The chalk cliffs of northern France are exposed to considerable erosion, resulting in a significant coastal retreat of up to 1 m per year on average. A key concern is how the mechanical properties of the chalk evolve under different weathering conditions. Previous research, based on a limited number of samples, has provided a preliminary understanding of potential changes in the mechanical properties of chalk in altered and unaltered states. As part of the DEPHY3GEO project, we conducted experiments on Saint Marguerite Chalk, which is part of the Newhaven Chalk Formation. Due to the inherent low strength of the chalk, some conventional displacement measurement techniques are inappropriate. Therefore, we used digital image correlation (DIC) coupled with basic acoustic measurements. Experiments were performed on chalk samples exposed to various weathering conditions, including salt solutions, fresh water, and thermal cycling. These experiments consisted of uniaxial and confined compression tests, which allowed us to evaluate the influence of weathering on the mechanical behavior of the chalk. Results show that water saturation of chalk rock significantly reduces by half or more its maximum compressive strength. Ongoing work will further investigate the weathering of chalk by studying its effect on micro- and macro-scale structures through structural analysis of cliff samples and scanning electron microscopy (SEM).

How to cite: Chalé, A., Bu, F., Jaboyedoff, M., Costa, S., Maquaire, O., and Kouah, M.: The Influence of Weathering on the Mechanical Properties of Chalk: Study of the Saint Marguerite Chalk , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11784, https://doi.org/10.5194/egusphere-egu25-11784, 2025.

Acoustic Emissions (AE), the laboratory analogs for seismic activity, offer a controlled environment to study deformation and failure mechanisms. By leveraging high-precision three-dimensional localizaton techniques, researchers can analyze ongoing these mechanisms during rock deformation experiments. The spatial resolution is thus crucial for increasing our capability to understand and predict failure modes in natural and engineered systems.

Despite recent advances in localization techniques, automated AE localization faces significant challenges. Conventional AE processing systems generally extract a limited set of parameters, such as arrival time defined as the first overcoming a given amplitude threshold. While these parameters provide the bulk information, they often overlook critical signal aspects, underestimating AE phenomena complexities and compromising the localization accuracy. In fact, amplitude thresholds may not capture accurately the signal onset, particularly in noisy or complex waveforms.

This study proposes a new methodology to improve source location accuracy and AE event classification by developing an automatic picking system tailored to seismic signal characteristics and set on Signal-to-Noise Ratio (SNR). The novel algorithm introduced here overcomes conventional amplitude-based thresholding by including broader waveform characteristics, source-receiver distance and wave propagation path. The analysis operates on multiple signal windows and provides uncertainty estimates, enabling more accurate AE location.

The AE source localization process was carried out using the Time Difference Of Arrival (TDOA) method, widely applied in rock deformation laboratory experiments. This approach considers signal arrival time differences at multiple transducers and, with a velocity model, estimates the three-dimensional coordinates of AE sources. The localization quality was assessed via four key parameters: 1) RMS (Root Mean Square) between observed and calculated arrival times, 2) localization errors along the three principal coordinates, 3) MAPE (Mean Absolute Percentage Error quantifying arrival time differences), and 4) the average azimuthal gap across three principal planes. These parameters quantify discrepancies in location accuracy and are employed to assess the final localization.

We applied our methodology to waveform data of AE recorded by an array of twelve 1 MHz piezo-electric transducers during conventional triaxial deformation of a 40 × 100 mm Darley Dale sandstone cylindrical sample (King et al., 2021) at 20 MPa confining pressure. We show that our approach reduces the localization errors and improves the AE detection and localization accuracy. By addressing conventional AE location limitations, this work advances AE-based monitoring toward more accurate and reliable results, providing higher resolution and improved information to describe the damage micro-mechanisms driving failure in stressed rocks. 

How to cite: Adinolfi, G. M., Chen, Y., and Vinciguerra, S. C.: Acoustic Emission Source Localization Techniques: A Methodology for Improved Localization Accuracy in Laboratory Deformation Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12108, https://doi.org/10.5194/egusphere-egu25-12108, 2025.

Mineralogical composition and porosity significantly influence the mechanical behavior of hydrocarbon reservoir rocks in crustal conditions. To understand the mechanical failure behavior and dynamic porosity changes with axial loading, triaxial compression experiments were conducted, in three distinct reservoir rock types: KG Basin Sandstone, Bombay High limestone, and Boise Sandstone. Initial porosities of collected samples varied; with KG sandstone at 30%-32%, Bombay limestone at 9.5%-12.7%, and Boise sandstone at 20.6%-25%, determined under dry testing conditions. A range of effective pressures (P_eff) was applied to investigate brittle faulting and cataclastic flow under dynamic axial loading. Both Boise and KG sandstones transitioned from a brittle dilatant regime to a compactive cataclastic flow regime, with Boise sandstone displaying brittle behavior up to 5 MPa P_eff and KG sandstone up to 50 MPa P_eff. Conversely, the Bombay limestone consistently exhibited compactive cataclastic flow at all tested experimental conditions. KG sandstone demonstrated highest peak strength relative to the other rock types due to its lower porosity. Additionally, Boise sandstone showed higher peak strength than Bombay limestone, primarily due to the presence of quartz within the sandstone matrix, which is more resistant relative to the carbonate minerals of limestone formations. In KG sandstone, there was a significant increase in porosity prior to sample failure, particularly up to 70 MPa P_eff, and the final porosity observed post-failure surpassed the initial porosity levels recorded, upto 50 MPa P_eff. In contrast, both Boise sandstone and Bombay limestone showed a continuous decrease in porosity with increasing differential loading throughout the experiment at all tested conditions. Deflection patterns of triaxial curves indicated a shift from shear-induced dilation to shear-enhanced compaction in both Boise sandstone (dilation up to 5 MPa P_eff) and KG sandstone (dilation up to 50 MPa P_eff), while the Bombay limestone predominantly displayed shear-enhanced compaction across all tested P_eff beyond a critical stress threshold. KG sandstone exhibited a transition from pre-failure dilatant to post-failure compactive inelastic strain with increasing P_eff, which contrasts with the post-failure compactive inelastic strain that was consistently observed in the case of the Bombay limestone.

    By integrating these petrophysical and mechanical parameters into reservoir management, hydrocarbon extraction can be made more effective and safer. These insights are essential for creating accurate geomechanical models, which are vital for strategic field operation planning, optimizing production rates, and maintaining reservoir stability.

How to cite: Dasgupta, A., Saha, P., Bal, A., and Misra, S.: Influence of triaxial compaction on the mechanical behaviour and porosity evolution of reservoir rocks., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6063, https://doi.org/10.5194/egusphere-egu25-6063, 2025.

Rocks exhibit astonishing time dependent mechanical properties, like memory of experienced stress or slow dynamics, which refers to a transient recovery of stiffness after a softening induced by almost any type of loading. This softening and transient recovery is observed in the subsurface and in buildings after earthquake shaking, or in laboratory samples.

Our investigation of anisotropy of the slow dynamics effect under uniaxial loading in dry sandstone samples shows that it is observed independent of propagation direction, while the loading effect shows the expected anisotropy originating from the opening and closing of cracks. These observations put a number of novel constraints on the enigmatic physics of slow dynamics.

We conclude that transient changes in bulk stiffness are caused by sliding of oblique grain-to-grain contacts and resulting changes in frictional properties as empirically described by rate-and-state friction and observed in laboratory experiments across block contacts.

Connecting the nonclassical nonlinearity of heterogeneous materials to the powerful framework of rate-and-state friction provides an elegant explanation for the long searched-for origin of slow dynamics and potentially adds a new perspective for the monitoring of very early stages of material failure when deformation is still distributed in the bulk and just starts to coalesce towards a fracture.

Similar experiments conducted are conducted with fluid saturation under varying confining and effective pressures illuminate the complex impact of pore fluids on the slow dynamics response of the material.

How to cite: Asnar, M., Sens-Schönfelder, C., Bonnelye, A., Curtis, A., Dresen, G., and Bohnhoff, M.: Anisotropy reveals contact sliding and aging as a cause of post-seismic velocity changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13545, https://doi.org/10.5194/egusphere-egu25-13545, 2025.

The formation of surface-parallel exfoliation fractures, or “sheeting joints,” in rock domes produces some of the most intriguing and celebrated landforms on Earth. In 1904, G.K. Gilbert outlined three mechanisms for exfoliation fracture formation: (1) contraction during original cooling, (2) decompression during exhumation, and (3) post-exhumation surface processes. Recent research, including direct measurements during spontaneous exfoliation, has emphasized the influence of solar heating on progressive (subcritical) fracture propagation. This study investigates the mechanisms driving exfoliation fracturing at Arabia Mountain, a biotite orthogneiss dome near Atlanta, Georgia (USA), which experienced a spontaneous exfoliation event in July 2023. Digital elevation model differencing and field observations revealed the event uplifted a ~250 m² area by ~30 cm, buckling sheets up to 12 cm thick, and leaving traces of rock fragments and dust thrown meters away from fractures. Following this event, we installed instrumentation during the summer of 2024 to monitor surface-parallel stresses, local seismic activity, and subsurface rock temperatures at the site. Multiple subsequent exfoliation events in June 2024 were captured in real-time, including direct observations of progressive fracture propagation and dynamic rupture during a period of high temperatures. Relative surface-parallel stresses measured at variable depths in two boreholes revealed clear daily cycles that appear correlated with diurnal patterns of solar heating. A decrease in stress magnitudes and rock temperatures and increased lag time with depth further supports links between rock stresses and solar heating at the surface.

These observations support the hypothesis that subcritical cracking can be initiated or propagated due to thermal stresses and highlight the critical role of solar heating in progressive rock fracturing. The implications of these findings extend beyond Arabia Mountain. The sensitivity of rock domes to thermally induced stresses elevate concerns for an increased hazard of spontaneous exfoliation and rockfall events with future climate warming. Understanding the interplay between thermal stresses, mechanical fracturing processes, and pre-existing damage is critical for predicting such hazards and improving mitigation strategies. Furthermore, our observations and monitoring results provide valuable insights into the fundamental mechanics of subcritical fracturing, which can aid in determining the influence of surface weathering and erosion—key processes impacting the evolution of rock domes. We highlight the importance of multidisciplinary research in advancing our understanding of exfoliation joint formation, and more broadly towards disentangling the impacts of climate change on rock dome stability and landscape evolution.

How to cite: Reynolds, A. N., Eppes, M. C., Collins, B. D., Peng, Z., and Lang, K. A.: Influence of solar heating on spontaneous rock exfoliation at Arabia Mountain, Georgia, USA, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12839, https://doi.org/10.5194/egusphere-egu25-12839, 2025.

Rock fracturing plays a key role in both the formation of mountain landscapes and natural hazards. Weathering agents, such as daily thermal variations and precipitation, are some of the main triggers of the weathering and fracturing process. However, the mechanisms involved are not well understood, and questions remain about the thermo-mechanical stress field in the near surface of cliffs. 

To better understand the effect of thermal variation on mechanical stress variation at centimeter to meter scales, long-term recordings were made using repeatable ultrasonic acoustic sensors to measure both esound velocity and waveform changes. The acoustic sensors were placed on a few square meters of a 50 m limestone cliff above Chauvet Cave in the Ardeche region of southeastern France.

The results show that daily cycles of velocity changes are evident and appear to correlate with thermal fluctuations and variations in solar radiation. We propose that the velocity variations are due to thermal stress variations in the rock. During the hottest part of the day, the velocity variation causes an increase in compressive hydrostatic stress. At the same time, the spectral analysis of the impulse response shows daily variations with the appearance of high frequency content during the hottest part of the day. Thus, we propose that the hottest times of the day have the effect of expanding the rock surface and thus closing the fractures, which increases the high frequencies content. Conversely, during cooling periods, we can detect tensile stresses that are likely to open fractures and contribute to progressive subcritical cracking within the rock mass.

This work was funded by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project

How to cite: Rousseau, R., Starke, J., Guillemot, A., Baillet, L., Moreau, L., and Larose, E.: Time-lapse ultrasonic testing for monitoring stress and structural changes in rock cliffs, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15953, https://doi.org/10.5194/egusphere-egu25-15953, 2025.

Through the application of Multi-temporal Interferometric Synthetic Aperture Radar (MTInSAR) for long-term monitoring of surface deformation, the Agency of Rural Development and Soil and Water Conservation (ARGSWC), Ministry of Agriculture, has identified 315 deep-seated landslide sites with protecting targets across Taiwan as of 2024. For this study, imagery from January 2022 to October 2023 from the ALOS-2 satellite, released by the Japan Aerospace Exploration Agency (JAXA), is used for the MTInSAR monitoring. Based on the MTInSAR surface displacement data, activity indices for the deep-seated landslide sites have been developed, including both the arithmetic mean and inverse area-weighted deformations. The k-mean clustering and risk matrix method are then employed to classify and rank the landslide activity. The analysis reveals that approximately 5.8%, 10.7% and 83.6% of the deep-seated landslide sites are classified as high, medium and low activity, respectively. In addition, statistical clustering techniques are applied to group the surface deformation data, which are then compared to slope units derived from the aerial LiDAR Digital Elevation Model (DEM) for the landslide sites. This approach helps to identify active landslide blocks or subzones within the landslide sites.

How to cite: Kuo, C.-Y., Au, S.-Y., Chan, Y.-H., Chen, R.-F., Chan, K.-C., Lin, E.-J., and Tsai, P.-W.: Monitoring Surface Deformation and Classifying Activity of Deep-Seated Landslide Sites Using Multi-Temporal InSAR, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14383, https://doi.org/10.5194/egusphere-egu25-14383, 2025.

Accurate initial geostress state is a prerequisite for the dynamic optimization design and construction of deep-buried tunnels, as well as for preventing disasters such as rockburst and large deformation. Inversion methods of the stress field for underground engineering, which utilize limited measurement data and numerical simulation techniques, have emerged as the primary approach. However, traditional inversion optimization models often treat stress components as independent scalars. This simplification overlooks issues of physical consistency and optimization complexity. Additionally, existing intelligent inversion methods, such as machine learning and heuristic optimization, require a large number of simulations, posing a challenge in balancing accuracy and efficiency. This issue becomes particularly problematic for large-scale tunnels in complex geological conditions where the computational time cost is exorbitantly high, thereby hindering the practical need for quick and precise stress field reconstruction.

To address these challenges, we propose a novel inversion method that integrates a tensor-based objective function with Bayesian optimization (TOF-BO). Concretely, this method regards the stress tensor as a whole, uses the Euclidean distance to measure the deviation between calculated and measured values during optimization, and formulates the objective function as the sum of deviations across all measurement points. Unlike traditional scalar objective functions, this tensor-based objective function preserves the physical correlations between stress components, reduces the dimensionality of the objective function, and effectively avoids the adverse effects of magnitude differences between components on optimization efficiency and accuracy. Given the objective function's dependency on costly simulation, we adopt Bayesian optimization, utilizing active learning to achieve global and efficient optimization.

This method was applied at the engineering site of a deep-buried tunnel under construction in southwestern China. The results showed that the TOF-BO method yielded satisfactory results (average accuracy=90.8%) with only 18 time-consuming numerical simulations, proving that the method can significantly reduce the demand for expensive simulations, effectively decrease computational costs, and possesses the ability to rapidly and reliably reconstruct the stress field within the study area. Compared to commonly neural network methods, the TOF-BO method improves accuracy by approximately 4.6% within the same time cost. In conclusion, the TOF-BO method provides an efficient and reliable solution for the inversion of geostress fields, demonstrating substantial potential for practical applications.

How to cite: Li, T. and Wei, D.: An Efficient Geostress Inversion Method and Its Application under Complex Geological Conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1749, https://doi.org/10.5194/egusphere-egu25-1749, 2025.

The Kırık Tunnel in Erzurum is an important route in the highway network designed to connect the northern and southern regions of Turkey. The tunnel, planned to be 7 km long, 10 m high and 7 m wide, passes through a geologically complex region. Preliminary investigations have revealed that the tunnel route is predominantly composed of clayey limestone and marl succession with variable slopes and orientations. During the tunnel construction works, a cave-in occurred during excavation after the water flow and a fault zone containing sandy-gravelly and organic matter was encountered at the 1+825+70 km section of the tunnel. The discovery of the crushed zone due to faulting necessitated a change in the tunnel design and it was decided to apply the back analysis method to understand the collapse mechanism. The deformation data affecting the tunnel were obtained with the help of load cells and back analysis was performed to determine the parameters of the collapse material from this deformation value at the time of collapse. Rock mass classifications were made using the parameters obtained from field and laboratory observations. As a result, with the help of Rocscience RS2, the collapse in the Kırık Tunnel was analyzed by finite element method and back analysis was performed. During the back analysis, the lateral vertical stress pressure acting on the faulted weak zone had to be re-evaluated. The k value changes in the numerical calculation model were examined from the existing approach methods and the comparison of these methods depending on the deformations in tunnel construction was given within the scope of the study. As a result of these data, new design parameters were found and grouting and support recommendations were given to overcome the stability problems in the tunnel. Accordingly, the lateral vertical stress ratio was taken as 1.6, which is normally 0.3, and the lateral pressure in the tunnel was determined as higher than the vertical pressure. With the new parameters, the unstable zone in the tunnel was theoretically crossed without any problems.

How to cite: Eslik, O., Undul, O., and Dogu, M. M.: The Effects of Unstable Fault Zones on Design Parameters of Tunnels (The Case of Erzurum Kırık Tunnel), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12997, https://doi.org/10.5194/egusphere-egu25-12997, 2025.

The tensile strength of brittle rocks is commonly determined experimentally using the Brazilian test (splitting tensile strength test). For spatial and temporal upscaling of these experimental results, numerical methods are necessary to predict rock behaviour under complex geomechanical conditions such as foundation, slope stability, underground space and reservoirs studies, and to analyse the influence of individual parameters on the outcomes.

Previous studies on phase-field modeling of rocks often employed a single open flaw to simulate fracture initiation, and two to four open flaws to represent fracture propagation and failure. These flaws were generally arranged in a geometrically ordered pattern. However, in natural rock formations, microfractures are predominantly closed and exhibit a more or less random distribution depending on texture.

This study used a phase-field model to simulate brittle fractures in granite containing multiple randomly distributed, closed pre-existing microcracks, employing the principal stress criterion as the driving force in the phase-field evolution equation.

This study focuses on analyzing the sensitivity of the model to crack density, spatial distribution, and phase-field parameters. The effect of displacement rate was investigated, and phase-field parameters were calibrated accordingly. To demonstrate the presented approach's accuracy, numerical simulations are compared to experimentally obtained results showing that the approach is in principle capable of temporal and spatial upscaling when microstructural features are considered.

How to cite: Asghari Chehreh, H., Witte, L., Duda, M., and Backers, T.: Simulation of displacement rate effect on tensile rock failure using a phase-field fracture approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15632, https://doi.org/10.5194/egusphere-egu25-15632, 2025.

After fifty years of mantle plume theory, the cause and effect remain subjects of intense debate. The scientific community is divided between those who believe that mantle plumes are fueled by a deep mantle source and those who argue for a shallow process related to plate tectonics. Although both the plume and plate hypotheses relate to the effect of thermal anomaly, few studies have attempted to explain the formation of initial thermal instability or the maintenance of magma sources for volcanism and large igneous provinces (LIPs). In this study, by considering the putative link between mantle dynamics, lithospheric breakup and flood basalts, we employ a three-dimensional spherical shell model to simulate the Earth's rifting process under thermal expansion. Based on the physical principles for the phase change of materials, we invoke a qualitative model of decompression-melting generation in rifting-induced abnormally-hot asthenosphere to explain that extra volume growth during the material phase change from solid to liquid can promote the coupling effect between pressure and temperature. Our fracture modeling shows that the shallow-based lithospheric process favors a no-root mantle plume as the source for volcanism. Continental rifting and breakup may be caused by the heat accumulation within the asthenosphere underneath the lithosphere, without the need for the existence of deep mantle plumes. Inversely, the fractures, particularly around triangle conjunctions, may release the stresses around their tips and accelerate the decompression melting, which in turn results in the thermal anomaly underneath the lithosphere inside which fractures develop. Our results reveal that pressure drops caused by uplift-induced rifting can accelerate decompression melting and provide the magma source for potential eruptions. Additionally, the significant increase in magma pressure, resulting from the abrupt volume expansion during the solid-to-liquid phase change, may act as the driving force of magma production. This process can become unstable if the coupling between melting pressure and temperature operates as a positive feedback loop. Such instability may lead to mantle dynamics emerging in a top-down pattern, offering insights into the rapid and voluminous magma eruptions characteristic of LIPs. Namely, the accumulated heat may result in expansion in the mantle which may promote uplifting, weakening and eventual breakup of the lithosphere, with huge outpouring of flood basalts, leading to the great events of LIPs. Furthermore, the associated cooling of the lithosphere occurs due to heat absorption during melting and heat loss during eruptions. This process provides a clear understanding of the short-lived yet massive nature of LIPs.

How to cite: Tang, C., Gong, B., and Chen, T.: Investigation into the formation of large igneous provinces from the perspective of Earth's breakup and induced decompression melting, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-146, https://doi.org/10.5194/egusphere-egu25-146, 2025.