EMRP1.14

Failure and fault slip in crystalline rocks is associated with dilation. When pore fluids are present and drainage is insufficient, dilation leads to pore pressure drops, which in turn lead to strengthening of the material. We conducted laboratory rock fracture experiments with direct in-situ fluid pressure measurements which demonstrate that dynamic rupture propagation and fault slip can be stabilised (i.e., become quasi-static) by such a dilatancy strengthening effect. We also observe that, for the same effective pressures but lower pore fluid pressures, the stabilisation process may be arrested when the pore fluid pressure approaches zero and vaporises, resulting in dynamic shear failure.

We use acoustic emission locations and our fluid pressure sensors to further detail dilatancy-induced stable failure by tracking the progression of the rupture front (i.e., creation of the fault) and the active slip patches of the newly formed fault. In doing so, we are able to link local pore pressure records to the position of the rupture front where dilation is strongest. We see minimal slip in the wake of the rupture front. Once the fault is completed, we observe that the entire fault slips for up to a few minutes, driven by pore pressure recharge of the fault zone. Hence, we directly observe decoupling of rupture and “after”-slip that would otherwise – in a dynamic failure – occur simultaneously.

All our observations are quantitatively explained by a spring-slider model combining slip-weakening behaviour, slip-induced dilation, and pore fluid diffusion. Using our data in an inverse problem, we estimate the key parameters controlling rupture stabilization: fault dilation rate and fault zone storage. These estimates are used to make predictions for the pore pressure drop associated with faulting, and where in the crust we may expect dilatancy stabilisation or vaporisation during earthquakes. For intact rock and well consolidated faults, we expect strong dilatancy strengthening between 4 and 6 km depth regardless of ambient pore pressure, and at greater depths when the ambient pore pressure approaches lithostatic pressure. In the uppermost part of the crust (<4 km), we predict vaporisation of pore fluids that limits dilatancy strengthening.

How to cite: Aben, F. and Brantut, N.: How dilatancy-induced pore pressure changes control rupture and slip during failure experiments in crystalline rock., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5711, https://doi.org/10.5194/egusphere-egu22-5711, 2022.

Superhot Rock (SHR) geothermal projects (e.g., Japan Beyond-Brittle Project, Iceland Deep Drilling Project, and Newberry Volcano) seek to extract heat from geothermal reservoirs where water reaches a supercritical state (≥ 400 °C). Exploiting such a resource could multiply the electrical power delivered by geothermal wells by almost an order of magnitude. However, SHR reservoirs are hosted in semi-brittle to ductile rocks where fluid flow, porosity, permeability, and rock mechanics are still poorly understood. We conduct experiments in a newly designed, internally heated, gas-confining triaxial apparatus (located at EPFL, CH) where we deform reservoir-type samples under realistic SHR temperature, pressure, and strain rate conditions. Deep well core samples (40 x 20 mm) of andesitic basalts (porosities of 8–10%) from Newberry Volcano (US), were subjected to increasing confinement pressure (25–100 MPa) and temperature (20–500 °C) while continuously recording gas permeability via harmonic permeability. Additionally, triaxial deformation experiments were done at strain rates of 10^-6 s^-1, confinement up to 100 MPa, temperature up to 500 °C, and up to 8% strain while recording permeability. Results were compared with granite samples from Lanhelin (Fr.). Samples were ductile (e.g., no localization of strain) at relatively low pressure–low temperature conditions (100 MPa, 200 °C). Moreover, permeability in samples subjected to hydrostatic conditions rapidly decreased several orders of magnitude from an initial value of 5.10^-20 m² to less than 10^-22 m²  at 50 MPa and 200 °C, effectively impermeable. Thus, permeability decreases rapidly in the ductile regime with strain to reach below measurable values at around 3% strain, and it remains so during subsequent semi-brittle flow up to 8% strain. We interpret this rapid decay of permeability as a result of the conjoined effect of ductile pore collapse and plastic deformation of the poorly crystalline matrix present in the sample. These insights further underline the need for advanced, sustainable reservoir engineering techniques in order to extract heat from high enthalpy geothermal reservoirs.

How to cite: Violay, M. and Meyer, G.: Permeability evoluation at the brittle to ductile transition in newberry volcano basalt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7387, https://doi.org/10.5194/egusphere-egu22-7387, 2022.

The fracturing state of rocks is a fundamental control on their hydro-mechanical properties at all scales and provides a descriptor of the evolution of brittle deformation around faults, underground excavations, and slopes. Its quantitative assessment is thus key to several geological, engineering and geohazard applications.

Descriptors of rock fracturing are diverse depending on considered scale, fracture topology (traces, surfaces) and sampling dimension (linear, areal, volumetric). A complete representation of fracture distribution and abundance in a 3D space can be obtained in the laboratory by non-destructive imaging techniques (e.g. X-ray CT), in terms of volumetric fracture intensity (P₃₂) and porosity (P₃₃). Nevertheless, geophysical imaging is usually unable to resolve small objects in fractured media at field scale. Window and scanline sampling strategies are easily applied in the field to measure fracture intensity descriptors (e.g. P₁₀, P₂₁) or empirical rock mass quality indices (e.g. GSI), but are affected by scale and fracture orientation biases. Some authors suggested that rock mass fracturing states can be characterized by measuring their heating and cooling response through infrared thermography (IRT), but a physically-based, generalized approach to prediction is lacking.

In this perspective, we carried out an experimental study on the thermal response of rock samples with known fracturing state. We studied cylindrical samples of gneiss (7) and schist (8), pre-fractured in uniaxial compression that produced complex fracture patterns constrained by rock composition and fabrics.

Using MicroCT (voxel: 0.625 mm) we reconstructed the 3D fracture network and computed the P₃₂ and P₃₃ of each sample. Then, we set up cooling experiments in both laboratory and outdoor conditions. In laboratory experiments, samples were oven-heated at 80°C and let cool in a controlled environment. Sample surface temperature during cooling was imaged in time lapse using a FLIR^TM T1020 IRT camera. In outdoor experiments, samples underwent natural solar forcing in a daily heating-cooling cycle.

The acquired multi-temporal thermal images were processed to extract: a) spatial temperature patterns corresponding to the response of individual features and fracture networks at different cooling steps; b) time-dependent cooling curves, described in terms of Cooling Rate Indices and a Curve Factor. These descriptors show statistically significant correlations with fracture abundance measures, stronger with P₃₃ than with P₃₂ and more robust for gneiss samples, characterized by more distributed fractures than schist. More fractured rocks cool at faster rates and the corresponding cooling curve shapes can be normalized to remove the effects of lithology and boundary conditions to obtain a predictive tool. Experimental results have been reproduced by 3D finite-element modeling of the cooling process in numerical samples including explicit fracture objects. Model results closely reproduce experimental data when fracture surfaces are included as convection surfaces, suggesting that overall sample cooling rates depend on the size of individual blocks forming the sample. Results of outdoor experiments show that differences in thermal response can be significantly detected even in natural conditions. Our results provide a starting point to develop an upscaled, quantitative methodology for the contactless in situ assessment of fracturing state of rock masses using thermal data.

How to cite: Franzosi, F., Casiraghi, S., Colombo, R., Crippa, C., and Agliardi, F.: Laboratory assessment of rock fracturing state using infrared thermography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4697, https://doi.org/10.5194/egusphere-egu22-4697, 2022.

Stromboli volcano, located in the north-easternmost island of the Aeolian archipelago (Southern Italy) and well known for its persistent volcanic activity, has experienced at least four sector collapses over the past 13 thousand years. The most recent activity resulted in the formation of the Sciara del Fuoco (SDF) horseshoe-shaped depression and a tectonic strain field believed to have promoted flank collapses and formed a NE / SW trending weakness zone across the SDF and the western sector of the island. The tectonic strain field interplayed with dyking and fracturing appears to control the episodes of instability and the onset of slip surfaces. This study presents new data identifying areas of damage that could promote fracturing via remote sensing and rock friction measurements taken on rocks around the SDF and the coupled “weak” zone. We have carried out a multiscale approach by integrating satellite observations with block and sample scale physical and mechanical properties and frictional tests carried out in triaxial configuration on cm scale slabs. Over 5000 individual fractures have been at first processed through the MatLab toolbox FracPaQ to determine fracture density, slip and dilatancy tendency around the collapse scarp with results showing that dilation and slip 0.6< is more common the northern side of the SDF as well as around areas of eruptive activity.

Key units have been sampled on the field (Paleostromboli, Vancori and Neostromboli) with reference to SDF and the weak zone. Physical and mechanical properties defined using elastic wave velocities, electrical resistivity, uniaxial compressive strength and elastic moduli have been assessed and inverted for comparison to field scale geophysical investigations. Finally, direct-shear tests in triaxial configuration were carried out to explore the frictional properties using rectangular basalt slabs at 5 – 15 MPa confining pressure in dry and saturated conditions. Preliminary results show a variation in the friction coefficient (µ) between 0.55 and 0.7 with a general µ decrease with increasing confining pressure and saturation. The most porous Neostromboli units show the lowest friction.  This suggests that the textural and pre-existing crack damage variability due to the complex and different magmatic history and cooling rates do control the evolution of the frictional properties and evolving fracturing processes. Further work will structurally quantify the slip evolution throughout post-mortem microstructural observation in order to interpret the relations to the field scale weakness zone and the SDF.

How to cite: Alcock, T., Vinciguerra, S., and Benson, P.: Multiscale analysis of physical rock properties at Stromboli Volcano: what controls the frictional properties?   , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1434, https://doi.org/10.5194/egusphere-egu22-1434, 2022.

The case study presented herein is located in the alpine environment of Austria (Hüttschlag), in the geologic unit of the Rauris Nappe, belonging to the Glockner Nappe System. The study site is composed of intensively foliated and fractured calc-mica schists and greenschists. Together with several generations of pre-existing discontinuity-sets, they form a rock mass, which has hosted multiple rock fall events since 2019. The rock fall events show a cumulative volume of 41 000 m³, with individual blocks of up to 200 m³ reaching the valley bottom.

In order to gain insights into the interplay between structural geology and the rock fall failure mechanism, we present a combined approach of methods. They act on multiple observation scales: At the micro-scale, intact rock samples are studied by petrographic microscopy of orientated thin sections. This provides insights into the mineralogy of the intact rocks and their inherent brittle and ductile microstructures (e.g. micro-cracks, folding).

In the field, advanced remote sensing techniques were applied, to perform medium- to large-scale investigations. For this purpose, a ground-based radar interferometer (GB-InSAR) was installed for several months. By this, the actual deformation of the unstable rock face and of the rock fall deposit at the slope´s foot was measured at mm resolution. Additionally, several campaigns of terrestrial laser scanning (TLS) enable us to derive high-resolution recordings of the inaccessible rock face, backed by 3D point cloud processing (LIS Pro 3D) tools. For additional displacement measurements and graphic representation of the results, unmanned aerial system photogrammetry (UAS-P) delivers a 3D model of the rock face.

Geological field investigations complete this combined approach, comprising the recording of lithological, hydrogeological and structural geological features. They embed the rock fall site in its geological setting and allow the creation of a 3D discontinuity network, validating the measurements derived from the advanced remote sensing techniques listed above.

The preliminary results promise interesting insights into the interplay between distinctive structural features and the failure mechanisms of the rock fall site in Hüttschlag, working on variable scales: From micro-structures to well-defined discontinuities, that may be reactivated in course of the rock fall process. This broad database serves as the basis for numerical modelling, intensifying the investigation of failure mechanisms. Furthermore, the high-resolution recordings of the instable rock face derived from UAS-P and TLS allow us to assess the potential failure volume of future rock fall events, contributing to the rock fall site´s hazard assessment subsequently.

How to cite: Gerstner, R., Kuschel, E., Valentin, G., Voit, K., Straka, W., and Zangerl, C.: Impact of structural geology on the failure mechanisms of a rock fall site in a metamorphic rock mass (Hüttschlag, Austria), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7622, https://doi.org/10.5194/egusphere-egu22-7622, 2022.

In deglaciating environments, rock slopes are affected by stress perturbations driven by mechanical unloading due to ice downwasting and concurrent changes in thermal and hydraulic boundary conditions. Since in-situ data is rare, the different processes and their relative contribution to slope damage remain poorly understood. Here we present detailed analyses of subsurface pore pressures and micrometer scale strain time-histories recorded in three boreholes drilled in a rock slope aside the retreating Great Aletsch Glacier (Switzerland). Additionally, we use monitored englacial water levels, climatic data, and annually acquired ice surface measurements for our process analysis.

At the timescale of days, diurnal meltwater cycles and rainfall infiltration into the glacier during summertime cause strong pressure fluctuations in the subglacial drainage channel that diffuse into the adjacent rock aquifer. We show that the pressure diffusion from the subglacial meltwater channel, through the fractured bedrock below the glacier ice, to the ice-free bedrock slope occurs under predominantly confined conditions. In the adjacent ice-free bedrock, rainfall infiltration can cause strong variations in the phreatic groundwater table of the slope. On the seasonal timescale, glacial hydraulic boundary conditions vary with high, relatively constant englacial water levels during wintertime and lower mean englacial water levels during summertime. Above ice elevations, snowmelt infiltration during springtime causes yearly maximum phreatic groundwater tables and a general recession over the rest of the year, that is interrupted by summertime rainfall infiltrations. The seasonality in hydraulic head levels of both the glacier and the rock slope controls the interaction of the two systems. On timescales of decades, phreatic groundwater levels in the rock slope are often assumed to be linked to the ice elevation of temperate glaciers. According to our data, this head boundary effect of the glacier is mainly effective during wintertime when it controls the minimum groundwater level in the slope.

Our results show that the variations in mechanical boundary conditions (or loads) caused by a temperate valley glacier on the adjacent rock slope are more complex than had been previously described. Our observed rapid bedrock strain signals coincide with some of the extreme englacial water level states, and are likely caused by rapid changes in the mechanical load of the glacier with an empty or water filled englacial drainage system. Similarly, but at seasonal timescales, the spring and fall transition time of the englacial hydrological system coincides with characteristic strain reactions in our bedrock slope. Our in-situ data show that these effects also promote progressive rock mass damage, probably similar to hydromechanical effects. Additionally, we show how a single extreme rainstorm event triggers hydromechanical damage exceeding the levels of two years exposition to all the other drivers for progressive rock mass damage in this environment.

The magnitude and impact of coupled cyclic processes in a paraglacial rock slope vary with location on the slope and the process considered. The strongest damage is observed directly at the actively reteating glacer margin and moves through the slope at relatively high speed.

How to cite: Hugentobler, M., Loew, S., and Aaron, J.: Hydromechanical Coupling and Damage at a Retreating Glacier Margin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7429, https://doi.org/10.5194/egusphere-egu22-7429, 2022.

The sustainability of geomineral resourses’ exploitation may be assured only in presence of adequate plans for the re-use and reclamation of old or abandoned sites. Among the most commonly used techniques, mining backfill is largely employed for the stabilization of underground sites. This technique recreates the original stress state of the underground, assuring the definitive stabilization of the hypogea volumes, and reduces the risks due to the interference between underground tunnels and ground surface (e.g. possible collapses and surface subsidences). Despite these obvious advantages, careful evaluations are needed to assure the environmental sustainability, with particular attention to the interaction between the hydro-geological and permeability features of the rock body and the chemical properties of the backfill material.

The present research proposes an analysis of the advantages and the risks connected with this technique, examining a case study of mining backfill in an underground gypsum quarry at the end of the active exploitation. The considered quarry is located in Monferrato (NW Italy) and is exploited within chaotic Messinian deposits made of gypsum blocks (from centimeter-size to kilometer-size) included in a marly matrix. The study includes a campaign of field and laboratory tests (i.e. geological and geo-structural mapping and modeling, geophysical surveys, mechanical and permeability tests) that aim at characterize the permeability and mechanical behaviour of the rock mass.

How to cite: Caselle, C., Bonetto, S. M. R., Mosca, P., Paschetto, A., Vianello, D., Garello, A., and Paletto, F.: Multiscale characterization of chaotic rock body for mining backfill remediation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9278, https://doi.org/10.5194/egusphere-egu22-9278, 2022.