Granitic Rock Mass Research Articles

Analysis of Granite Deformation and Rupture Law and Evolution of Grain-Based Model Force Chain Network under Anchor Reinforcement

In actual underground rock engineering, to prevent the deformation and damage of the rock mass, rock bolt reinforcement technology is commonly employed to maintain the stability of the surrounding rock. Therefore, studying the anchoring and crack-stopping effect of rock bolts on fractured granite rock mass is essential. It can provide significant reference and support for the design of underground engineering, engineering safety assessment, the theory of rock mechanics, and resource development. In this study, indoor experiments are combined with numerical simulations to explore the impact of fracture dip angles on the mechanical behavior of unanchored and anchored granite samples from both macroscopic and microscopic perspectives. It also investigates the evolution of the anchoring and crack-stopping effect of rock bolts on granite containing fractures with different dip angles. The results show that the load-displacement trends, displacement fields, and debris fields from indoor experiments and numerical simulations are highly similar. Additionally, it was discovered that, in comparison to the unanchored samples, the anchored samples with fractures at various angles all exhibited a higher degree of tensile failure rather than shear failure that propagates diagonally across the samples from the regions around the fracture tips. This finding verifies the effectiveness of the numerical model parameter calibration. At the same time, it was observed that the internal force chain value level in the anchored samples is higher than in the unanchored samples, indicating that the anchored samples possess greater load-bearing capacity. Furthermore, as the angle αs increases, the reinforcing and crack-stopping effects of the rock bolts become increasingly less pronounced.

The role of different explosives in reducing energy and emissions in mining comminution for sustainable improvement

In the mining industry, comminution represents one of the most energy-intensive activities that generate large amounts of CO2, making it a great challenge to the industry. Using high explosives in the blasting operation can cover this comminution-related challenge. In this study, the effect of blasting on the comminution was investigated considering the mentioned challenge by blasting samples of granite rock mass using three types of common explosive materials, namely ANFO, emulite, and pentolite, at different powder factors, contrasting with other studies that focus primarily on increasing the powder factor of a specific type of explosive. Based on the results of this study, it was observed that compared to ANFO, other explosives tested in this work increased the energy consumption per ton of rock mass in the blasting stage by 1.1–1.8 times, with CO2 emission for the ANFO, emulite, and pentolite measured at 0.78 × 10−3 kg, 3.10 × 10−3 kg, and 5.14 × 10−3 kg, respectively. Also, Using ANFO, emulite, and pentolite led to 4.47 %, 6.45 %, and 7.07 % relative advantages in terms of saving total mining energy in the comminution stage. Finally, CO2 emissions for the comminution stage were estimated using Bond's indices and regional electricity data.

Kinetic simulation of uranium migration in granite fissure media of beishan, gansu, China: A case study based on the Laplace transform and inverse transform methods

Deep geological disposal is currently considered the most practical and feasible method for disposing high-level radioactive wastes (HLWs). One of its key scientific issues is the migration of nuclides in fissure media. However, studies on the migration of nuclides like U-238 are relatively limited. In this study, the granite rock masses in Beishan, Gansu were selected to construct a physical and mathematical model of U-238 migration using the Laplace transform and inverse transform methods. Based on the numerical methods, the kinetic migration of nuclide U-238 in the fissure media of granite was simulated, and the effects of parameters such as fissure width, hydraulic gradient, diffusible area ratio and rock porosity on the migration of U-238 were investigated. The following insights were obtained: (1) After 100,000 years, U-238 has a very limited diffusion depth in the matrix domain, with a diffusion range of only a few tens of millimeters, whereas it migrates relatively quickly in the fissure domain, with a maximum migration distance of about 1500 m. (2) Under the same migration time and distance, the relative concentration of nuclide U-238 in the fissure domain increases with larger gap width, hydraulic gradient, and diffusible area ratio, but decreases with higher rock porosity. (3) In the same time range, the rock masses with larger gap widths, hydraulic gradients, and diffusible area ratios have larger migration ranges, while those with higher porosities have smaller migration ranges. (4) While selecting a site for diposal of HLWs, it is recommended to choose rock masses in the granite area of Beishan with no fissures or few fissures; additionally, areas with smaller fissure widths, hydraulic gradients, and diffusible area ratios but higher rock porosity should be prioritized. This study can provide important theoretical support for understanding nuclide migration in the future geological disposal of HLWs.

A series of regression models to predict the weathering index of tropical granite rock mass

In the recent past, several weathering indicators have been developed to describe its state of weathering. The state of rock weathering is a useful indicator to estimate the integrity of tropically weathered rock material and mass which weatherability plays an important role in a tropical region. Through a ground assessment tool, the strength and durability of the rock mass could be estimated and complex or adopted to simplify the early prediction of the complex engineering parameter. This paper presents several models of the Weathering Index (WI) using selected significant parameters using statistical analysis. For this purpose, several sites have been chosen to represent granitic rock mass. Forty (40) numbers of samples were collected and tested comprising from four (4) sites in Malaysia. Several laboratory tests have been conducted such as Point Load Index (Is(50)), dry density, Slake Durability 1 (SD1), Slake Durability 2 (SD2) and moisture content. The field and laboratory data sets are used to determine the WI by using simple regression and MLR analysis Significant parameters found to be useful in determining the WI are selected namely SD1, dry density, Is(50), and block volume. These parameters were selected based on stepwise analysis using Statistical Package for the Social Sciences (SPSS). Following the models’ implementation, the models were evaluated and the best prediction model was selected after considering statistical coefficients, such as coefficient of determination (R2), variance account for (VAF), and root mean squared error (RMSE), as well as utilizing a straightforward ranking approach. The findings of this study could contribute to the more accurate prediction of WI using a more simplistic field and laboratory parameters. Therefore, the WI is useful during the initial stages and planning of rock excavation work and provides a good description of weathering grade and rock mass properties, which will affect excavatability in granitic areas.

Disc-cutter induced rock breakage mechanism for TBM excavation in rock masses with different joint shear strengths

When tunnel boring machines (TBMs) excavate through jointed rock masses, the cutting efficiency is strongly affected by the shear strength of joints, the mechanism of which, however, remains poorly understood. In this study, a series of disc-cutter indentation tests were conducted on granite rock mass specimens with different joint shear strengths. During the indentation, the cracking process was recorded by a digital image correlation (DIC) system. The deformation and strength of specimens, cracking behavior, rock breakage mode and cutting efficiency were quantitatively investigated. In addition, to investigate the combined effects of joint shear strength, orientation and spacing on the rock breakage mechanism, numerical rock mass models were established based on a particle flow code PFC2D. Experimental results reveal that the cracking of primary and secondary cracks changes from the mixed shear-tensile to tensile mode in the initial stage, while the joint shear strength does not affect the cracking mode in the subsequent propagation process. The rock breakage mode is classified to an internal block breakage mode, a cross-joint breakage mode and a cutters-dependent breakage mode. The cross-joint breakage mode is optimal for improving the cutting efficiency. Numerical simulation results reveal that the increase in the joint shear strength changes the internal block breakage mode to cross-joint breakage mode for rock masses of particular ranges of joint orientation and spacing. These findings provide basis for improving the TBM cutting efficiency through jointed rock masses.

Comprehensive data set of in situ hydraulic stimulation experiments for geothermal purposes at the Äspö Hard Rock Laboratory (Sweden)

Abstract. In this article, a high-resolution acoustic emission sensor, accelerometer, and broadband seismometer array data set is made available and described in detail from in situ experiments performed at Äspö Hard Rock Laboratory in May and June 2015. The main goal of the hydraulic stimulation tests in a horizontal borehole at 410 m depth in naturally fractured granitic rock mass is to demonstrate the technical feasibility of generating multi-stage heat exchangers in a controlled way superiorly to former massive stimulations applied in enhanced geothermal projects. A set of six, sub-parallel hydraulic fractures is propagated from an injection borehole drilled parallel to minimum horizontal in situ stress and is monitored by an extensive complementary sensor array implemented in three inclined monitoring boreholes and the nearby tunnel system. Three different fluid injection protocols are tested: constant water injection, progressive cyclic injection, and cyclic injection with a hydraulic hammer operating at 5 Hz frequency to stimulate a crystalline rock volume of size 30 m × 30 m × 30 m at depth. We collected geological data from core and borehole logs, fracture inspection data from an impression packer, and acoustic emission hypocenter tracking and tilt data, as well as quantified the permeability enhancement process. The data and interpretation provided through this publication are important steps in both upscaling laboratory tests and downscaling field tests in granitic rock in the framework of enhanced geothermal system research. Data described in this paper can be accessed at GFZ Data Services under https://doi.org/10.5880/GFZ.2.6.2023.004 (Zang et al., 2023).

Measurement of granite resistivity under deep underground conditions based on van der Pauw method

With the growing demand for sustainable energy and infrastructure development in the country, the utilization of deep geothermal resources and the development of deep underground space engineering are becoming more and more important. Under the background that the country vigorously promotes the development of deep rock and soil underground space and the utilization of resources. The experiment will be performed on granite, one of the deep subsurface rocks, a rock type widely used in geotechnical research projects because of its high strength, low permeability and thermal stability. However, its properties under the extreme temperature and chemical conditions deep in the earth are unknown, and how to efficiently convert deep rocks into geothermal resources for utilization is still unknown. Therefore, studying the influence of temperature and chemical corrosion on the resistivity of granite is very important to ensure the safety and durability of underground structures and effectively utilize thermal energy resources in deep underground. This experiment will control the influence of temperature, pressure, chemical corrosion and the comprehensive effect of the three on the resistivity of granite, and indirectly measure the amount of heat absorbed in the granite rock mass. Simulate the deep underground granite under high confining pressure, high ground temperature and corrosion conditions the nature of the performance. Record, count and analyze the data, comprehensively judge the environmental interference of the granite rock formation under actual conditions and compare the destructive effects of temperature and chemical corrosion and the change of the resistivity of the rock mass. Help scientifically judge and accurately analyze the underground construction problems that may be encountered in future projects and the effective utilization of geothermal energy and provide a basis for the application of key technologies and methods.

Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

AbstractThe in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault‐associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture‐induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr‐Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12°∼18°), and a maximum horizontal principal stress (SHmax) oriented N100∼120°E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi‐quantitative estimation of the in‐situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications.

Influence of hole diameter on mechanical properties and stability of granite rock surrounding tunnels

Nowadays, the development and utilization of more and more engineering construction are closely related to granite. However, many granite rock masses in Qingdao contain natural hole defects, which have a great impact on the mechanical properties of granite. It may even cause instability of surrounding rock and safety accidents. Therefore, in this paper, we discuss the influence of the hole diameter on the mechanical properties and stability of granite rock surrounding tunnels. Uniaxial compression experiments were conducted on granite with different hole diameters, and monitoring was carried out using the acoustic emission system and the XTDIC (Xintuo 3D Digital Image Correlation) three-dimensional–full-field strain-measurement systems. The relationship between the strength, deformation, and hole size of granite was investigated. In addition, using the Yangkou tunnel as the prototype and the PFC2D (Particle Flow Code of 2D) particle-flow–numerical-simulation program, a working tunnel model with different hole sizes was established to simulate the influence of natural hole defect sizes on the stability of rock. The results show that: (1) with an increase in hole diameter, the uniaxial compressive strength and elastic modulus of the granite sample gradually decreased. The brittleness of the granite samples gradually decreased, and the ductility gradually increased. (2) Under the action of axial stress and with an increase in the hole diameter, the sample was more likely to produce a stress concentration around the hole defect, which increased the deformation localization band, development, and expansion, as well as the intersection degree. As a result, granite samples are more likely to develop new cracks. These cracks increase in number and size, reducing the compressive strength of the granite sample. (3) The size of the hole defects significantly affected the damage and mechanical properties of the model surrounding rock. When increasing the hole diameter, the defect area increased and the tensile stress concentration near the hole in the localized rock became more evident. In addition, the stability of the rock surrounding the tunnel was significantly reduced, and its bearing capacity was weakened, leading to easier crack initiation and rock damage.

Preliminary analysis of the mechanisms, characteristics, and causes of a recent catastrophic structurally controlled rock planar slide in Esposende (northern Portugal)

A catastrophic rock planar slide occurred in the parish of Palmeira de Faro, Esposende (N Portugal), on November 23, 2022, in the middle of the night, killing two people while they were asleep in the first floor of a building close to the affected cut slope. The collapsed slope presented a maximum height of about 23 m, and was excavated in a granitic rock mass strongly weathered, in some areas by mechanical means and, locally, with the support of blasting. The displaced material, with an estimated volume of 2000 m3, described an oblique trajectory with respect to the dip direction of the slope with a maximum length of about 44.6 m. The direction of the trajectory could be explained by the strong structural control of the instability trough three discontinuity sets which delimit the slid rock mass causing multiple damage on the above-mentioned building. Although further detailed studies are necessary to elucidate the causes of the landslide, the unfavorable geological conditions, the anomalous accumulated rainfall during the previous two months (more than double than in the historic records), and some excavations apparently performed on the cut slope could be related to this instability. Therefore, this recent landslide highlights the important influence of the rock mass structure on the development of rock planar slides.

Evolution characteristics of acoustic emission and strain energy for deep granite under different damage stages

The first underground research laboratory (URL) as the research platform for the deep geological disposal of high-level radioactive waste (HLW) in China is being built in a deep granite rock mass. The damaged characteristics and mechanical properties of the deep granite under loading seriously influence the URL’s construction safety and disposal function for HLW. Therefore, this paper investigates the damaged characteristics and mechanical properties of the Beishan deep granite used in various confining pressure levels through a series of triaxial compression and acoustic emission (AE) tests. Based on the crack volumetric strain model, the crack stress thresholds of the deep granite specimens under various confining pressure levels were acquired. The damage development of the specimens used in five confining pressure conditions was analyzed, and the evolution of AE parameters and strain energy of the deep rock under different damage stages were explored. The studied results shown that the crack stress thresholds of the deep granite rose linearly with the increasing confining pressure, but their stress ratios remained relatively constant under various confining pressure levels. Under the rock loading tests, the designed confining pressure levels influenced the AE hit count and AE accumulative count development under the rock damage process. The high-frequency points of the AE hit count coincide with the AE accumulative count increasing, and the maximum value of the AE hit count of the specimens used in different confining pressure conditions appears in the postpeak stress‒strain stage. Additionally, with the axial strain of the loading rock rising in the prepeak stress‒strain stage, all the strain energy (including total energy U, elastic energy Ue and dissipated energy Ud) continues to rise, and in the postpeak stress‒strain stage, Ue stored inside the rock is quickly released, resulting in the Ue/U decreasing and Ud/U increasing.

Assessing the tunnel stability in brittle rocks based on strain bursting assessment

Strain bursting is a phenomenon associated with deep underground excavations and tunnelling in brittle hard rocks that result in a sudden and violent failure, posing high risks and damage. This work aims to assess the strain bursting and the energy response characteristics and develop a guideline that can be utilized in the preliminary investigation stage. The obtained results are intended to enable predicting different brittle behaviours in deep geological environments. Elastic stress analysis using the DISL approach based on boundary element method is established (EX3-software) for the assessment of two tunnel geometries (circular, horseshoe) in granitic rock mass. Two different stress states are modelled to simulate the potential stress regime. The analysis’ results demonstrate that different geometries affect the energy storage capacity and result in different behaviour regarding the bursting potential of the rock mass.

Seismically safe parameters of confined blasting in levelling dry dock bottom

During construction of a dry dock, it became necessary to level the bottom of gneissic granite rock mass to 7 m by blasting. The blasting site appeared to be spaced at 8.5–20.0 m from a reinforced concrete caisson and a sand cofferdam reinforced with soil–cement piles. Aiming to ensure the required levelling at 16.2 m, the preset fragmentation quality and the reduced seismic safety, the parameters associate with the drilling-and-blasting pattern were calculated. The values of PPV caused by blasting-induced seismic waves in the caisson and cofferdam were found from the constructed formulas. The experimental measurement of ground vibrations induced by blasting used seismic recorder Vibracord DX. Al in all, 20 large-scale blasts were performed, and the blasted rock volume totaled 9 Km3. The comparison of the actual and calculated values of PPV in the caisson and cofferdam proves reliability of the presented formulas. It is calculated that PPV caused by the blasting-induced seismic waves at the farther wall of the caisson and at the nearest wall of the soil–cement piles are 2.0–27.0 mm/s and 2.0–22.0 mm/s, respectively, which is much less than the allowable PPV of 100–500 mm/s for the hydraulic engineering structures. It has been determined during blasting that the caisson preserves its integrity and no increase in the water inflow from the gulf is observed.

In-situ Primary Stress Detection Based on Seismic Tomography Measurements and Numerical Back-analysis for an Underground Radwaste Repository

In order to understand how the in-situ primary stress state has evolved with subsidence and uplift in a granitic rock mass for anticipated of a radioactive waste repository in Hungary, the authors investigated the applicability of seismic tomography as an interpretive tool. Very high P wave velocity (Vp) values were obtained during the tomographic scanning of the study area of the repository, and these were compared with existing findings of in-situ and laboratory seismic measurements.Apart from seismic tomographic survey, dynamic FEM numerical modelling, empirical calculations of residual stresses, laboratory measurements of compression wave (ultrasonic) velocities on intact rock cores, in-situ primary stress measurements as well as site geological model were integrated to evaluate the use of seismic tomography for identifying possible in-situ stress increases around the excavation.A detailed calibration modelling was carried out based on the site seismic tomography measurements and during the large-scale modelling. It was observed that the increasing Vp is directly related to simulated increasing directional loadings on the rock mass. Using a measured wave raypath it was possible to check the different in-situ stress parametrizations which resulted in the best approximation to the measured Vp values.It was concluded that the rock mass under investigation to extend the repository must have higher in-situ stress values than the area of the constructed deposition chambers nearby. The results of this research indicated that seismic tomography is a useful tool for determining relative stress around and within the vicinity of underground excavation.

Estimation of the representative elementary volume of three-dimensional fracture networks based on permeability and trace map analysis: A case study

A three-dimensional (3D) discrete fracture network (DFN) was generated to evaluate the representative elementary volume (REV) of granitic rock masses based on the statistical geometry parameters mapped from outcrop surveys at the Xinchang site, China. Then, extensive numerical simulations were performed to investigate the effect of fracture density on the side length of models of the REV size (LREV). The relationship between LREV of 3D models and the geometric parameters of two-dimensional (2D) cutting planes parallel and perpendicular to the direction of hydraulic gradient variation were analyzed. This relationship was determined by calculating fluid flow through 90 original 3D models, 810 3D submodels, and 270 2D cutting models. Finally, two multivariable regression equations were proposed for predicting the LREV of the 3D fracture network and equivalent permeability of the REV size. The results show that the clustering results obtained from the 3D DFN model are similar to those obtained from field measurements. The values of LREV of the 3D DFN model based on statistical geometry parameters mapped from outcrops are 18.89 m, 19.56 m, and 24.86 m in three flow directions, respectively, which clearly highlights the anisotropic characteristics of fractured rock masses. The LREV decreases with increasing fracture density following an exponential relationship. The multivariable regression function estimates the evolution of the LREV of 3D DFN models with a wide range of fracture densities from 0.53 to 1.33 m/m2. The predicted results are more accurate than those predicted by the fitting function between the fracture density of 2D cutting planes perpendicular to the z-axis and LREV. The proposed model provides a method to approximate the LREV of 3D DFN models and equivalent permeability of the REV size (KREV) using geometric parameters of the 2D cutting planes obtained from trace map analysis of fractured rock masses.

Characteristics and rippability conditions of near-surface lithologic units (Penang Island, Malaysia) derived from multimethod geotomographic models and geostatistics

The nature and rippability conditions (rock mass quality [RMQ]) of the near-surface lithologic units of Batu Maung, Penang Island, Malaysia, were investigated to develop sustainable groundwater and infrastructure. This study used a novel approach that combined seismic refraction tomography (SRT), electrical resistivity tomography (ERT), borehole drilling, and simple linear regression (SLR) analysis. The seismic P-wave velocity (Vp) and resistivity models revealed four distinctive units: residual soils (silty sand), very poor-to-good (weathered) granite, and the fresh granitic unit, including the fractured/faulted zones. The thicknesses of the residual and weathered materials ranged from a few centimeters to 15 m and 1.0–16 m, respectively. The developed empirical relationship effectively predicted rock quality designation (RQD) from Vp data through SLR analysis, with a prediction accuracy of 96% and a p-value<0.05. Also, the results from five key regression assumptions: linear relationship, multivariate normality, no multicollinearity, no autocorrelation, and homoscedasticity, suggested an accurate and statistically significant empirical relationship for use in granitic environments. The RMQ statistical model accurately classified the lithologic units beneath the area into Classes I–VI. Due to the low load-bearing of the rippable residual soils, very poor-to-fair weathered granitic rock masses, and fractured/faulted zones based on their Vp and resistivity values, and the steep slopes in the northern section of the study area, all intended infrastructures, particularly high-rise buildings and buildings with continuous footing foundations, should be piled to rest on the non-rippable fresh granitic units, with RQD and Vp values of >90% and >2100 m/s, respectively, in the central to the northcentral section in the study area. Conversely, the deep-weathered/fractured zones of depths >35 m, beneath Line 1, towards the central part of the area, with resistivity and Vp values of 100–900 Ω-m and <1900 m/s, respectively, were identified as potentially water-containing zones for sustainable groundwater abstraction.

Responses of Pre-Holed Granite under Coupled Biaxial Loading and Unloading Stress Condition

Underground excavation is a necessary process for constructing mines, tunnels and depots in granite rock mass. In this study, the numerical granite specimens were established by the discrete element method and confirmed by laboratory experiments in order to investigate the peak stress, cracking development and failure properties of pre-holed granite under coupled biaxial loading and unloading conditions. The results show that, for the specimens containing D-type and square holes, the peak biaxial unloading strengths first decrease, then increase and finally decrease as the inclination angles of the holes increase. For the specimens with elliptical holes, the peak biaxial unloading strengths first decrease and then increase with the increases in the inclination angles of the holes. The biaxial unloading strengths of specimens containing elliptical, circular, D-type and square holes decrease in that order. The cracks initially appear near the crossover points between the X-type shear fracturing plane and the pre-hole in the center and gradually expand along the X-type shear direction, which indicates that the failure of pre-holed granite is primarily shear failure. When the overall length of cracks expanding along the X-type shear direction extends to the size of the pre-hole in the center, the failure of the pre-holed specimen occurs. When the existing pre-hole in the center of the granite specimen extends to connect with the shear slip in the vicinity of the hole, this triggers overall failure.

Deformation Behaviors and Control Measures of Tunnels in Jointed and Altered Granite Rock Mass with High In Situ Stress: A Case Study

AbstractLarge deformation of the surrounding rock may also occur in hard rock tunnels under certain circumstances. Taking the construction of the jointed and altered granite stratum in the Zangga T...

Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis.

The hydraulic fracturing technique (also termed mini-frac test) is commonly used to estimate the in situ stress field. We recently conducted a mini-frac stress measurement campaign in the newly-established Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Four vertical boreholes, dedicated for stress characterization of the granitic rock mass, hosted a total of 19 mini-frac test intervals. Systematic pressure transient analysis was performed to carefully estimate the magnitude of the least principal stress (S_mathrm {3}). We compared five different methods (inflection point, bilinear pressure decay rate, tangent, fracture compliance, and jacking pressure) to identify an adequate approach best suited for our test scale and the host rock mass. We found that the methods used to determine the fracture closure pressure underestimate the magnitude of S_mathrm {3}, presumably due to the rapid closure of the hydraulic fracture after shut-in. The most consistent results were found using the inflection point and bilinear pressure decay rate method, which both determine the (instantaneous) shut-in pressure as the proxy for the S_mathrm {3} magnitude. The determined shut-in pressure, or S_mathrm {3} magnitude, is 14.6pm 1.4 MPa from the inflection point method. This allowed us to further estimate the stress environment around the BedrettoLab, which is transitional between normal and strike-slip faulting. The measured local pore pressures from extended shut-in periods are between 2.0 and 5.6 MPa, significantly below hydrostatic. A combination of drainage, cooling, and the excavation damage zone of the tunnel may have significantly perturbed the in situ stress field in the vicinity of the BedrettoLab.

Natural instability phenomena in granitic rock masses: slopes in decomposed granite, boulder fields and irregular large granitic boulders

Based on the experience gathered by the authors, this article presents a review of the instability phenomena typically associated to natural slopes in granite terrains, focusing on conditions encountered in the northwest of the Iberian Peninsula, particularly in the region of Galicia, characterized by a very humid oceanic climate with high rainfall. Accordingly, a brief introductory review of geomorphological aspects of granitic terrains is presented, in which different slope morphologies are defined that can give rise to varied instability phenomena. Three types of potentially unstable slope environments are defined, namely high areas with large ellipsoidal shaped individual granite boulders, mid-slope regions formed by groups of average size granitic boulders and mild slopes formed by highly or completely decomposed granite, locally known as ‘jabre’. In all these cases, the weathering of granite materials plays an important role, so we briefly explain how the granite rock masses are weathered and eventually eroded. Subsequently, and for each of these three cases, the authors present a typical description of the phenomena to be studied, the characteristic instability mechanisms observed and that need to be analyzed and the most appropriate characterization techniques to be applied. A case study concerning the stability analysis of a particular scenario of every type of instability phenomenon is then presented.