 116 sets of directional shear test data were used to train and validate four machine learning algorithms. The intermediate principal stress coefficient, principal stress axis orientation angle, mean principal stress, and temperature were selected as input variables, while frozen clay strength was taken as the output. Model performance was systematically evaluated using cross-validation and further verified through comparison with supplementary experimental data. Based on the optimal model, the distribution of frozen clay strength within a multi-dimensional input parameter space was analyzed. In addition, model interpretability techniques were employed to conduct sensitivity analysis, enabling quantitative evaluation of the relative importance of different input parameters. The results demonstrate that machine learning approaches can accurately reproduce the stress–strain behavior and failure strength of frozen clay, while effectively capturing the complex nonlinear relationships between strength and controlling factors. Overall, this study shows that machine learning provides a robust and efficient alternative for predicting frozen soil mechanical parameters. The proposed framework enhances prediction.
How to cite: Wang, D.: Prediction of Frozen Clay Strength Under Different Temperature Conditions Using Machine Learning Approaches, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11103, https://doi.org/10.5194/egusphere-egu26-11103, 2026.
Granular materials compact, increase in density, and degas as they accumulate. This changes the material properties, storage capacities, and fracture mechanics. We developed a mechanical model for compacting granular old snow. Based on minimal assumptions and data, we address a general phenomenon in compacting granular medium: propagating ruptures or “firnquakes”.
Compacting snow becomes firn then ice. As the snowpack consolidates, it transitions from a non-homogeneous granular material to a more elastic continuum material. We propose that the granular legacy produces spatial variations in density, stiffness, and pre-stress. This creates an internal structure of supports in unconsolidated snow at depth. Firn can quake when these supports collapse. By combining granular with brittle fracture mechanics and making use of statistical percolation theory, we can explain the conditioning, triggering, and progression of firnquakes in a bulk homogeneous material, with near constant boundary conditions.
Our model provides means to assess ruptures in granular materials, which unlike firnquakes, can have hazardous consequences, like landslides, avalanches, powder tailing failure. It also provides mechanistic explanations and statistical approaches to assess storage structure and capacity, which, in the case of Antarctic’s firn, has been linked to icesheet disintegration.
How to cite: Voigtländer, A. and Gee, B.: Why firn (old snow) quakes - a continuum mechanics theory with granular legacy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1649, https://doi.org/10.5194/egusphere-egu26-1649, 2026.
During uniaxial compression testing, the sample fails macroscopically in the form of longitudinal splitting. This indicates that, although only axial load was applied, internal tensile forces caused the sample to fail. In the classic Mohr-Coulomb failure criterion, these tensile forces are not taken into account and the minimum principal stress is considered to be zero. In our modified Mohr-Coulomb failure criterion, we assume that during a uniaxial compression test, tensile stresses are generated in the rock, causing the specimen to fail. Under this assumption, it is possible to extend the stress state during a compression test into the tensile range. The hypothesis is that during a uniaxial compression test, failure is also determined by the tensile strength perpendicular to the load axis. Based on Mohr-Coulomb theory, it is now possible to determine the cohesion and internal friction coefficient from this stress state, knowing only the compressive and tensile strength of the rock.
This method has been tested for various rock types with known values for cohesion, internal friction coefficients, and tensile and compressive strength. Our method provides a good estimate of the intrinsic rock properties.
We present the theoretical basis for our modified Mohr-Coulomb failure criterion and its applicability to various rock types.
How to cite: Blöcher, G. and Cacace, M.: Estimation of cohesion and internal friction coefficient using a modified Mohr-Coulomb failure criterion., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5143, https://doi.org/10.5194/egusphere-egu26-5143, 2026.
With the extension of mining and tunneling engineering into deep complex water-bearing strata, the interaction between groundwater and rock mass has become a critical factor governing mechanical excavation efficiency. The presence of water fundamentally alters the rock fragmentation characteristics, and understanding this hydro-mechanical coupling is prerequisite for optimizing conical pick performance. In this study, a comprehensive experimental framework combining macroscopic indentation tests and microscopic characterization was established to evaluate the breakability of twenty distinct lithologies under dry and saturated conditions. The variation of Peak Indentation Force (PIF) and cutting work was monitored, alongside micro-analysis using SEM and XRD to reveal the intrinsic controls of mineral composition and pore structure. The results demonstrate a lithology-dependent bifurcation: porous sedimentary rocks exhibit significant degradation in strength due to pore pressure wedging and chemical softening, whereas dense magmatic rocks remain largely insensitive to saturation. Furthermore, to bridge the gap between experimental data and field application, an Extreme Gradient Boosting (XGBoost) model was used. Feature importance analysis reveals that under water-saturated conditions, the Brittleness Index surpasses hardness as the dominant predictor for rock breakability. This study quantifies the water-weakening mechanism and provides a data-driven approach for predicting cutter performance and improving excavation efficiency in water-bearing environments.
How to cite: Shi, X. and Wang, S.: Experimental investigation and machine learning prediction of water-weakening effects on rock breakability by conical pick, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8640, https://doi.org/10.5194/egusphere-egu26-8640, 2026.
Progressive damage and failure in rock masses is governed by multi-scale processes ranging from micro-crack growth to meter-scale fracture opening. We present an active acoustic monitoring approach that captures these evolving processes through time-lapse waveform fingerprinting, providing a quantitative measure of the temporal evolution of rock mass stiffness and scattering properties.
We deployed acoustic sensors on three highly fractured rock cliffs (two limestone sites in southern France and one gneiss site in western Switzerland) and conducted repeated active acoustic measurements every few minutes over periods of several weeks to more than one year. Each source-receiver path yields a unique acoustic response whose complexity increases with fracture density and scattering. By tracking phase shifts and waveform distortions, we 'draw' time-lapse waveform fingerprints that are highly sensitive to small changes in crack density, fracture aperture, and contact stiffness.
The waveform fingerprints reveal strong repeatability under similar meteorological conditions, with coincident patterns observed on days sharing comparable temperature and moisture regimes. Distinct fingerprints emerge under different rock cracking and damage states reflecting reversible thermo-hydro-mechanical effects. Some rocks are indeed more reactive to external forcings than others. At longer timescales, partial but incomplete recovery of the fingerprints is observed. In the one-year data set, major fingerprint features reappear under similar climatic conditions, but with persistent residual changes, indicating the accumulation of irreversible damage within the rock mass.
Future work could apply diffuse acoustic wave spectroscopy and acoustic correlation-based imaging to spatially locate damage and quantify fracture growth, enabling the transition from qualitative fingerprints to quantitative maps of rock degradation.
How to cite: Starke, J., Rousseau, R., Rolland, A., Baillet, L., and Larose, E.: Acoustic Fingerprints - Tracing Irreversible Damage in Natural Cliffs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17611, https://doi.org/10.5194/egusphere-egu26-17611, 2026.
Seismic monitoring is an effective tool for studying rock mass stability, playing a crucial role in detecting the precursory assessment of damage and cracking processes preceding and accompanying macroscopic failures.
We first present the seismic monitoring results from the Lorgino Quarry in Crevoladossola (NW Italy) where a 6000 m³ rock-fall occurred on January 26, 2023 shortly after deploying a small-aperture array (about 100 meters) of three seismic stations, equipped with a tri-axial, velocimetric sensor and data-loggers sampling at 250 Hz. The rock fall took place about a month after the site-specific seismic array installation at the lithological contact between folded gneisses and a dolomitic limestones unit, mainly composed of dolomites and dolomitic saccharoid marbles. The rockfall seismic signature lasted 15s and the spectral analysis shows the occurrence of multiple sub-episodes of slip triggered by the initial rupture.
As no obvious correlations between precursory activity and the rockfall occurrence were observed via traditional seismological approaches, we applied an unsupervised deep-learning method that combines a deep scattering network, for automatic feature extraction, with Gaussian mixture model clustering. This approach successfully identified low-amplitude signals occurring nearly one hour before the rockfall, nearly undetectable in raw seismic records and likely associated with a nucleation phase occurring well before the acceleration to failure.
In order to investigate the physical mechanisms driving the nucleation phase, we carried out rock deformation laboratory experiments, where marble cylindrical samples (100x40mm) from the quarry were triaxially loaded in compression to failure at constant effective pressure (20MPa) while an array of 16 Piezoelectric Transducers recorded the ongoing Acoustic Emissions (AE). The time and spatial distribution of AE reveal the nucleation and growth of patches led by limited occurrence of low energy AE events and the coalescence of microfractures into cm-scale macroscopic ruptures planes leading to AE clustering and stress drop and a peak in number of events and energy. Preliminary source mechanism analysis, carried out by developing an automated focal mechanism inversion workflow for AE based on P-wave first-motion, integrating polarity and amplitude measurements, suggests that the inverted focal mechanisms are stable and broadly consistent with the imposed stress conditions, highlighting the potential of the workflow to improve source mechanism quality by identifying and excluding unreliable solutions.
How to cite: Vinciguerra, S., Adinolfi, G. M., Zhou, W., and Comina, C.: Unravelling Precursory Rockfall seismic signatures via multiscale clustering analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7457, https://doi.org/10.5194/egusphere-egu26-7457, 2026.
Yosemite National Park (California, USA) is characterized by high granitic rock walls affected by diffuse rock slope instabilities. These pose high rockfall hazards to roads and threaten the lives of millions of people that every year access the park to visit the natural beauties of Yosemite Valley, walk trails and climb iconic rock walls like El Capitan. Here, rockfalls are chiefly triggered by the progressive failure of portions of exfoliation sheets (“flakes”) bound to the cliff by rock bridges. In this context, identifying potentially unstable flakes is crucial for risk mitigation, yet the field characterization of such flakes remains difficult, highlighting the need for remote sensing mapping methods.
At the southeastern face of El Capitan, in situ time-lapse infrared thermographic (IRT) surveys, conducted in October 2024, revealed that exfoliation sheets cool faster than the surrounding rock mass heated by the same daily solar forcing. To lay foundations for a remote detection methodology, we carried out a combined laboratory and numerical study of the IRT signature of daily heating and cooling of exfoliation sheets and the underlying physical processes.
We conducted 37 laboratory experiments in a controlled setup, where the cooling of 20 cm by 20 cm granite slabs with variable thickness (1-6 cm) and opening of a simulated exfoliation joint (2-54 mm), oven-heated at 85°C, is monitored by contact thermocouples and a high resolution thermal camera. For each tested combination of slab thickness and joint aperture, we recorded detailed temperature time series and modelled cooling curves using the lumped capacitance solution of Newton’s law of cooling.
Experimental results show that, until a threshold value of the thickness/aperture ratio is reached, IRT can detect a dependence between the cooling rate of the external slab face and the aperture of the simulated exfoliation joint, with two contrasting trends. For very small aperture, cooling speed decreases with aperture. Beyond a certain aperture value, varying with slab thickness, the slab face cools faster as joint aperture increases.
To investigate the physical processes underlying this behaviour, we reproduced our experiments by 2D and 3D finite-element numerical simulations with the software Temp/W-GeostudioTM, considering different conditions (i.e. initial temperature of the cliff rock behind the flake, conduction, and air convection parameters). Model results suggest that convective heat transport in the open simulated joint strongly controls the thermal energy dissipation within the cooling flake. For very small joint apertures or limited convective circulation, the insulating effect of air results in slower flake cooling. However, for increasing joint aperture and thus greater air convection, the results indicate more effective heat dissipation and associated faster cooling. Our study provides a quantitative framework towards the development of remote mapping of unstable rock features upon proper methodology upscaling to in situ conditions.
How to cite: Giorgi Spreafico del Corno, F., Agliardi, F., Castellanza, R., Stock, G. M., Bruschetta, R., and Collins, B. D.: Investigating the thermal behavior of exfoliation sheets in granitic cliffs (Yosemite, USA) through laboratory experiments and numerical modeling , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11286, https://doi.org/10.5194/egusphere-egu26-11286, 2026.
Excavating large-scale tunnels in tectonically active regions, such as the Himalayan seismic zone, challenges the stability of underground structures due to high in-situ and induced stresses. The Diamer-Basha Dam (DBD) project involves complex tunneling through heterogeneous bands of granite and diorites, necessitating an engineered support system to mitigate progressive rock mass failure. In Pakistan, the tunnel support often relies on empirical classifications like the Rock Mass Rating (RMR) and Q-system. These systems provide a useful initial estimate; however, their direct application without site-specific calibration frequently results in conservative or over-designed support systems. This study investigates an optimized support framework by integrating empirical characterization with numerical Finite Element Method (FEM) analysis. Using geological data acquired from the site, including face maps and borehole logs, we classified rock mass and simulated its response to excavation using RS2 software. The research specifically evaluates the mechanical efficacy of Fiber Reinforced Shotcrete against optimized combinations of plain shotcrete and active rock bolts. Numerical simulations indicate that the in-situ rock mass possesses sufficient self-supporting capacity in specific zones to allow for a reduction in shotcrete thickness when supplemented with bolting. The models demonstrate that optimized designs maintain the required structural stability while reducing material consumption. These findings suggest that a hybrid empirical-numerical framework offers a cost-effective engineering solution for large excavations. By validating support performance through numerical modelling, this study provides a repeatable framework for optimizing tunnel support in complex geological environments.
How to cite: Aman, U., Ali, Z., Hussain, S. N., Nazeer, S., and Ayub, M.: Optimization of Tunnel Support Systems in High-Stress Geological Zones: A Case Study of Diamer-Basha Dam, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11234, https://doi.org/10.5194/egusphere-egu26-11234, 2026.
