Catastrophic failure is a critical phenomenon present in Earth systems on a variety of scales, and is associated with the evolution of damage leading to system-size failure. Laboratory testing of rock failure permits characterization of fracture network evolution at the micro-scale to understand the interaction of cracks, pores and grain boundaries to an applied stress field, and the relationship between deformation and seismic response. Previous studies have relied on acoustic emissions (hearing) or X-ray imaging (seeing) to study the process of localization, which involves spontaneous self-organization of smaller cracks along faults and fractures on localised zones of deformation. To combine hearing and seeing of the microscopic processes and their control of system-sized failure, a novel x-ray transparent cell was used for deformation experiments of rock samples, which permits integration of acoustic monitoring with fast synchrotron x-ray imaging. To increase temporal characterization of damage beyond the temporal resolution of the fast 3D synchrotron system, acoustic emission (AE) feedback control was used to regulate the applied stress and slow down the deformation processes. As a result, there is increased temporal resolution of the incremental deformation between successive x-ray scanned states allowing synchronized comparison of acoustic emissions to x-ray scans. Here, we present the seismic analysis used to characterize the velocity evolution of the rock samples, and the location and characteristics of individual AE events in relation to microscopic deformation processes. Time-lapse velocity measurements are linked to internal stress changes and structural damage corresponding to seismic and aseismic deformation processes, while acoustic emissions are a direct indication of local cracking.  We show that we can successfully locate AE events in 3D using only two sensors on either end of the sample, based on ellipsoid mapping, and x-ray image to event correlation. We explore temporal and spatial statistics of AE signatures and how those are linked to the strain field in the samples measured with incremental digital volume correlation between pairs of recorded x-ray tomograms. The direct observation of AE and X-ray images enables quantification of relevant seismic (local cracking leading to AE) to aseismic (elastic loading and silent irreversible damage) processes, with information extracted over fine temporal resolution throughout the deformation process through the AE-feedback control.

How to cite: Mangriotis, M.-D., Curtis, A., Cartwright-Taylor, A., Andò, E., Main, I., Bell, A., Butler, I. B., Fusseis, F., Rizzo, R., Ling, M., Marti, S., Leung, D. D. V., Singh, J., and Magdysyuk, O.: Microfracture evolution leading to catastrophic failure observed by hearing and seeing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15691, https://doi.org/10.5194/egusphere-egu21-15691, 2021.

Sedimentary crustal porous rocks span a wide range of grain size distributions – from monodisperse to highly polydisperse. The distribution of grain size depends on the location and conditions of rock formation, the chemico-physical processes at play, and is influenced by subsequent geological processes. Well-sorted granular rocks, with a grain size distribution close to monodisperse, and granular rocks with a more polydisperse grain size distribution, have repeatedly been subjected to laboratory experiments. And yet the natural variability from sample to sample and structural heterogeneity within single natural samples all conspire to prevent us from constraining the effect of grain size polydispersivity. While a few studies have focused on the influence of grain size, the control of grain size distribution on the mechanical behavior of rocks has scarcely been studied, especially in the laboratory. In this study, we address this knowledge-gap using synthetic samples prepared by sintering glass beads with controlled polydisperse grain size distributions. When heated above the glass transition temperature, the beads act as viscous droplets and sinter together. Throughout viscous sintering, a bead pack evolves from an initial granular discontinuous state into a solid connected porous state, at which the microstructural geometries and final porosity are known. Variably polydisperse individual samples were prepared by mixing glass beads with diameters of 0.2, 0.5, and 1.15 mm in various proportions, which were sintered together to a final porosity of 0.25 or 0.35. Hydrostatic and triaxial compression experiments were performed for each combination of polydispersivity. The samples were water-saturated, deformed at room temperature, and deformed under drained conditions (with a fixed pore pressure of 10 MPa). Triaxial experiments were conducted at a constant strain rate at effective pressure corresponding to the ductile (compactive) regime. Our mechanical data provide evidence that polydispersivity exerts a significant control on the compactive behavior of porous rocks. Insights into the microstructure were gained using scanning electron microscopy on thin sections prepared from samples before and after deformation. These data allow for the observation of the different deformation features, and by extension the deformation micro-mechanisms, promoted by the different type and degree of polydispersivity. Overall, our data show that, at a fixed porosity, increasing polydispersivity decreases the stress required for compactant failure.

How to cite: Carbillet, L., Heap, M., Wadsworth, F., Baud, P., and Reuschlé, T.: From monodisperse to polydisperse: the influence of grain size distribution on the mechanical behavior of porous synthetic rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2629, https://doi.org/10.5194/egusphere-egu21-2629, 2021.

In this research, failure and deformation processes of andesitic rocks are investigated through laboratory and discrete element modeling (DEM) analysis to reveal the transition of the cracking, namely from microscale to mesoscale (lab scale). For this purpose, the mechanical properties of Ankara andesites were initially investigated by performing uniaxial - triaxial compressive and indirect tensile laboratory tests. Further, these properties were used as reference parameters for the calibration process in a numerical model, generated through a three-dimensional open source code (Yade) based on the discrete element method (DEM). Our results show that during the linear-elastic region of the stress-strain curve, the major mechanism of rock behavior is driven by tensile cracks. When the crack damage threshold is reached, as a result of plastic strain, the strength related to the inherent cohesion significantly decreases and damage in the rock cannot be prevented anymore. At the peak stress of the curve, both tensile and shear cracks accumulate, intensively. Even the mesoscale failure mechanism is controlled by shearing at the residual stage of the yielding, based on the micro-scale process in the DEM model, the number of micro shear cracks is very limited compared to the tensile ones. This finding shows that the friction takes the control of the damage process as the only driving force during residual phase time.  

How to cite: Dinç Göğüş, Ö., Yılmaz, D., Avşar, E., and Kayabalı, K.: Investigation of failure and damage processes of andesitic rocks , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-172, https://doi.org/10.5194/egusphere-egu21-172, 2020.

The localisation of structural damage, in the form of faults and fractures, along a distinct and emergent fault plane is the key driving mechanism for catastrophic failure in the brittle Earth. However, due to the speed at which stable crack growth transitions to dynamic rupture, the precise mechanisms involved in localisation as a pathway to fault formation remain unknown. Understanding these mechanisms is critical to understanding and forecasting earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as failure of man-made materials and structures. We used time-resolved synchrotron x-ray microtomography to image in-situ damage localisation at the micron scale and at bulk axial strain rates down to 10^-7 s^-1. By controlling the rate of micro-fracturing events during a triaxial deformation experiment, we deliberately slowed the strain localisation process from seconds to minutes as failure approached. This approach, originally established to indirectly image fault nucleation and propagation with acoustic emissions, is completely novel in synchrotron x-ray microtomography and has enabled us to image directly processes that are normally too transient even for fast synchrotron imaging methods. Here, we first present the experimental apparatus and control system used to acquire the data, followed by damage localisation and shear zone development in a sample of Clashach sandstone viewed in unprecedented detail. Time-resolved microtomography images demonstrate a strong intrinsic correlation between shear and dilatant strain in the localised zone, with bulk shear strain accomodated by the nucleation and rotation of en-echelon tensile microcracks within a grain-scale shear band. Rotation is accompanied by antithetic to synthetic shear sliding of neighbouring crack surfaces as they rotate. The evolving 4D strain field, measured with incremental digital volume correlation between pairs of recorded x-ray tomographic volumes, independently confirm the correlation between shear and dilatant strain and show how strain localises spontaneously, first through exploration of several competing shear bands at peak stress before transitioning to failure along the optimally-oriented final fault plane. In order to ‘ground-truth’ inferences made from bulk measurements and seismic waves (the primary method of detecting deformation at the field-scale where direct imaging of the subsurface is impossible), we (a) compare rupture energy estimates from local slip measurements with those from bulk slip data, and (b) use AE source location estimates to identify individual cracks and other local changes in the microstucture that may explain the AE source.

How to cite: Cartwright-Taylor, A., Main, I. G., Butler, I. B., Fusseis, F., Mangriotis, M.-D., Curtis, A., Bell, A., Ling, M., Andò, E., Rizzo, R., Marti, S., Leung, D., and Magdysyuk, O.: Seeing and hearing quasi-static shear band localization in a sandstone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9048, https://doi.org/10.5194/egusphere-egu21-9048, 2021.

The Jharia region of lower Gondwana in India is one of the largest Underground Coalmine Fires (UCF) affected coalfield in the world. The UCF induced small scale as well as large-scale surface fracturing often creates the life-threatening conditions to coal miners and local surroundings. So, there is a need to understand the thermomechanical behaviour of coal measures rocks to predict the land disturbances in thermo-environmental conditions. It will provide an insight into the UCF induced subsidence mechanism and its preventive measures. The Jharia coal field predominantly consists of sandstone (75-80% by volume) and rest is composed of coal, shale and carbonaceous shale. The present study focuses on thermo-mechanical behaviour of Barakar sandstone (BS) under elevated temperatures. The cores of BS sample were prepared according to the ISRM standards. Further, samples were grouped and thermally treated in temperature range of 25°C, 100°C, 150°C, 300°C, 400°C, 500°C, 600 °C, 700°C and 800°C at a heating rate of 5°C/min for 24 hours in furnace.  Then, these thermally treated BS samples were subjected to laboratory test for stress-strain characteristics. In the process of deformational characteristics evaluation, effect of mineralogical changes and mode of fracture pattern were also studied at the mentioned elevated temperature. Based on the obtained results, the deformational behaviour of thermally treated BS specimens can be grouped into three zones, viz., zone 1 (25-300°C), zone 2 (300-500°C) and zone 3 (500-800°C). In zone 1, the characteristics of the stress-strain curve is similar to those under air dried sandstone specimen. However, small increment in stiffness were observed upto 300°C. The stress-strain curves in this zone shows dominantly brittle fracturing. The increment in stiffness may be related to evaporation of pore water that increases the cohesion between the mineral grains resulting higher stiffness value. In zone 2, the deformation pattern again shows brittle fracturing with continuous decrement in stiffness. The reduction in stiffness may be related to thermally induced porosity and increased microcrack density. In zone 3, the stress strain curve is observed to be concave upward. It indicates the pseudo-ductile behaviour of the thermally treated BS specimens. The observed results suggest a typical behaviour of deformation pattern under UCF induced rock fracturing which may be useful in predicting the land subsidence in UCF affected areas. Present research outcome may be used to design the support measures to reduce the associated hazards.

How to cite: Tripathi, A., Gupta, N., Singh, A. K., Rai, N., and Pain, A.: Deformational characteristics of thermally treated sandstone from an underground coalmine fire region, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14332, https://doi.org/10.5194/egusphere-egu21-14332, 2021.

Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. To date, investigation of sample heterogeneity, specifically tensile strength and strength anisotropy has focused on layered rocks, such as shales, sandstones and gneisses. Data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites, even though these frequently occur in geo-energy settings.

We have performed Brazilian Disc tests on limestone samples containing planar heterogeneities, investigating Brazilian test Strength (BtS) and the effects of orientation on strength. We used prefractured Indiana limestone to represent a planar heterogeneity without cohesion and Treuchtlinger Marmor samples with central stylolites to represent a planar heterogeneity of unknown strength (as an example of a sealed fracture). The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ (steep angle) is parallel to the principal loading direction, and 0⁰ (low angle) to the horizontal axis of the sample. All experiments were filmed, and where possible Particle Image Velocimetry was used to determine internal particle motion. Moreover, we used a 2D Comsol model in which we simplified the stylolite surface as a sinusoid. The model was used to qualitatively determine how i) a different period of the sinusoid and ii) relative strength of sinusoid/matrix affect the results.

Our results show that all imperfect samples are weaker than intact samples. The 2D Comsol model indicates that the qualitative results remain unaffected by changing the period (assumed to be representative of roughness) of the cohesive heterogeneity, nor by the relative strength contrast: the location of the first fracture remains unaffected. For both heterogeneity types, the fracture patterns can be divided into four categories, with two clear endmembers, and a more diffusive subdivision in between.

For a cohesion-less heterogeneity:

• steep angles lead to frictional sliding along the interface, and only a small hypothesized permeability increase.
• Intermediate angles lead to a combination of tensile failure of the matrix and sliding along the interface, where for steeper angles more new fractures form which follow the path of the existing fracture.
• Low angles lead to closure of the old fracture and new tensile failure.

For a cohesive heterogeneity of unknown cohesion:

• Steep angles lead to intensive failure of the heterogeneous zone, attributed to the presence of a stress concentrator.
• Intermediate angles lead to partial failure along the heterogeneous zone, and the formation of new fractures in the matrix, potentially instigated by mode II failure to accommodate motion.
• Low angles lead to the formation of a new fracture plus opening within the heterogeneous zone.

These results imply that hydrofracture (i.e. creating tensile stresses) of a stylolite-rich zone will lead to more fractures than fractures in a homogeneous zone, where the orientation of the stylolites and bedding will control the orientation of the permeable pathways.

How to cite: Pluymakers, A., Barnhoorn, A., and Bakker, R.: Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5959, https://doi.org/10.5194/egusphere-egu21-5959, 2021.

A postmortem technique is introduced to investigate the fracture connectivity evolution under elevated confining pressures via a sensitivity analysis. Three Onagawa shale samples are deformed under brittle, ductile, and transition conditions, by increasing the confining pressures. Brittle deformation is characterized by longitudinal splitting of the sample at 3% axial strain, and the onset of transition from brittle to ductile deformation is between 4% ~ 5% axial strain. The ductile deformation is characterized by a distributed conjugate fracture network and strain hardening. In completion of the deformation, the samples are scanned in a commercially available X-ray CT machine. The grayscale values of the primary 2D images were reversed, stacked, and surface rendered to obtain the 3D volume distribution of the fractures. Reversing and surface rendering allowed the acquisition of volume and surface data of the fractures along with their direct visualization. Further, utilizing a residual analysis, the voxel value density distribution that fabricated the fracture network is extracted (Residual histogram). Thresholding of the residual histogram generated volume segments of the final fracture network demonstrating the sensitivity of the fracture network to the choice of threshold. Voxel volumes of fractures alone are obtained by thresholding post-peak voxel values of the residual histogram and consecutive post-peak thresholding shows that the generated volume segments of the fracture network can be utilized to interpret, possible nucleation sites after strain localization, propagation of fractures, and coalescence. Fracture connectivity is quantified by means of relative entropy from information theory, and the relative entropy of size distribution of fracture volumes showed that it is closer to zero with the fractures being well connected. Moreover, the cumulative fracture volume shows a power-law growth towards the failure after a unique threshold to each sample. These results have been validated by previous acoustic emission studies and a 4D tomographic investigation on strain localization of shale. Therefore, despite the postmortem nature of the investigation, the new technique has opened possibilities to investigate the fracture properties and their evolution under elevated confining pressures.

How to cite: Jayawickrama, E., Muto, J., Sasaki, O., and Nagahama, H.: A postmortem approach via X-ray computed tomography and thresholding to investigate fracture network evolution in Onagawa shale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11553, https://doi.org/10.5194/egusphere-egu21-11553, 2021.

Large rock slopes instabilities form over long timescales through progressive rock mass strength weakening of initially stable slopes. Progressive rock mass damage is driven by environmental loads and is thus strongly dependent on the local setting and environmental conditions of the rock slope, which can vary over time. It is often assumed that the strong variations of the thermal and hydraulic boundary conditions during deglaciation in combination with unloading due to ice downwasting cause enhanced rates of rock mass damage. However, in-situ observations to quantify deformation, damage and the relevance of different drivers in such environments are rare. This presentation is related to the contribution of Oestreicher et al., presenting in the same session, addressing similar questions, but at different scales and based on different field data and analysis.

In this contribution we analyze continuous pore pressure, temperature and micrometer-scale deformation time series from a subsurface monitoring system comprised of three, 50 m deep, highly instrumented boreholes in a crystalline rock slope which is located beside the rapidly retreating glacier tongue of the Great Aletsch Glacier (Switzerland). We compare high-resolution reversible and irreversible deformation signals with potential drivers, including locally measured pore pressure fluctuation, rock temperature variations, and nearby earthquakes. We show that shallow (10 - 15 m deep) deformations in our rock slope are dominated by thermo-mechanical forcing, whereas deformation measured below this depth is mainly driven by hydro-mechanical effects related to pore pressure fluctuations. Both reversible deformation and irreversible damage events occur more frequently during the snow-free summer season, when we observe higher dynamics in thermal and hydraulic boundary conditions. In our 2.5 years long time series, we do not find any significant deformation event coinciding with a nearby earthquake. Additionally, we discuss differences in the deformation signal with respect to the stability state and the rock mass quality at the different monitoring locations. Also, we assess longer term impacts of glacier retreat and ice downwasting on rock slope deformation and damage. Such information is critical for an improved understanding and quantification of factors contributing to the formation of paraglacial rock slope instabilities.

How to cite: Hugentobler, M., Aaron, J., and Loew, S.: Drivers for micrometer-scale deformation and progressive rock mass damage in a deglaciating rock slope – insights from subsurface in-situ monitoring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5192, https://doi.org/10.5194/egusphere-egu21-5192, 2021.

Stromboli volcano has experienced four sector collapses over the past 13 thousand years, resulting in the formation of the Sciara del Fuoco (SDF) horseshoe-shaped depression and an inferred NE / SW striking rift zone across the SDF and the western sector of the island. These events have resulted in the formation of steep depressions on the slopes on the volcano where episodes of instability are continuously being observed and recorded. This study aims to quantify the fracture density inside and outside the rift zone to identify potential damaged zones that could reduce the edifice strength and promote fracturing. In order to do so we have carried out a multiscale analysis, by integrating satellite observations, field work and seismic and electrical resistivity analyses on cm scales blocks belonging to 11 lava units from the main volcanic cycles that have built the volcano edifice, ie. Paleostromboli, Nestromboli and Vancori. 0.5 m resolution Pleiades satellite data has been first used to highlight 23635 distinct linear features across the island. Fracture density has been calculated using Fracpaq based on the Mauldon et al (2001) method to determine the average fracture density of a given area on the basis of the average length of drawn segments within a predetermined circular area. 41.8 % of total fracture density is found around intrusions and fissures, with the summit area and the slopes of SDF having the highest average fracture density of 5.279  . Density, porosity, P- wave velocity in dry and wet conditions and electrical resistivity (in wet conditions) were measured  via an ultrasonic pulse generator and acquisition system (Pundit) and an on purpose built measuring quadrupole on cm scale blocks of lavas collected from both within and outside the proposed rift zone to assess the physical state and the crack damage of the different lava units.  Preliminary results show that P-wave velocity between ~ 2.25 km/s < Vp < 5km/s decreases with porosity while there is high variability electrical resistivity with 21.7 < ρ < 590 Ohm * m. This is presumably due to the lavas texture and the variable content of bubble/vesicles porosity and crack damage, that is reflected by an effective overall porosity between 0 and 9 %. Higher porosity is generally mirrored by lower p-wave velocity values. Neostromboli blocks show the most variability in both P-wave velocity and electrical resistivity. Further work will assess crack density throughout optical analyses and systematically investigate the UCS and elastic moduli. This integrated approach is expected to provide a multiscale fracture density and allow to develop further laboratory testing on how slip surfaces can evolve to a flank collapse at Stromboli.

How to cite: Alcock, T., Vinciguerra, S., Benson, P., and Vagnon, F.: Multiscale fracture density analysis at Stromboli Volcano, Italy: implications to flank stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8872, https://doi.org/10.5194/egusphere-egu21-8872, 2021.

The Fracture Induced Electromagnetic Radiation (FEMR) technique has gradually progressed in the past decade as a useful geophysical tool to determine the direction and magnitude of recent crustal stresses, visualize the modification and realignment of stresses inside tunnels thus proving to be an important precursor for geohazards, earthquake forecasting, as well as delineate landslide-prone slip planes in unstable regions. Its working principle is based on the generation of geogenic electromagnetic radiation emanating from the brittle rock bodies that are fractured being subjected to an incremental increase of the differential stress in the near-surface of the Earth’s crust. The “Process zone” at the fractured crack tip contains numerous microcracks which subsequently creates dipoles due to the polarization of charges on such microcrack tips which rapidly oscillates emitting FEMR waves of frequencies between KHz to MHz range. The coalescence of the microcracks eventually leads to a macro failure dampening the amplitude of the FEMR pulses. The attenuation of FEMR pulses is comparatively lesser than seismic waves making it a more efficient precursor to potential tectonic activities indicating an upcoming earthquake a few hours/days before the actual event. In the current study, we have attempted to exploit this technique to identify the locations of the potential active faults across the tectonically active Narmada-Son Lineament (NSL), Central India. Although the first tectonic stage involved rifting and formation of the NSL during the Precambrian time, the rifting continued at least till the time of Gondwana deposition. Later, tectonic inversion took place as a result of the collision between the Indian and the Eurasian plate resulting in reverse reactivation of the faults. Episodic reverse movement along NSL caused recurrent earthquakes and linear disposition of the sediments that were deposited at the foothills of the Satpura Horst. Although the origin of East-West trending NSL dates back to the Precambrian time, it is very much tectonically active as manifested by recent earthquakes. The study has been conducted by taking linear FEMR readings across 3 traverses along the NSL which on analysis provides an idea about the potential active faults, their locations, and frequency of occurrence. The accumulation of strain in the brittle rocks that can eventually lead to a macro failure is demarcated as an anomalous increase in the amplitude of the FEMR pulses indicative of an upcoming tectonic episode in the region. To further corroborate the analysis, we have attempted to determine the neo-tectonic activity in the region by calculating the morphometric parameters across the Khandwa-Itarsi-Jabalpur region, Central India. Finally, we attempt to comment on the tectonic evolution of Central India in the recent past. We also encourage researchers to adapt the novel technique of FEMR which is swift, affordable, and feasible compared to conventional techniques deployed to survey the active tectonics of a region.

How to cite: Das, S. and Mallik, J.: Delineating active faults across the Narmada-Son Lineament (NSL), India using the technique of Fracture Induced Electromagnetic Radiation (FEMR), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2370, https://doi.org/10.5194/egusphere-egu21-2370, 2021.

Rock mass damaging has become a topic of great interest in the engineering-geology research community during the last decades as it can significantly influence slopes stability. In this sense, the study of mechanics and dynamics of jointed rock masses represents a challenge because it will allow to better understand how external continuous and transient stressors can influence the short- to long-term stability controlling their pre-failure behavior. Consequently, the detection of permanent changes in physical and mechanical parameters, due to periodic or transient stressors, is an important target to mitigate the related geological risk as it can potentially lead rock masses to failure, especially when infrastructures and natural or cultural heritages are exposed elements. In this framework, the Acuto field laboratory (Central Italy) has been designed and implemented in 2016 within an abandoned quarry by employing an integrated geotechnical and geophysical monitoring system, with the aim of investigating how natural and anthropic conditioning factors could lead fractured rock masses to failure. The integrated monitoring system, which is installed on a potentially unstable 20-m³ jointed rock block, is composed of several strain devices (i.e., strain gauges -SG- and jointmeters -JM-), one fully equipped weather station, one rock thermometer, eight high-sensitivity microseismic uniaxial accelerometers and optical and InfraRed Thermal cameras. The acquisition of long-term monitoring time-series, coupling multimethodological approaches, allowed to establish cause-to-effect relationships among different environmental stressors and induced strain effects, highlighting the continuous action of thermal stresses on rock mass deformations both at the daily and seasonal timescales. In fact, while the analysis of thermal and strain monitoring data allowed to characterize the cyclic contraction and relaxation response of major rock fractures and microcracks to temperature fluctuations, the microseismic monitoring array was able to detect during thermal transient (i.e., freezing conditions) the occurrence of microseismic emissions potentially related to the genesis or progressive growth of pre-existing cracks.

Starting from 2018, experimental activities at the Acuto field lab are supported by the “Dipartimento di Eccellenza” project of the Italian Ministry of Education Universities and Research funds attributed to the Department of Earth Sciences of the University of Rome “Sapienza”.  In this framework, the Acuto filed laboratory will undergo a structural upgrade that will be aimed at the investigation of two new sectors of the abandoned quarry. These new sectors will be instrumented with innovative thermal profiles probe, fiber Brag grating sensors and traditional SG and JM for detailed stress-strain monitoring, acoustic emission sensors and high-frequency and low-frequency geophones for ambient seismic noise monitoring and microseismic events detection as well as accelerometers for evaluating the rock mass response in the case of seismic shaking. The main goal of such an improvement will be both technical and methodological, and will shed light on the application of integrated geophysical and geotechnical monitoring approaches in investigating the multiscale rock mass damaging process as well as the detection of rock mass failure precursors by using non-conventional combinations and configurations of geotechnical and broad-band geophysical devices.

How to cite: Grechi, G., D'Angiò, D., Fiorucci, M., Iannucci, R., Lenti, L., Marmoni, G. M., and Martino, S.: Integrated geophysical and geotechnical monitoring for multiscale rock mass damaging investigation at the Acuto Field-Lab (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9403, https://doi.org/10.5194/egusphere-egu21-9403, 2021.

Retreating glaciers around the world lead to rapid and profound changes in the surrounding landscapes. In the Alps, many glaciers are rapidly retreating and downwasting, substantially modifying stresses and hydro-thermal boundary conditions on the adjacent slopes. There is an increase in observations of bedrock responses and the formation of large-scale instabilities in paraglacial environments, but still a little knowledge about the underlying preparatory factors and drivers. This presentation is linked to the one from Hugentobler et al. in the same session. Both studies take place in the same catchment and address the same questions at different spatial scales, with other techniques and datasets.

We analyse surface deformation data monitored in a crystalline bedrock catchment, on the recently deglaciated slopes of the Great Aletsch Glacier (Valais, Switzerland). Our monitoring system has been in operation for six years and comprises 93 reflectors, 2 robotic TPS, and 4 cGPS stations distributed on both sides of the glacier tongue. This unique dataset allows studying the main processes involved at relevant spatial and temporal scales. The response of potential drivers for reversible and irreversible deformation is evaluated through combined multivariate (vbICA) and cross-correlation statistical analysis. We found that the variability in deformation near the glacier tongue is primarily controlled by glacier unloading through melting and seasonal groundwater fluctuations. At the catchment scale, the later effect is poroelastic and hence reversible, but we argue that it could also induce hydromechanical fatigue. By investigating the deformation's spatial pattern, we observed that the reversible deformation is mostly controlled by discrete structures such as hectometer-scale brittle-ductile shear zones striking subparallel to the valley axes and the main Alpine foliation. Field mapping and pressure monitoring during borehole drilling suggest that infiltration into the fractured rockmass is very heterogeneous and mainly controlled by the presence of interconnected tensile fractures.

How to cite: Oestreicher, N., Roques, C., Hugentobler, M., Aaron, J., and Loew, S.: Drivers of reversible and irreversible slope deformations in a paraglacial environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8082, https://doi.org/10.5194/egusphere-egu21-8082, 2021.

The nature of the boundary between deforming rock masses and stable bedrock is a significant issue in the scientific debate on Deep-Seated Gravitational Slope Deformations (DSGSDs). In many DSGSDs the deforming masses move on a continuous sliding surface or thick basal shear zone (BSZ) [1-3]. This last feature is due to viscous and plastic deformations and was observed (or inferred) in many worldwide sites [4]. However, no clear evidence has been documented in the geological context of the Apennine belt, despite the several cases of DSGSDs documented in this region [5-6].

This work describes a peculiar case of a BSZ found in the central part of the Apennine belt and observed at the bottom of a DSGSD which affects the Meso-Cenozoic carbonate ridge overhanging the Luco dei Marsi village (Abruzzi region). The NNW-SSE oriented mountain range is a thrust-related Miocene anticline, edged on the east by an intramountain tectonic depression originated by Plio-Quaternary normal faulting. The BSZ appears on the field as a several meters-thick cataclastic breccia with fine matrix developed into Upper Cretaceous, biodetritic limestone and featuring diffuse rock damage.

The gravity-driven process was investigated through field survey, aerial photo interpretation and remote sensing (SAR interferometry) and framed into a geological model which was reconstructed also basing on geophysical evidence from the CROP 11 deep seismic profile. The effects on slope deformation determined by progressive displacements along normal faults and consequent unconfinement at the toe of the slope was analysed by a multiple-step numerical modelling constrained to physical and mechanical properties of rock mass.

The model results outline the tectonic control on DSGSD development at the anticline axial zone and confirm the gravitational origin of the rock mass damage within the BSZ. Gravity-driven deformations were coexistent with Quaternary tectonic processes and the westward (backward) migration of normal faulting from the basin margin to the inner zone of the deforming slope.

References

[1] Agliardi F., Crosta G.B., Zanchi A., (2001). Structural constraints on deep-seated slope deformation kinematics. Engineering Geology 59(1-2), 83-102. https://doi.org/10.1016/S0013-7952(00)00066-1.

[2] Madritsch H., Millen B.M.J., (2007). Hydrogeologic evidence for a continuous basal shear zone within a deep-seated gravitational slope deformation (Eastern Alps, Tyrol, Austria). Landslides 4(2), 149-162. https://doi.org/10.1007/s10346-006-0072-x.

[3] Zangerl C., Eberhardt E., Perzlmaier S., (2010). Kinematic behavior and velocity characteristics of a complex deep-seated crystalline rockslide system in relation to its interaction with a dam reservoir. Engineering Geology 112(1-4), 53-67. https://doi.org/10.1016/j.enggeo.2010.01.001.

[4] Crosta G.B., Frattini P., Agliardi F., (2013). Deep seated gravitational slope deformations in the European Alps. Tectonophysics 605, 13-33. https://doi.org/10.1016/j.tecto.2013.04.028.

[5] Discenza M.E., Esposito C., Martino S., Petitta M., Prestininzi A., Scarascia-Mugnozza G., (2011). The gravitational slope deformation of Mt. Rocchetta ridge (central Apennines, Italy): Geological-evolutionary model and numerical analysis. Bulletin of Engineering Geology and the Environment,70(4), 559-575. https://doi.org/10.1007/s10064-010-0342-7.

[6] Esposito C., Di Luzio E., Scarascia-Mugnozza G., Bianchi Fasani G., (2014). Mutual interactions between slope-scale gravitational processes and morpho-structural evolution of central Apennines (Italy): review of some selected case histories. Rendiconti Lincei. Scienze Fisiche e Naturali 25, 161-155. https://doi.org/10.1007/s12210-014-0348-3.

How to cite: Di Luzio, E., Discenza, M. E., Putignano, M. L., Minnillo, M., Di Martire, D., and Esposito, C.: The Luco dei Marsi deep-seated gravitational deformation: first evidence of a basal shear zone in the central Apennine mountain belt (Italy) , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5083, https://doi.org/10.5194/egusphere-egu21-5083, 2021.

Rainfall-induced landslides pose a substantial risk to people and infrastructure worldwide, but their mechanical behavior is not well understood. As a result, hazard predictions for these landslides, especially for rainfall and slope-failure correlations, remain an active area of research. Many operational rainfall-induced landslide hazard maps still assume a classical Coulomb type failure criterion where slope-failure must occur either before or at peak subsurface pore pressure reached during a precipitation event. Using satellite-derived surface precipitation data and soil infiltration simulations over a 15 day period preceding 121 rainfall-induced landslides across India, we find that these events occurred systematically 2-12 days after the simulated peak pore pressures on the inferred failure slope nucleating between 0.5 and 5 m depth. These observations cannot be explained with the Coulomb failure criterion, since failure on these slopes is significantly delayed behind the occurrence of the inferred strength minimum. Instead, in this study, we investigate whether a slope failure model with time- and slip-variable shear strength, governed by the rate-state friction (RSF) equations widely used in earthquake mechanics, can explain the observed ranges of time-delays between slope failure and inferred peak pore pressure.

To concentrate on the role of the constitutive behavior of the failure surface, we examine spring-slider dynamics under a classical RSF framework driven by variable on-slope and far-field pore pressure and flux time histories. We derive analytical expressions for the time-to-failure of such a spring-slider under simple pore pressure perturbation histories and find that the delay-times can vary significantly depending on the laboratory derivable RSF parameters, soil bulk properties, and particulars of the pressure history. We further examine the roles of dilatancy strengthening and pore compaction in determining the time-lag between peak pore pressure and slope failure. We find that dilatancy can have either a stabilizing or a destabilizing effect on slope failure depending on the hydrological and mechanical properties of the failure plane and the soil column. Finally, we show with numerical simulations that periodic pore pressure or flux oscillations can also drive asynchronous repeated slope failures in both the presence and absence of the coupling of pore pressure and shear deformation. Our results show that the observed rainfall-landslide correlations for these 121 landslides can be explained with inherently time- and slip-dependent shear strength prescriptions like the RSF equations. This, in turn, implies that realistic landslide hazard monitoring might require the examination of soil shear strength under the experimental protocols widely used in rock friction experiments to determine whether the constant friction assumption inherent in the Coulomb criterion needs to be revised in favor of RSF or similar constitutive equations for shallow landslides.

How to cite: Paul, K., Bhattacharya, P., and Misra, S.: Investigation of The Observed Time-lag Between the Simulated Peak Pore Pressures and Slope Failures in Rainfall Induced Landslides: A Numerical Approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15352, https://doi.org/10.5194/egusphere-egu21-15352, 2021.

In 2009, a large-scale landslide was triggered by typhoon rainfall and buried an entire village, which named Hsiaolin and located in Taiwan. 
After that, Soil and Water Conservation Bureau (SWCB) has promoted a national project for the prevention work of large-scale landslide. The national project includes with the investigation of potential area, the design of monitoring system, and the design of warning system, etc. 
The investigation of potential large-sclae landslide was based on the  digital elevation model with 1 meter resolution. However, the investigation of the underground was lack and not clear enough. Therefore, the specific landslide's body is hardly to estimate and it causes difficulty in follow-up works. 
This study applied two methods to investigate the scenario of slope failure. The first method is based on the limited equilibrium method, which proposed by Yoshino and Uchida (2019). The method was used to search the specific region of unstable slope based on a series of high-resolution digital elevation models. After the specific region of unstable slope was confirmed, the landslide can be simulated by a numerical model, which this study proposed to represent the entire landslide process from occurrence to post-failure . 
These proposed methods were applied at Baolai area, south Taiwan to track the evolution of the potential area. The failure scenario could be evaluated by the proposed numerical model. By this study, the investigation of underground can be evaluated and these results are very important information for the design of monitoring system.

How to cite: Tsai, Y. and Lee, W.: Application of the high-resolution digital elevation model in an integrated numerical method for the occurrence mechanism and post-failure behavior of the landslide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10764, https://doi.org/10.5194/egusphere-egu21-10764, 2021.

In the pre-feasibility study stage, only a small amount of borehole data can be obtained. Since the available geological information is insufficient, the engineering geological conditions of the project can only be preliminarily and approximately estimated during this stage. In this study, we attempt to seek a method to make a preliminary analysis and evaluation of the stability of the surrounding rock masses of an underground rock carven project, which makes full and optimum use of the limited borehole data to accomplish the assessment of the investigated site. The basic information on rock fractures is extracted from the borehole Television logging data and the fracture extension directions are also determined. Providing that the cracks detected in the borehole would extend to the cavern area, the cracks with appropriate direction, larger width and larger hydraulic conductivity can be selected. These selected cracks are considered in the numerical model established using ANSYS, and the stability of surrounding rock of cavern is analyzed under this situation. In the absence of large amount of borehole data, this method, which set up an extreme case, can be used to analyze possible failure of rock mass under extreme adverse conduction in advance. In general, the proposed method for stability analysis could contribute to the design and construction practice of a tunnel project constructed in fractured rock masses.

How to cite: Tan, L. and Xiang, W.: Stability analysis of the surrounding rock of cavern under extreme situation based on limited borehole data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-230, https://doi.org/10.5194/egusphere-egu21-230, 2020.

The novel geophysical remote imaging method of muography is based on cosmic-ray induced muon particles that are detected after passing through the media of interest. If the studied objects are solid, their sizes can vary from meters to up to kilometres. In terms of penetration capability, muography can be placed between methods based on X-rays and those using seismic waves. The most famous objects imaged with muography are pyramids (e.g., Khufu's Pyramid at Giza in Egypt) and volcanoes (e.g., Mt Etna in Italy). One clear advantage of muography compared with seismic methods is that muons, unlike seismic waves, do not reflect from geological interfaces. In addition, the scattering phenomenon is a minor issue and needs consideration only at low-energy muons. Raw data must be corrected according to topography. On the basis of extensive numeric simulations of Hivert et al. (2017), the lowest density variations observable for muography with a significant level of 3σ (a typical significance level in physics) are around 2% at 150 m, 4% at 300 m, and 10% at 700 m of depth, respectively. If these numbers are extrapolated to depths below 100 m, the mean density differences in the range of 1% are likely within the observation capability of muography. It is also worth to note that the 1% difference in a mean rock density results in an approximately 3% difference in the muon flux. This indicates that muon flux measurements are very sensitive to the density variations of rocks.

In underground tunnelling, muography has at least four applications: (1) muography can be used to detect a potential risk (such as a water reservoir, a weak zone with loose rocks, boulders, etc.) before or during tunnelling, (2) muography can be employed to monitor overburden rock behaviour during tunnelling operation to avoid risks like the roof cave-ins, (3) muography can be applied to monitor the overburdening rock masses in tunnels after they are excavated to predict and avoid the collapse of rock mass, and (4) muography can be used to estimate the size and volume of a rock mass collapse in a tunnel since the volume of the collapsed rocks must have markedly smaller density than original overburden rock mass. In an excavating tunnel project using a tunnel boring machine (TBM), a muon detector can be installed in the TBM during tunnelling. If there occurs a tunnel cave-in, muography can be employed in undamaged tunnels nearby (sideways or below) the collapse. If possible, the collapse can also be approached safely via an undamaged part of the collapsed tunnel. If none of these are available, borehole muography can be applied as a substitute solution. Whereas an undamaged underground tunnel is either filled by air or water, a collapsed tunnel segment is characterized by air and rock, or water and rock. In either case, the average density of the tunnel segment is increased. We are currently planning simulations and real-world tests to validate these assumptions.

How to cite: Kuusiniemi, P., Holma, M., and Zhang, Z.: Cosmic-ray muography applications in underground tunnelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9667, https://doi.org/10.5194/egusphere-egu21-9667, 2021.