The pore size and the distribution of individual or connected pores contribute to the porosity in a rock which is closely related to rock weathering degree and rock strength. The chemical reaction is normally higher for the larger specific surface area which is closely related to the pore size distribution in a rock. The variation of pore size distribution in sedimentary rocks from Gyeongsan basin in Korea was determined by the laboratory artificial acceleration weathering experiment using peristatic pumps. The pore size distribution of rock specimens was measured by the nitrogen gas adsorption method using BELSORP-max II of Microtrac MRB. The pore characteristics were measured on the outer surface and the innermost part of rock samples to determine the variation of pore size distribution since the outer surface was directly affected by weathering processes while the innermost part was not. The high-purity nitrogen gas is used to evaluate the pore size distribution with different methods such as BET, BJH, and HK. The overall pore volume and size have been increased by the weathering experiment for the tested sedimentary rocks-sandstone, conglomerate, and shale. The increase of macropore in sandstone by weathering experiment leads mainly to the increase in pore volume, while the rise of micropore and mesopore in conglomerate drives the increase of pore volume. 

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(NRF-2020R1F1A107576412).

How to cite: Woo, I.: Pore Size Redistribution by Laboratory Weathering Tests on Sedimentary rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1518, https://doi.org/10.5194/egusphere-egu23-1518, 2023.

In rocks and concrete, dynamic excitation leads to a fast softening of the material, followed by a slower recovery process where the material recovers part of its initial stiffness as a logarithmic function of time. This requires us to exit the convenient framework of time independent elastic properties, linear or not, and investigate non-classical, non-linear elastic behavior.

These phenomena can be observed during seismic events in affected infrastructure as well as in the subsurface. Since the transient material changes are not restricted to elastic parameters but also affect hydraulic and electric parameters as well as material strength, as documented for instance by long lasting changes in landslide rates, it is of major interest to characterize the softening and recovery phases. It may help us gain more insight in hazard prediction from both a geological and engineering perspective.

The underlying physics behind those non-classical, non-linear effects, sometimes referred to as “Nonlinear Mesoscopic Elasticity”, are not agreed upon. There is a lack of experiments that would allow us to discriminate between the existing models.: we aim to contribute to filling that knowledge gap.

Our experiments are made on a sample of Bentheim sandstone, initially dry and then fully saturated, in a triaxial cell. We subject the sample to loading and holding cycles in the microstrain range, while also varying confining pressure and pore pressure. Active acoustic measurements during those loading cycles with an array of 14 piezoelectric sensors allow us to monitor relative velocity changes during the experiment by using Coda Wave Interferometry (CWI).

We observe the dynamic softening as well as the recovery processes in the sample during repeated loading phases of different durations. We find that characteristics of the observed velocity changes vary depending on the observed sensor combination, indicating spatial variability of the response, as well as depending on the lapse time and frequency content of the acoustic measurements that we perform the CWI on.

These experiments serve to estimate the exact capabilities of our experimental setup in terms of signal quality, signal stability and lapse time dependent decorrelation of coda waves. We expect our results to inform a future series of similar but more refined experiments addressing the pore pressure dependence of the non-classical response of rocks.

How to cite: Asnar, M., Sens-Schönfelder, C., Bonnelye, A., and Dresen, G.: Non-classical, non-linear elasticity in rocks: experiments in a triaxial cell with pore pressure control, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8050, https://doi.org/10.5194/egusphere-egu23-8050, 2023.

The presence of water causes a dramatic reduction of the strength of most rocks. Under compressive stress conditions, fracture mechanics models show the strength of a rock sample is in particular controlled by frictional parameters and the fracture toughness of the material. Previous studies suggested that these parameters could change significantly in the presence of water, but there is a paucity of data quantifying this. Here, we report fracture toughness, frictional and uniaxial compression tests performed on five sandstones and five limestones under dry and water-saturated conditions, that provide new insight into the mechanical influence of water on sedimentary rock strength. Our new data showed that on both sandstones and limestones, the presence of water causes a reduction of both the fracture toughness (from 0 to 50%) and the static friction coefficient (from 0 to 40%), suggesting that water weakening in these sedimentary rocks is mostly due to a reduction of these two parameters under the relatively high strain rate conditions investigated here. While for sandstone we found a reduction of the Uniaxial Compressive Stress between 0 to 35%, it was less variable in limestone, in most cases around 40%. The measured fracture toughness and frictional parameters were then introduced into two well-known micro-mechanical models (the pore-emanating cracks model and the wing crack model), which provide simple theoretical expressions for the Uniaxial Compressive Strength. We found that the predicted water-weakening based on our toughness and friction parameter measurements is in overall agreement with our strength measurements on dry and wet samples.

How to cite: Violay, M., Noel, C., and Baud, P.: Effect of water on sandstone and limestone, fracture toughness, frictional parameters and brittle strength., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4578, https://doi.org/10.5194/egusphere-egu23-4578, 2023.

Pressure, temperature, and infilling fluids influence the petrophysical properties and the associated damaging processes of rocks at all scales. Moreover, each fluid-rock system possesses peculiar mechanical behaviours being these particularly complex in carbonate rocks hosting fluids. In this work, we analyze the laboratory results of deformed clean and hydrocarbon-filled limestones under varying pressure and temperature, providing links between recorded physical properties (seismic velocity), fluid behavior, and damaging. We focus on carbonate-bearing reservoir (Bolognano Formation) rocks, sampled in the Majella massif (Central Italy) that represents a very good analogue for buried carbonate reservoirs. This reservoir is composed by calcarenites with connected porosity of about 20% saturated by hydrocarbon in the solid state at the outcrop conditions. We performed hydrostatic, triaxial and true-triaxial deformation tests up to a temperature of 100º C and a confining pressure up to 100 MPa on both clean and naturally hydrocarbon-filled limestone samples. Results show increasing seismic velocity and Young’s modulus with increasing confining pressures for both clean and saturated samples as expected. However, different results are observed when the temperature is increased. At low temperatures saturated samples show larger seismic velocity and rigidity with respect to clean samples whilst at higher temperatures the opposite occurs. In particular, when temperature is rised up to 100º C the Young’s modulus of the saturated samples dramatically decreases, being this coupled by a clear volume reduction even during hydrostatic tests (no differential stress applied). Accordingly, microstructural observations highlight grain crushing related to a large amount of randomly distributed cracks within saturated samples. On the contrary, clean samples are characterized by few microfractures, pointing out the primary role played by liquid hydrocarbons. These observations are in good agreement with meso and microstructural features observed on outcropping hydrocarbon-filled carbonate-bearing faults. The presence of fluid hydrocarbons (high temperature) severely weakens the rock promoting fracturing whilst at lower temperature the presence of solid hydrocarbons increases the mechanical properties of hydrocarbon-bearing rocks. These observations have a large impact for the petrophysical characterization of reservoirs and for the understanding microscale to mesoscale mechanisms of deformation and fluids movement along deformed rock volumes.

How to cite: Trippetta, F., Ruggieri, R., Motra, H. B., and Collettini, C.: Mechanical behavior of porous carbonates as a function of pressure, temperature, and fluid content from laboratory experiments and correlation with larger scale structures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14779, https://doi.org/10.5194/egusphere-egu23-14779, 2023.

Rocks are very much susceptible to deformation in tension, especially under elevated temperatures. Therefore, the study of the dynamic tensile behaviour of rock exposed to high temperature is highly significant to understand the tensile deformation behaviour in dynamic loading conditions which will be proved useful in a variety of engineering problems such as quantifying the blast load impact in fire affected underground/opencast coal mine regions; assessment of ground subsidence due to coalmine fire coupled with blast loading etc. The Jharia coalfield region, known as the coal capital of India, is affected by pervasive underground coalmine fire for decades resulting in small to large-scale surface fracturing. So,the present study focuses on the effect of high temperature on dynamic tensile behaviour and its relation with micro-mineralogical properties of subsurface coal-bearing sandstone samples from a fire-affected mine.  To achieve the objective, the prepared samples were kept in the furnace for 24h with a heating rate of 5°C/min and then allowed to cool down naturally within the furnace. Samples were divided into nine groups based on the thermal treatment at 25 °C, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C. Using the Split-Hopkinson Pressure Bar (SHPB), the indirect dynamic tensile strength was measured for each group. Based on the obtained results, the indirect dynamic tensile strength of heat-treated specimens is characterized into three zones; viz.: Zone 1 (25-400°C), Zone 2 (400-600°C) and Zone 3 (600-800°C). In zone 1, an increase in average indirect dynamic tensile strength is observed with elevated temperature. However, in zone 2, a sharp decreasing trend in indirect dynamic tensile strength was observed with increasing temperature. This zone is characterised by a progressive increase in thermal cracks and porosity which is possibly the prime reason for a sharp transition in thermal properties. An overall reduction in indirect dynamic tensile strength is observed within zone 3, however, the rate of reduction is gentle. The plasticity that occurred due to high temperature was responsible for a slow rate of reduction in indirect dynamic tensile strength.

How to cite: Tripathi, A., Khan, M. M., K. Singh, A., and Pain, A.: Dynamic tensile behaviour of Barakar sandstone under high-temperature conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5667, https://doi.org/10.5194/egusphere-egu23-5667, 2023.

3D printing is a rapidly evolving technology that has proven useful for a wide variety of disciplines and industries. However, knowledge of its applicability to the fields of rock and soil physics remains limited. 3D printing allows the design of samples with any desired microstructural composition, enabling independent control of properties such as pore space fabric, size, and density; a feat impossible to accomplish with naturally occurring geomaterials. The use of 3D printed samples is therefore highly attractive for relating the effective properties of heterogeneous materials to their microstructural arrangement, a key subject in the fields of rock and soil physics.

This study aims to characterize the physical properties of 3D printed materials (i.e., elasticity parameters, porosity, permeability) and evaluate whether they are suitable to be used as proxies for heterogeneous geomaterials. Two distinct 3D printing technologies were employed for this purpose: the Fused Deposition Modelling (FDM), and Stereolithography (SLA) methods. The FDM method constructs 3D objects by superposing layers of polymer-based filament through a heated nozzle, whereas the SLA method, also known as resin 3D printing, uses a laser light source to cure liquid resin into hardened plastic.

Samples with a variety of pore shapes (sphere, needle, penny shaped), sizes, and pore densities were designed and printed as cylindrical samples of 25 mm diameter and 62.5 mm height. Samples were then subjected to uniaxial compression to measure their effective elastic parameters (elasticity modulus and Poisson’s ratio), and these measurements were compared with theoretical predictions. Preliminary results indicate that the FDM printing method is inadequate for representing a heterogeneous solid composed of an isotropic matrix and void space, due to the intrinsically anisotropic fabric resulting from the layer-by-layer printing method. Additionally, samples with a porous microstructure appear to be effectively stiffer than the intact material, which is attributed to enhanced material sintering surrounding the edges of the void spaces. On the other hand, SLA printing appears to hold more promise and be able to represent a composite material composed of an isotropic matrix with a heterogeneous void space. Further measurements need to be made to confirm these preliminary findings, and this work is currently in progress. 

How to cite: Adamus, F., Stanton-Yonge, A., Mitchell, T., Healy, D., and Meredith, P.: Physical properties of 3D printed materials and their applicability as proxies for heterogeneous geomaterials, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16739, https://doi.org/10.5194/egusphere-egu23-16739, 2023.

At present, a reliable method for forecasting earthquakes has not been developed yet, as the physical mechanisms that generate them are very complex and still not completely understood. To overcome the difficulties of retrieving direct observations and measurements in the field, here we employ laboratory and numerical models to investigate and better understand strain localization preceding mainshocks.

We perform a failure test on an intact and dry sample of Berea sandstone confined at 20 MPa with a triaxial machine (LabQuake). Employing in-house developed, conical-type and fully calibrated piezo-electric transducers (PZT), we are able to investigate the acoustic emission (AE) clouds by relocating the single events and by computing their focal mechanisms and scalar seismic moments. The PZT sensors are also used actively to allow for the construction of inhomogeneous and anisotropic velocity models. We further employ distributed strain sensing (DSS) with optical fibers to capture the heterogeneous spatial distribution of the surface strain by gluing the fibers on the sample surface. We observe AE clustering in two regions located at the top and bottom of the rock specimen throughout the majority of the experiment. As the test approaches the main failure, AE localize at one side of the sample in the lower half before obliquely propagating upwards by forming a macro-fracture. Surface strain heterogeneities are detected during the experiment, and regions of higher extensional strain correlate in time and space with rock volumes experiencing high AE activity. Numerical simulations, which are conducted using a two-dimensional continuum-based and fully coupled seismo-hydro-mechanical poro-visco-elasto-plastic modelling tool (H-MEC), are validated with both AE and DSS data. The combination of laboratory and numerical investigations allows us to individuate and study physical mechanisms (e.g., visco-plastic compaction of pores and shear banding) that explain the processes responsible for both surface strain concentration and the generated AE clouds. These findings suggest that the deformation in the interior of the sample is mainly occurring inelastically and is localized along an obliquely forming shear band. We estimate the partitioning between seismic and total deformation to be ~0.09 % and this effectively confirms the previous evidences related to the irreversible localization of strain within the rock specimen. 

How to cite: Bianchi, P., Selvadurai, P. A., Salazar Vásquez, A., Dal Zilio, L., Madonna, C., Gerya, T., and Wiemer, S.: Evidence of Strain Localization Preceding Rock Failure: Insights From Laboratory and Physics-Based Poroelastic Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11447, https://doi.org/10.5194/egusphere-egu23-11447, 2023.

The reliable assessment of the fracturing state of rock masses is a fundamental step towards the evaluation of their geomechanical quality and the quantification of their hydraulic and mechanical properties. Traditional field discontinuity mapping techniques remain fundamental to collect statistical populations of discontinuity attributes and characterize rock mass structure and quality. However, point-like field surveys are strongly biased by scale and orientation. The development of 3D surveys allowed to partly overcome this problem by providing high-resolution point clouds. These allow a robust characterization of fracture geometry but require significant mapping efforts. Here we proposed a quantitative contactless approach to rock mass fracturing assessment by the use of Infrared Thermography (IRT).

IRT is increasingly used in rock-mechanics to characterize rock porosity/fracturing and to monitor rock mass stability, by measuring the thermal response of rock materials to heating or cooling. However, existing IRT applications to the geomechanical study of rock masses are mostly qualitative and lacking sound theoretical and experimental foundations. Starting from the laboratory scale, studying the thermal behaviour of rock samples with different fracture degrees, we propose a quantitative approach to quantify rock mass fracturing, that combines IRT rock temperature monitoring during cooling with the quantification of different descriptors of fracturing state suitable for different analysis scales (laboratory vs in situ).

As a field laboratory we used the Mount Gorsa porphyry quarry (Trento, North Italy), characterized by a homogeneous rock type but strongly variable fracturing states related by complex structurally-controlled and progressive slope damage processes.

We performed a field campaign on quarry front making a) Geometrical UAV surveys and b) field Geological Strength Index (GSI) evaluation on typical spots, c) we carried out IRT monitoring during night cooling using FLIRT1020 thermal camera.

During data processing d) thermal data acquired were corrected by environmental effects (blue sky radiation, slope inclination etc.) adopting original and ad hoc calibrated filters to skim the thermal response from geometrical and external biases. Finally we try to find a correlation between the thermal response of rock-mass outcrop to their quality index.

Our results support the possibility to upscale the analysis to field conditions in order to account for the radiative characteristics of natural environments, the limitations of the technique and upscaling issues typical of fractured rock-mass, taking into account that fracturing metrics (used in laboratory phase) at rock-mass scale, should influence block size distributions, which is fundamental in the evaluation of quality indices, e.g the Geological Strength Index (GSI) widely used in engineering applications.

Emphasising all these issues, the goal of our work is to investigate the relationship between the thermal response of rock mass quality index, through an experimental method developed at laboratory scale and upscaled to in situ conditions.

How to cite: Franzosi, F., Crippa, C., Garzonio, R., Casiraghi, S., Derron, M.-H., Jaboyedoff, M., and Agliardi, F.: In situ assessment of rock mass fracturing using infrared thermography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14869, https://doi.org/10.5194/egusphere-egu23-14869, 2023.

Heterogeneities control rock properties, especially hydraulic and geophysical properties. Complex systems typically include multiple porosities at embedded scales, from the micro/meso cracks and pores to geological macro-fractures and karsts. This complex network play coupled roles and introduces difficulties in the characterization of the whole formation.

In order to constrain these coupled effects, we use seismic to acoustic data to characterize a multi-scale double porosity network and to understand the corresponding flow and mechanical properties of a shallow aquifer reservoir. The study focusses on the platform “Observatoire des transferts dans la Zone Non-Saturée” (O-ZNS, Orléans, France), an artificial excavation in the karstified and fractured limestone formation of Beauce aquifer. It is composed by an exceptional well (20 m-depth, 4 m-diameter) surrounded by 8 cored boreholes.

Two seismic refraction profiles crossing the O-ZNS site were carried out to determine P-wave velocities. The profiles delineated three main geological units: (i) a clayey soil (0-2 m), (ii) a weathered and karstified limestone layer (2-7 m), and (iii) massive limestone down to the underlying Molasse du Gâtinais layer at a depth of 25 m. In consistence with the lithological log, a thin layer of more massive limestone is highlighted around 5 m-depth. In addition, we also observed that the increase in P-wave velocity slows down after 15 m. This effect is consistent with the increasing fracture density and karst development observed on the direct log imagery and on the well 3D scan. In the massive thin limestone layer of 5 m-depth, the interpreted relative crack density is low, around 0.08. However, in the last layer from 15 to 20m-depth, the relative crack density is much more important, even so discrepant, with maximal values around 0.4.

In parallel to large scale field investigation, mechanical tests and elastic wave velocities have been measured on representative core samples. A strong discrepancy is observed, whatever the property. For example, at 16 m-depth, P-wave velocities are distributed from 3,650 to 5,700 m.s^-1 and the corresponding mechanical parameter of crack density ranges from 0 to 0.5. In addition, extreme values of crack density, above 1 are observed around 19 m-depth. These large discrepancies and crack density values are consistent with mechanical behavior and microstructure observation made directly on core samples, even though some samples are more porous than cracked and the distinction need to be kept. Samples are then classified through image processing in three categories: the porous ones, the cracked ones, and the mixed ones allowing to discuss and organize the heterogeneity distribution of the O-ZNS.

To complete the study, an intermediate characterization is running on metric blocs sampled at different depth from the O-ZNS well. Their analyses include 3D external scans at high resolution (as for the surface of O-ZNS well) and P-wave velocity measurements at intermediate frequencies. These blocs are then iterativelly, cut into smaller blocs and re-characterized in order to obtain the distribution of heterogeneity size and characteristics with depth targeting the determination of a REV (Relative Elementary Volume) for future modeling developments.

How to cite: Mallet, C., Laurent, G., and Azaroual, M.: Seismic to acoustic characterization of geomechanical and microstructural properties of a vadose zone heterogeneous limestone formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3096, https://doi.org/10.5194/egusphere-egu23-3096, 2023.

How to implement rock bridges and rock bridge failure in slope stability analysis is an ongoing discussion within the rock mechanic and landslide community. Although there has been intensive research over several decades, there is still a lack of knowledge on how to measure intact rock bridges on rock slopes, how to quantify their impact on rock mass strength, and how they affect the initial failure mechanism. Therefore, we present the analysis of a rock fall case study located in the alpine environment of southern Salzburg (Austria), where a rock slope composed of a polymetamorphic rock mass hosted three rock fall events in the year 2019. The primary aim of this study is the reconstruction of the multiphase failure event and the investigation of the influence of the discontinuity network with its intact rock bridges on the initial failure mechanism.

In our study, we performed a detailed reconstruction of the rock fall process by helicopter-borne event documentation. Moreover, we identified the rock fall failure mechanism by analysing a video capturing the first rock fall event.

Furthermore, we developed a high-resolution digital surface model of the complex post-failure topography by unmanned aerial vehicle photogrammetry (UAV-P) with real-time kinematics (RTK). Based on this model, we map the location, orientation and persistence of pre-existing discontinuities and identify failed intact rock bridges on the rupture surface of the unstable rock slope.

Additionally, we conducted point load and direct shear tests in the rock mechanic laboratory. We applied the former on block specimens to derive the uniaxial compressive strength of the intact rock. The latter allowed us to estimate the Mohr-Coulomb shear strength properties of intact rock and of failure planes, which formed sub-parallel to foliation planes in course of the test procedure.

After the third rock fall event of 2019, a ground-based interferometric synthetic aperture radar (GbInSAR) was installed for 166 days to monitor the actual deformation of the rock slope. We analysed the obtained deformation data at mm resolution to detect zones of ongoing slope movements.

Finally, we integrate the topographical and geological model, the structural inventory, and the geomechanical properties into a 2D numerical model based on the distinct element method (UDEC). We use Voronoi tessellation to allow the development of any failure path within intact rock bridges. By varying the persistence of pre-existing discontinuities and the shear-strength properties of rock bridges, we study the impact of rock bridge location, spatial distribution, and strength on the initial failure mechanisms of the rock slope. We validated the distinct element model by comparing its outcome with the essential characteristics of the rock fall observed in the event reconstruction and deformation monitoring.

By this integrated approach of methods applied to a polyphase rock fall process, we show that the initial rock fall failure mechanism is sensitive to the spatial distribution of rock bridges and their assigned shear strength properties.

How to cite: Gerstner, R., Kuschel, E., Fey, C., Voit, K., Valentin, G., and Zangerl, C.: Rock bridge control on the failure mechanism of a rock fall in a metamorphic rock mass, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6698, https://doi.org/10.5194/egusphere-egu23-6698, 2023.

We present a novel apparatus designed to investigate the mechanical and chemical processes active during the nucleation and the subsequent propagation of a seismic rupture. The earthquake is experimentally represented by the sudden frictional sliding of two blocks caused by either: i) the passage of a rupture front from a nearby seismogenic source at prescribed slip velocity, or ii) by the sudden release of strain energy cumulated during the slow (tectonic) loading stage preceding the nucleation of seismic rupture. M.E.E.R.A. (Mechanics of Earthquake and Extended Rupture Apparatus) is a biaxial horizontal machine installed at the laboratories of the Istituto Nazionale di Geofisica e Vulcanologia of Rome (Italy) thanks to a grant funded by the Italian Dipartimento di Protezione Civile. MEERA works on two blocks sized 320x80x50 mm³ put in frictional contact under a normal load up to 30 MPa. Blocks can be either rocks or analogue materials. The normal load and the shear stress are supplied by 6 hydraulic piston cylinders. One piston applies the tangential force up to 150 kN and up to 40 mm/s of slip rate. The other 5 cylinders modulate the normal force on the 320 x 50 mm² contact surface. The 6 pistons are mounted on a rigid stainless-steel vessel that can be closed by a top built in plexiglass, which enables the environmental chamber for fluid confinement. The plexiglass top resists up to 6 MPa of fluid pressure exerted and controlled by using two ISCO pumps.  MEERA is designed following the outline described in McLaskey and Yamashita (2017) and introduces three novelties: the control in displacement and displacement rate of the tangential piston up to 1kS/s; the environmental chamber; the rigid stainless-steel frame. MEERA is designed to study how the tectonic loading of a frictional interface composed of natural rocks determine the stress state and shear stress evolution governing seismogenic processes. To this end, the simulated fault in MEERA is equipped with acoustic sensors, strain gauges, optical fibers and high velocity cameras to measure and constrain rupture nucleation processes and earthquake source parameters, including directivity and rupture velocity, the dynamics of seismic ruptures and the earthquake energy budget at different scales. We aim at comparing the laboratory observations and the signals collected by MEERA with those collected by the newly developed on-fault observatory of the ERC FEAR project in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Swiss Alps) to provide novel insights in earthquake mechanics.

How to cite: Spagnuolo, E., Cornelio, C., Aretusini, S., Pozzi, G., Cocco, M., Selvadurai, P., and Di Stefano, G.: A novel apparatus to study the mechano-chemical processes active during the nucleation and propagation of earthquakes (MEERA)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8807, https://doi.org/10.5194/egusphere-egu23-8807, 2023.

Talc is an important product of several hydration and dehydration reactions in deep faults and subduction zones. The unique weakness of talc along its basal planes makes it an essential component in understanding various fault slip behaviors (e.g., episodic vs continuous slip, seismic vs aseismic) or realistic geodynamic models. A recent experimental study by Boneh et al. (2023) on talc mechanical behavior at high P-T conditions highlighted: (i) talc’s low friction coefficient under all conditions (<0.14), with thermal weakening down to µ~0.01 at 700 °C. (ii) Grain-scale microstructures demonstrate a component of fracturing and microcracking under all conditions tested. And (iii) pressure-dependence of talc strength decreases at higher temperatures, where there is also a greater tendency for localization. A vital part of depicting mineral rheology is the understanding of their underlying mechanisms of deformation associated with the observed bulk mechanical and microstructural behavior. To reveal the underlying deformation mechanism/s we analyzed the deformed samples through high-resolution transmission electron microscopy (TEM) at Utrecht university of samples prepared using a focus ion beam (FIB). Five talc samples were examined – an undeformed sample, and samples deformed at 400, 600, and 700°C under 1 GPa, and at 400 °C under 1.5 GPa.

Seven FIB lamellae sampled areas adjacent to the main fracture (if exists) or high damage zones. The starting material shows talc flakes with a thickness of ~100-400 nm without a sample-scale preferred alignment. The sample deformed under 400°C and 1.5 GPa exhibits distributed deformation with opening cracks along talc basal planes and pervasive kinking normal to the basal planes. The sample deformed at 400°C and lower pressure (1.0 GPa) exhibits thin lamination (~50 nm) well oriented with the orientation of the main fracture plane. The sample deformed at 600°C exhibits crystal delamination along the basal cleavage (forming grain fragments <10 nm in width) along the main fracture. The sample deformed at 700°C exhibits more areas of high damage, possibly due to the similar basal-cleavage delamination. A key incentive is to relate the observed nano-scale crystal defects with the bulk mechanical behavior and with processes that might promote the localization of deformation. Pressure-dependent strength can be accounted for by kinking and kinking-induced porosity while thermal weakening can be related to temperature-dependent mobility of crystal defects leading to delamination along the basal cleavage. We will discuss possible physical mechanisms of talc deformation and the prospect of extrapolating the mechanical behavior of talc achieved at the lab to the range of conditions expected in natural settings.

How to cite: Boneh, Y., Ohl, M., Plümper, O., Hirth, G., and Peč, M.: The Weakest Link – Revealing the microphysical deformation mechanisms of talc under P-T conditions associated with fault creep and slow slip events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11449, https://doi.org/10.5194/egusphere-egu23-11449, 2023.

Morphology of rock joints (faults, fractures) has been recognized as the key factor controlling their mechanical behaviour, including the pre-peak and post peak phases as well as dilatancy. It also appears playing a significant role in two mechanical behaviours that have been observed on natural fracture during shearing: (i) joint dilation, or (ii) joint closure in association with asperities crushing, and rock matrix plastic deformation. We examine how (i & ii) occur in the joint, discussing their relationship with normal stress, joint morphology and intact matrix mechanical properties. To do this, two innovative methodologies based on 3DP technologies using a sand and phenolic binder on one side and a polymer (PA12) and binder jetting technology on the other one are applied to built fractures in a weak matrix, and in a strong matrix respectively. Joint surface roughness are built as fractal property with a self–affine replication, in accordance with natural observations. Results of direct shear tests under constant normal stress reveal that the mechanical behavior of the joints is first controlled by the mechanical parameters of the material (UCS/σ_n ratio), then by the joint geometry. In the case where the UCS/σ_n ratio is high (>40) corresponding to a strong material compares to the mechanical solicitation, no significant damage is notice on the joint and the maximal dilation angle is controlled by the steepest angles of the shorter wavelength asperities, which may only represent a small percentage of the surface roughness. In a the case of a weak material the joint behaviour is more complex, and is controlled by a specific range of asperities sizes. Three behaviours were observed depending on the applied normal stress: (i) at low normal stress the larger wavelengths asperities cause dilation since they are not sheared off; (ii) at normal stress over 40% of the UCS value, tensile and/or slip cracks were observed around those asperities, leading to their crushing and beheading; (iii) at normal intermediate stress, the two mechanisms were conjointly observed. In the second case (ii) joint closure is observed and the permeability increases in the surrounding matrix. Those results implies that the UCS/σ_n ratio plays an key role in fault shear behaviour, off-fault damage propagation and fluid circulation.

How to cite: Conin, M., AbiAad, E., Deck, O., and Jaber, J.: Investigation of fault behaviour during shear process for weak and strong materials using 3D printing technologies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9823, https://doi.org/10.5194/egusphere-egu23-9823, 2023.

Earthquakes introduce long-lasting transient mechanical damage in the subsurface that can take years to recover to a new elastic steady-state. The associated transient perturbation of the elastic moduli can cause postseismic hazards such as enhanced landsliding.  This dynamics is linked to relaxation, a phenomenon observed in a wide class of materials after straining perturbations. In this study, we analyze the successive effect of two large earthquakes (the 2017 Mw7.7 Tocopilla and the 2014  Mw8.2 Iquique earthquakes) on ground properties through the monitoring of seismic velocity from ambient noise interferometry in the Atacama desert in Chile. The absence of rainfall in this area allows study of the mechanical state of the subsurface by limiting the potential effect of variations in groundwater content. We show that relaxation timescales are a function of the current state of the subsurface when perturbed by earthquakes, rather than ground shaking intensity. Our study highlights the predictability of earthquake damage dynamics in the Earth's near-surface and potentially other materials. We propose to reconcile this paradigm with existing physical frameworks by considering the superposition of different populations of damaged contacts. 

How to cite: Illien, L., Turowski, J. M., Sens-Schönfelder, C., Berenfeld, C., and Hovius, N.: Predictable healing rates in near-surface materials after earthquake damage in Chile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5402, https://doi.org/10.5194/egusphere-egu23-5402, 2023.

The propagation of micro-cracks will cause the change of rock infrared radiation (IR) information, which provides the possibility to study the rock damage behavior and failure precursor using IR. In this paper, a new quantitative characterization method of rock damage evolution using IR is proposed. Firstly, the maximum classes square error and median filter methods are used to separate the temperature increment caused by crack development in IR images. On this basis, a new index, Damage Infrared Energy Response (DIER), is proposed to describe the crack evolution state and recognize the failure precursor of rock. It is found that the change characteristics of DIER and Acoustic Emission (AE) count are consistent: the DIER remains at the level in the compaction and elastic stages, rises gradually in the stable crack propagation stage, and increases sharply in the unstable crack propagation stage and fluctuates with the appearance of AE count “peak”. The change characteristics of DIER in unstable crack propagation stages can be regarded as the failure precursors of rock, about 84.10% of peak stress. Then, according to the continuum damage mechanics theory, the DIER is used to establish a theoretical characterization of the damage variable for rock, which can accurately describe the damage evolution process of the rock under uniaxial compression. The research results can provide experimental and theoretical support for monitoring slope and rock engineering stability by IR.

How to cite: Liu, W. and Jaboyedoff, M.: Theoretical damage characterization and failure precursor recognition of the rock under uniaxial compression using infrared radiation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7857, https://doi.org/10.5194/egusphere-egu23-7857, 2023.

At the nanoscale, fracturing creates surface area and flow pathways, which control the rates of fluid-rock interactions. However, how fractures form at the nanoscale remains enigmatic. Here, we implement molecular dynamics simulations to reproduce fracture propagation in quartz and basalt. These simulations require large systems and long simulation times and are therefore currently depending on interatomic potentials. In the recent years, machine learning approaches have been established as a way to fit interatomic potentials, where the potentials are trained with quantum-mechanical data obtained from ab initio molecular dynamics simulations. We have developed machine-learned interatomic potentials for silica and basalt that allow using molecular dynamics simulations to simulate fracture propagation at the nanoscale. The interatomic potentials reproduce the mechanical properties of bulk silica and basalt sand have also been trained to account for fracture propagation. First, we trained a potential on silica to verify the fitting procedure, and then we used the same procedure to train an interatomic potential for basalt. By training the potential with water and carbon dioxide as fluids, we aim to study how a dynamic fracture damage basaltic glass and how the water and carbon dioxide enter these fractures in the wake of rupture. Our results are relevant for carbon mineralization where a coupling between dissolution of the basalt and precipitation of carbonate minerals can lead to nanofracturing of the rock.

How to cite: Guren, M. G., Sveinsson, H. A., Malthe-Sørenssen, A., Caracas, R., and Renard, F.: Machine-learned interatomic potentials for modelling nanoscale fracturing in silica and basalt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6256, https://doi.org/10.5194/egusphere-egu23-6256, 2023.

Neural networks have proved to be able to capture the relevant and informative features of a wide variety of data types and predict the desired output for different regression or classification problems. Finding a mapping between materials’ structure and a given physical property of those systems is an example of a problem that could be approached with machine learning methods like neural networks. Especially when we are dealing with systems with a very large design space where using classical computational methods like molecular dynamics can be very time and resource-consuming for the study of a very large number of systems, a well-trained neural network can be greatly faster and more efficient for computing the relevant properties. In this work, we study α-quartz crystals with one porous layer with simplex noise as the shape of porosity. Simplex noise is a gradient based procedural algorithm that can produce irregular geometries with surface morphology resembling what is observed in nature. The property that we want the neural network to learn is the yield stress of these systems under both shear and tensile deformation. Molecular dynamics simulations are used for a randomly selected sample of possible structures in order to generate the ground truth to be used as the training data. We employ deep convolutional neural networks (CNN) which are commonly used when dealing with image or image-like data since the input data for the problem in hand is a binary 2-D structure of the porous layer of the systems. The trained model is compared with a basic polynomial fit of stress versus porosity. The trained CNN is successful in predicting the yield stress of a system based on the geometry of that given system, with lower variability and higher precision compared to the base polynomial regression method. The saliency maps created with the trained model show the model to be successful in capturing the physics of the problem when compared with the stress fields calculated using molecular dynamics simulations. This method of modeling materials can be further developed for the inverse design of structures with desired properties without the need for a huge number of simulations on a wide domain of possible systems.

How to cite: Najafi, F., Andersen Sveinsson, H., Dreierstad, C., and Malthe-Sørenssen, A.: Modeling the relationship between mechanical yield stress and material geometry using convolutional neural networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13293, https://doi.org/10.5194/egusphere-egu23-13293, 2023.

The Universal Distinct Element Code (UDEC) has been rising much prevalence and is a validated technique applied to simulations in varied branches of geotechnics. Nevertheless, the characterization of water-weakening effects on a rock model remains elusive, resulting in a poor constraint referring to water-induced simulations by UDEC. In this context, previous research has been attempting to conduct an intuitive link between modelling parameters and saturation degrees, S_r, to implement a water-weakening process in UDEC, leading to a detrimental potential devoid of the basic logic that modelling parameters determine macroscopic properties of a rock model in contrast to dominance by S_r owing to the discrepancy in a physical sense and spatial scale. To fill in this gap, a new methodology coupled with a parametric study is first proposed with procedures that macroscopic properties of actual rock with different S_r are input into parametric relations to acquire predicted modelling parameters, which will be sequentially calibrated and adjusted until simulations are in line with actual tests. Utilizing this methodology, water-weakening effects on macroscopic properties, mechanical behaviours, and failure configurations of numerical models in UDEC are commensurate with tested ones to the utmost with noticeable computational expediency harnessing the benefit of the parametric study, indicating the feasibility and simplicity of the methodology. Thus, in the implementation of a water-weakening process in UDEC, we suggest converting an embarking from an intuitive link between modelling parameters and S_r into a parametric analysis to determine modelling parameters according to macroscopic properties under different S_r.

How to cite: Bu, F., Jaboyedoff, M., Derron, M.-H., and Xue, L.: Parametric study to characterize water-weakening effects in UDEC, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12979, https://doi.org/10.5194/egusphere-egu23-12979, 2023.

The deformation and failure processes of rocks under stress are primarily induced by microcracking. Detecting this micro-interaction phenomenon before the ultimate failure has paramount importance for predicting the post-failure rock damage characteristics. In this study, we aim to quantify the evolution of microcracking through fractal analyses of scanning electron microscope (SEM) images, captured from three different rock types subjected to uniaxial loading at various stress levels. In terms of uniaxial compressive (UCS) and tensile strength (UTS) values, the rocks range from the strongest to the weakest as being diabase, ignimbrite, and marble, respectively.  All rock samples are uniaxially loaded up to critical stress thresholds as crack initiation (σ_ci), crack damage (σ_cd), and peak stress (σ_p) levels, considering their pre-defined characteristic stress-strain curves. Using the box-counting technique, the fractal dimension values (D_B) of cracking intensity, induced by loading are determined for all these three stages. Here, it should be noted that higher fractal dimensions represent more intense microcracking according to the fractal theory. The results show that the D_B values are increasing with the increasing amount of microcracks and the greatest D_B values are calculated for Diabase due to its highest strength ratio (UCS/UTS). Although the marble has the weakest strength values, it presents a higher D_B value than that of ignimbrite (D_Bmarble = 1.215 and D_Bignimbrite = 1.133) once the σ_cd stress threshold is reached. Furthermore, the D_Bmarble value is also greater than the D_Bignimbrite value for the σ_p stress level. It is because marble has a higher UCS/UTS ratio than the ratio of ignimbrite. Our results highlight the important role of rock texture on brittleness which exerts a primary control on fractal dimensions (D_B). A decrease in volumetric rigidity is more dramatic in marble than in ignimbrite with incremental loading. The insights provide a better understanding of the microcracking process that leads to macro-scale deformations in rock engineering.

How to cite: Dinç Göğüş, Ö., Avşar, E., Develi, K., and Çalık, A.: Progressive failure characteristics of different rock types through fractal analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-254, https://doi.org/10.5194/egusphere-egu23-254, 2023.