| 1. |
- Castro-Filgueira, Uxia, et al.
(author)
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Particle flow code simulation of intact and fissured granitic rock samples
- 2020
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In: Journal of Rock Mechanics and Geotechnical Engineering. - : Elsevier BV. - 1674-7755. ; 12:5, s. 960-974
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Journal article (peer-reviewed)abstract
- This study presents a calibration process of three-dimensional particle flow code (PFC3D) simulation of intact and fissured granite samples. First, laboratory stress-strain response from triaxial testing of intact and fissured granite samples is recalled. Then, PFC3D is introduced, with focus on the bonded particle models (BPM). After that, we present previous studies where intact rock is simulated by means of flat-joint approaches, and how improved accuracy was gained with the help of parametric studies. Then, models of the pre-fissured rock specimens were generated, including modeled fissures in the form of "smooth joint" type contacts. Finally, triaxial testing simulations of 1 + 2 and 2 + 3 jointed rock specimens were performed. Results show that both elastic behavior and the peak strength levels are closely matched, without any additional fine tuning of micro-mechanical parameters. Concerning the post-failure behavior, models reproduce the trends of decreasing dilation with increasing confinement and plasticity. However, the dilation values simulated are larger than those observed in practice. This is attributed to the difficulty in modeling some phenomena of fissured rock behaviors, such as rock piece corner crushing with dust production and interactions between newly formed shear bands or axial splitting cracks with pre-existing joints.
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| 2. |
- Cristóbal, Andrés, et al.
(author)
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Automatic Characterization of Block-In-Matrix Rock Outcrops through Segmentation Algorithms and Its Application to an Archaeo-Mining Case Study
- 2024
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In: Geosciences. - : MDPI AG. - 2076-3263. ; 14:2
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Journal article (peer-reviewed)abstract
- The mechanical behavior of block-in-matrix materials is heavily dependent on their block content. This parameter is in most cases obtained through visual analyses of the ground through digital imagery, which provides the areal block proportion (ABP) of the area analyzed. Nowadays, computer vision models have the capability to extract knowledge from the information stored in these images. In this research, we analyze and compare classical feature-detection algorithms with state-of-the-art models for the automatic calculation of the ABP parameter in images from surface and underground outcrops. The outcomes of this analysis result in the development of a framework for ABP calculation based on the Segment Anything Model (SAM), which is capable of performing this task at a human level when compared with the results of 32 experts in the field. Consequently, this model can help reduce human bias in the estimation of mechanical properties of block-in-matrix materials as well as contain underground technical problems due to mischaracterization of rock block quantities and dimensions. The methodology used to obtain the ABP at different outcrops is combined with estimates of the rock matrix properties and other characterization techniques to mechanically characterize the block-in-matrix materials. The combination of all these techniques has been applied to analyze, understand and try, for the first time, to model Roman gold-mining strategies in an archaeological site in NW Spain. This mining method is explained through a 2D finite-element method numerical model.
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| 3. |
- Estevez-Ventosa, Xian, et al.
(author)
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Scale effects on triaxial peak and residual strength of granite and preliminary PFC3D models
- 2022
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In: Geomechanics and Engineering. - : TECHNO-PRESS. - 2005-307X .- 2092-6219. ; 31:5, s. 461-476
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Journal article (peer-reviewed)abstract
- Research studies on the scale effect on triaxial strength of intact rocks are scarce, being more common those in uniaxial strength. In this paper, the authors present and briefly interpret the peak and residual strength trends on a series of triaxial tests on different size specimens (30 mm to 84 mm diameter) of an intact granitic rock at confinements ranging from 0 to 15 MPa. Peak strength tends to grow from smaller to standard-size samples (54 mm) and then diminishes for larger values at low confinement. However, a slight change in strength is observed at higher confinements. Residual strength is observed to be much less size-dependent. Additionally, this study introduces preliminary modelling approaches of these laboratory observations with the help of three-dimensional particle flow code (PFC3D) simulations based on bonded particle models (BPM). Based on previous studies, two modelling approaches have been followed. In the first one, the maximum and minimum particle diameter (Dmax and Dmin) are kept constant irrespective of the sample size, whereas in the second one, the resolution (number of particles within the sample or phi v) was kept constant. Neither of these approaches properly represent the observations in actual laboratory tests, even if both of them show some interesting capabilities reported in this document. Eventually, some suggestions are provided to proceed towards improving modelling approaches to represent observed scale effects.
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| 4. |
- González-Fernández, Manuel A., et al.
(author)
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Size effects on strength and deformability of artificially jointed hard rock
- 2024
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In: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier Ltd. - 1365-1609 .- 1873-4545. ; 176
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Journal article (peer-reviewed)abstract
- An extensive experimental study is presented including around 140 triaxial compressive strength tests on artificially jointed hard rock. The experiments were performed on cylindrical and jointed specimens with either 1 sub-vertical and 2 sub-horizontal (1 + 2) or 2 sub-vertical and 3-sub-horizontal (2 + 3) joint sets. The specimen diameter consisted of 38, 54 and 84 mm and the rock was Blanco Mera granite. The confining pressures ranged between 0.2 and 15 MPa and such a testing program complements the previous study in this regard with about 100 triaxial tests on intact rock specimens. A complete set of stress-strain curves were obtained from the experiments followed by extracting the peak and residual strengths as well as indicative deformability parameters (elastic and Poisson's ratio). For the jointed specimens, the indirect strain measurement was corrected to implicitly calculate the elastic parameters following the energy-based concept. It was found that the peak strength is joint set dependent as per the conventional rock mass classification systems. For jointed specimens a moderate strength size effect was observed in which the peak strength tended to increase with size from 38 mm to 54 mm and then decreased with a rise in specimen diameter. Also, it was concluded that the effect of size on peak strength become less prominent in cases with more joints and the influence of additional jointing becomes less significant for larger specimens; both are novel findings which have not been explored before. The approximated variable elastic parameters of jointed specimens exhibited size independency but joint and confinement dependency, while the variable Poisson's ratios specifically, appeared to be size and joint set independent. The presented stress-strain data from granite specimens suggests that the mechanical parameters of jointed rock are largely controlled by the rock structure where the specimen size can play a significant role, particularly at low confinements.
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| 5. |
- Gui, Jing Yun, et al.
(author)
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Analytical and experimental studies on toppling behavior of blocks misaligned with the slope face
- 2024
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In: Bulletin of Engineering Geology and the Environment. - : Springer Nature. - 1435-9529 .- 1435-9537. ; 83:4
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Journal article (peer-reviewed)abstract
- The stability of potentially toppling rock blocks has been typically addressed for the case of regular block geometries showing symmetry planes and resting on surfaces aligned with the slope face, i.e., the strike of the slope is parallel to the block sides and their basal planes. However, these simple geometries with ideally oriented blocks are not often found in nature. This study aims to bridge this knowledge gap through analytical and experimental approaches, in the form of laboratory physical models, to study toppling cases for non-conventional scenarios. An engineering geology review was first conducted, identifying potential non-standard cases in nature. Then, the current analytical solutions for both the factor of safety and critical toppling angle were modified for these misaligned cases, focusing on single block and standard block toppling, and considering the effect of block-edge rounding. Physical models support the analytical approach. It is concluded that toppling stability is slightly improved when the orientation of the basal plane of the blocks differs with the strike of the slope. In this way, for single blocks, the critical toppling angle increases a few tenths of a degree when the difference between strikes is 10°, about 1° with strike differences about 20° and 2 to 3° for misalignments about 30°. Similar increases are observed for block toppling cases. Accordingly, small differences in strike of basal planes and the slope contribute to slightly increasing the factor of safety of slopes prone to block toppling. This approach improves practical engineering toppling stability analyses.
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| 6. |
- Pérez-Rey, Ignacio, et al.
(author)
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Assessment of Direct Tensile Strength Tests in Rock Through a Multi-laboratory Benchmark Experiment
- 2024
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In: Rock Mechanics and Rock Engineering. - : Springer Nature. - 0723-2632 .- 1434-453X. ; 57, s. 3617-
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Journal article (peer-reviewed)abstract
- This study aims to experimentally assess repeatability and reproducibility of direct tensile strength (DTS) tests with deformability measurements on two types of rocks: Blanco Mera granite (Spain) and Cotta sandstone (Germany). The tests were conducted in four rock mechanics laboratories located in different countries (Canada, Germany, Spain and Sweden). A total of 51 tests were performed on cylindrical specimens of the two rocks, using different test equipment and measuring devices. Mean and standard deviation DTS values were determined in the four laboratories for the granite (5.70 ± 0.32, 6.06 ± 0.11, 3.84 ± 0.50 and 6.76 ± 0.10 MPa) and for the sandstone (1.88 ± 0.07, 1.96 ± 0.06, 1.15 ± 0.32 and 1.74 ± 0.19 MPa), together with Young’s moduli and Poisson’s ratios in tension, being statistically analysed to evaluate the variability and compare the main results obtained from the participating laboratories. The findings indicate that the DTS test with deformability measurements on cylindrical rock specimens is operationally feasible. However, certain shortcomings have been identified during the course of the experiments with the existing methodologies, such as the one suggested by the ISRM for DTS tests. The results have also shown to be sensitive to appropriate test and strain measurement configurations. The objective of this study was to shed light on these issues and provide new insights for potential future improvements of the existing testing methods.
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| 7. |
- Perez-Rey, Ignacio, et al.
(author)
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Experimental observations and variability assessment of the basic friction angle of rock and concrete saw-cut surfaces under different testing conditions
- 2022
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In: Arabian Journal of Geosciences. - : Springer Nature. - 1866-7511 .- 1866-7538. ; 15:3
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Journal article (peer-reviewed)abstract
- Prediction of the shear strength of rock discontinuities is usually performed by resorting to empirical shear strength criteria that depend on different inputs, some of which can be estimated through laboratory tests. Even though these tests are usually developed with rock materials, concrete is often used to make joint replicas. A relevant parameter among those affecting the shear strength behaviour of discontinuities is the basic friction angle, representative of a planar surface and clearly related to the angle of repose of granular materials and solid bodies placed on an inclined plane. Many efforts were carried out by several researchers to suggest a straightforward and simple procedure to obtain this parameter for rock surfaces in a rigorous way by means of tilt tests, which crystallized into an 'ISRM Suggested Method'. Aiming at complementing basic friction angle results from tests developed under different scenarios and with different materials (rock and concrete), this paper presents an experimental program encompassing more than 500 tilt tests carried out in dry, water-saturated and submerged conditions. Complementarily, a detailed assessment of both rock and concrete surfaces was carried out, in order to study the possible implications between surface topography, wear, mass loss and evolution of the basic friction angle with repeated testing. The basic friction angle of a granite and a concrete has been estimated for three different testing scenarios related to the water content of specimens. A relevant effect of the environmental conditions of the laboratory (humidity and temperature) on results has been detected. Experimental conclusions are complemented with a statistical assessment of results.
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| 8. |
- Perez-Rey, Ignacio, et al.
(author)
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Influence of Microroughness on the Frictional Behavior and Wear Response of Planar Saw-Cut Rock Surfaces
- 2020
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In: International Journal of Geomechanics. - : American Society of Civil Engineers (ASCE). - 1532-3641 .- 1943-5622. ; 20:8
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Journal article (peer-reviewed)abstract
- Saw-cut rock surfaces, classically utilized to estimate basic friction angle of discontinuities by means of tilt test and other procedures, may seem planar to the naked eye. Nevertheless, they actually present roughness at a micrometric scale. Aiming at characterizing some of these saw-cut rock surfaces and assessing the possible implications between their microscale topography and the resulting tribological behavior, the authors of this study resorted to the 3D focus-variation technique to analyze different surface-texture parameters. Tilt tests were carried out on specimens cut on three rock types, and the involved sliding surfaces were evaluated at a microscale for different testing stages (prior to any test and after two series of repeated tests). An apparently logical inverse correlation between repeated testing and friction angle has been observed, more marked for the smoother surfaces. Higher roughness at the scale of the analysis tends to produce lower friction-angle values, as otherwise observed for mismatched natural rock surfaces. In addition, saw-cut rock surfaces present systematically negative skewness and high values of kurtosis for their height distributions, indicating the occurrence of narrow and deep pits or valleys. Directional hybrid parameters and, in particular, the root mean square (RMS) of the gradient of the surface in the direction of sliding correlates rather well with the measured sliding angle. The authors concluded that the 3D focus-variation technique represents a powerful tool to assess surface-texture parameters of saw-cut rock surfaces, in addition to being useful for understanding some features of the tribological, or wear and frictional, behavior of these type of surfaces.
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