| 1. |
- Castro-Filgueira, Uxia, et al.
(författare)
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Particle flow code simulation of intact and fissured granitic rock samples
- 2020
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Ingår i: Journal of Rock Mechanics and Geotechnical Engineering. - : Elsevier BV. - 1674-7755. ; 12:5, s. 960-974
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Tidskriftsartikel (refereegranskat)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. |
- Chan, T., et al.
(författare)
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DECOVALEX III BMT3/BENCHPAR WP4 : The thermo-hydro-mechanical responses to a glacial cycle and their potential implications for deep geological disposal of nuclear fuel waste in a fractured crystalline rock mass
- 2005
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier BV. - 1365-1609 .- 1873-4545. ; 42:5-6, s. 805-827
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Tidskriftsartikel (refereegranskat)abstract
- A number of studies related to past and on-going deep repository performance assessments have identified glaciation/ deglaciation as major future events in the next few hundred thousand years capable of causing significant impact on the long term performance of the repository system. Benchmark Test 3 (BMT3) of the international DECOVALEX III project has been designed to provide an illustrative example that explores the mechanical and hydraulic response of a fractured crystalline rock mass to a period of glaciation. The primary purpose of this numerical study is to investigate whether transient events associated with a glacial cycle could significantly influence the performance of a deep geological repository in a crystalline Shield setting. A conceptual site-scale (tens of kilometres) hydro-mechanical (HM) model was assembled based primarily on site-specific litho-structural, hydrogeological and geomechanical data from the Whiteshell Research Area in the Canadian Shield, with simplification and generalization. Continental glaciological modelling of the Laurentide ice sheet through the last glacial cycle lasting approximately 100,000 years suggests that this site was glaciated at about 60 ka and between about 22.5 and 11 ka before present with maximum ice sheet thickness reaching 2500 m and maximum basal water pressure head reaching 2000m. The ice-sheet/drainage model was scaled down to generate spatially and temporally variable hydraulic and mechanical glaciated surface boundary conditions for site-scale subsurface HM modelling and permafrost modelling. Under extreme periglacial conditions permafrost was able to develop down to the assumed 500-m repository horizon. Two- and three-dimensional coupled HM finite-element simulations indicate: during ice-sheet advance there is rapid rise in hydraulic head, high transient hydraulic gradients and high groundwater velocities 2-3 orders of magnitude higher than under nonglacial conditions; surface water recharges deeper than under nonglacial conditions; upon ice-sheet retreat, the gradients reverse; fracture zone network geometry, interconnectivity and hydraulic properties significantly influence flow domain response; residual elevated heads are preserved for 10,000s in the low-diffusivity rock; and no hydraulic jacking or shear failure occurs at depth. It was found that transient coupled modelling is necessary to capture the essence of glacial effects on Performance Assessment. Model dimensionality also significantly affects simulated results.
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| 3. |
- Darcel, C., et al.
(författare)
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Rock mass effective properties from a DFN approach
- 2018
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Ingår i: 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018. - : American Rock Mechanics Association (ARMA).
-
Konferensbidrag (refereegranskat)abstract
- Rock mass mechanical properties are strongly controlled by the fractures they contain. Their determination raises strong issues for many rock-engineering applications, like underground repository safety assessment, support design, slope stability or mine caving. To compensate the impossibility to perform direct in-situ measures of these properties at appropriate scales, empirical approaches classically aim to determine the rock mass equivalent properties from simple indicators. Here we propose an approach based on the complete representation of the rock mass as an intact rock with a population of discrete fractures through it (the Discrete Fracture Network). The core of the approach is the definition, at the rock mass scale, of the deformation induced by each fracture locally, including the fracture mechanical and geometrical parameters, the remote stress conditions and the interactions with the rest of the fracture population. Depending on the conditions, the resulting scaling and anisotropic effects can be critical. The method is applied to the Forsmark site in Sweden. We show that two main scaling regimes occur, where the shift from the one to the other is controlled by the ratio between the intact rock modulus, the typical fracture stiffness and the DFN size distribution. Beyond the scaling issue we quantify the resulting level of anisotropy.
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| 4. |
- Davy, P., et al.
(författare)
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DFN, why, how and what for, concepts, theories and issues
- 2018
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Ingår i: 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018. - : American Rock Mechanics Association (ARMA).
-
Konferensbidrag (refereegranskat)abstract
- DFN – Discrete Fracture Network – is primarily a modeling framework for fractured geological systems that aims to integrate field data into simulations of flow and/or deformation. It is complementary to, or competing with, continuum methods with both advantages of easily integrating the statistical properties of fracture networks, and of not assuming any homogenization scale. The core element is the DFN conceptual model, which makes a functional link between data from different sources, prior knowledge and medium models. We discuss some fundamental issues about this conceptual model, namely (i) the upscaling of small-scale measurements to site-scale relationships, (ii) intrinsic variability versus geological determinism, (iii) the way to incorporate a priori knowledge, (iv) the transformation of a statistical description into a medium model, (v) the critical characteristics (length scales, scaling laws or physical properties) of fractures for a given DFN application. The main product of the DFN conceptual model is medium models, whose role is to extrapolate/interpolate data with a faithful representation of the geological system. The way in which fracture correlations are taken into account, or not, in the generation process plays an important control on the connectivity and flow properties of medium models.
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| 5. |
- Davy, P., et al.
(författare)
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Elastic Properties of Fractured Rock Masses With Frictional Properties and Power Law Fracture Size Distributions
- 2018
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Ingår i: Journal of Geophysical Research - Solid Earth. - : American Geophysical Union (AGU). - 2169-9313 .- 2169-9356. ; 123:8, s. 6521-6539
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Tidskriftsartikel (refereegranskat)abstract
- We derive the relationships that link the general elastic properties of rock masses to the geometrical properties of fracture networks, with a special emphasis to the case of frictional crack surfaces. We extend the well-known elastic solutions for free-slipping cracks to fractures whose plane resistance is defined by an elastic fracture (shear) stiffness k(s) and a stick-slip Coulomb threshold. A complete set of analytical solutions have been derived for (i) the shear displacement in the fracture plane for stresses below the slip threshold and above, (ii) the partitioning between the resistances of the fracture plane on the one hand and of the elastic matrix on the other hand, and (iii) the stress conditions to trigger slip. All the expressions have been checked with numerical simulations. The Young's modulus and Poisson's ratio were also derived for a population of fractures. They are controlled both by the total fracture surface for fractures larger than the stiffness length l(S) (defined by k(s) and the intact matrix elastic properties) and by the percolation parameter of smaller fractures. These results were applied to power law fracture size distributions, which are likely relevant to geological cases. We show that if the fracture size exponent is in the range -3 to -4, which corresponds to a wide range of geological fracture networks, the elastic properties of the bulk rock are almost exclusively controlled by k(s) and the stiffness length, meaning that the fractures of size l(S) play a major role in the definition of the elastic properties.
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| 6. |
- De Simone, Silvia, et al.
(författare)
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Equivalent Biot and Skempton Poroelastic Coefficients for a Fractured Rock Mass from a DFN Approach
- 2023
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Ingår i: Rock Mechanics and Rock Engineering. - : Springer Nature. - 0723-2632 .- 1434-453X. ; 56:12, s. 8907-8925
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Tidskriftsartikel (refereegranskat)abstract
- A quantitative and analytical approach is adopted to estimate two important parameters for coupled hydro-mechanical analysis at the scale of a fractured rock mass, namely the equivalent Biot effective stress coefficient α¯ and Skempton pore pressure coefficient B¯ . We derive formal expressions that estimate the two equivalent poroelastic coefficients from the properties of both the porous intact rock and the discrete fracture network, which includes fractures with different orientation, size, and mechanical properties. The coefficients are equivalent in the sense that they allow effectively predicting the volumetric deformation of the fluid-saturated fractured rock under an applied load in drained and undrained conditions. The formal expressions are validated against results from fully coupled hydro-mechanical simulations on systems with explicit representation of deformable fractures and rock blocks. We find that the coefficients are highly anisotropic as they largely vary with fracture orientations with respect to the applied stress tensor. For a given set of fracture and rock properties, B¯ increases with the ratio of normal to average stress undergone by the fractures, while the opposite occurs for α¯ . Additionally, both α¯ and B¯ increase with fracture density, which directly impacts the deformation caused by a load in undrained conditions. Because the effective stress variation is proportional to the applied load by (1 - α¯ B¯) , a factor that partly compensates for the decrease in equivalent rock stiffness caused by the fractures, a fully saturated fractured rock may deform less than an intact rock in undrained conditions, while the opposite occurs in dry conditions.
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| 7. |
- Doolaeghe, D., et al.
(författare)
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Controls on fracture openness and reactivation in Forsmark, Sweden
- 2023
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- In crystalline bedrock, the open fraction of the fracture network constitutes the main pathways for fluids. Many observations point out that the state of stress influences the open fraction, likely indicating recent reactivation. But how this occurs is still unresolved. We analyse the conditions for fracture reactivation from fracture data collected in the uppermost 1 km of bedrock in Forsmark, Sweden. The open fraction is mainly correlated to the stress acting normally on the fracture but even away from critical failure, leading us to analyse the potential fluid pressure required for reactivation, P-c. We observe that 100% of the fractures are open when P-c is hydrostatic, and the ratio decreases exponentially to a plateau of similar to 17% when P-c is lithostatic and above. Exceptions are the oldest fractures, having a low open fraction independent of P-c. We suggest that these results reflect past pressure build-ups, potentially related to recent glaciations, and developing only if the preexisting open fraction is large enough.
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| 8. |
- Estevez-Ventosa, Xian, et al.
(författare)
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Scale effects on triaxial peak and residual strength of granite and preliminary PFC3D models
- 2022
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Ingår i: Geomechanics and Engineering. - : TECHNO-PRESS. - 2005-307X .- 2092-6219. ; 31:5, s. 461-476
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Tidskriftsartikel (refereegranskat)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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| 9. |
- Figueiredo, B., et al.
(författare)
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Effects of the initial stress and spalling strength on spalling around deposition holes and tunnels
- 2023
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Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315.
-
Konferensbidrag (refereegranskat)abstract
- Spalling in the deposition holes and tunnel stability are of concern for the design of deep geological repositories for spent nuclear fuel. A 3D numerical stress model, including a single deposition tunnel and multiple deposition holes to analyse the potential for spalling resulting from the excavation phase, was developed for the proposed repository location at the Forsmark site. Several potential initial stress field cases and spalling strengths of the intact rock were considered. The magnitudes of the factor of safety, the spalling depth, the minor principal stress, and the differential stress were evaluated. The results showed that no spalling in the central deposition hole is indicated for the most likely stress field, and the occurrence of spalling and the spalling depth are minimised for a maximum horizontal stress aligned approximately parallel to the tunnel axis. For spalling occurrence and depth of spalling, the magnitude of the maximum horizontal stress is more critical than its orientation, and the spalling strength is more critical than the magnitude of the maximum horizontal stress. No instability problems were indicated in the vicinity of the roof for all analysed stress cases.
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| 10. |
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| 11. |
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| 12. |
- Ivars, Diego Mas, 1974-, et al.
(författare)
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A case study of the Odenplan station in the Stockholm City Link project - Analysis of in situ stresses and observed ground behaviour.
- 2016
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Rapport (refereegranskat)abstract
- The Odenplan Station in the Stockholm City Link project is located directly under theexisting subway station with a minimum rock cover of approximately 7 m. The span of theOdenplan Station is 25 m. When the Odenplan station was excavated, deformations largerthan anticipated in previous calculations were observed. The main reason for this discrepancyis believed to be high horizontal in situ and/or persistent fractures in the area parallel withthe excavation.The excavation of the Odenplan station constitutes a case study with well-documented rockmass characteristics, excavation sequences, support sequences and measured deformations.It therefore provides a unique project to analyse. Based on this case study, the main objectivesof this study was to (1) suggest a general technique for numerical back analyses based onmeasured deformations under geological conditions as those present in the Stockholm area,and (2) analyse the range of possible in situ stresses at Odenplan Station in the City Linkproject.The complex three-dimensional 3DEC model lead to rather long computational times. Ageneral technique for a real back analysis was therefore not possible to obtain in this study,since it would require unrealistically long computational time. Instead, a sensitivity study onthe in situ stresses and different types of continuum and discontinuum approaches wereperformed. The results from the analyses gave valuable insight into which types of modelsthat were able to properly recreate the observed ground behavior at Odenplan. The resultsfrom the study clearly showed that a continuum approach does not capture the true behaviorof the blocky rock mass at Odenplan. It is therefore recommended that discontinuumapproaches are used in the future when structurally controlled block movements could beexpected. However, the exact cause behind the discrepancy between the continuum and thediscontinuum model has not been determined in this report.The performed sensitivity analysis suggests that the in situ stresses are high and in the rangeof those previously measured. Maximum horizontal in situ stresses are probably in the rangeof 5.7-7.2 MPa at a depth of 0-20 m, which agrees well with the results of previous rockstress measurements, which were, on average, 8.1 MPa at a depth of 30 m.The encouraging results from this project indicate that further stress sensitivity analysesshould be performed to have a better estimation of the range of in situ stresses in theStockholm area by fitting displacements obtained from modeling to those measured in situ in different projects. These analyses are important in order to understand our present abilityto perform numerical simulations reflecting true rock mass behavior.
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| 13. |
- Ivars, Diego Mas, et al.
(författare)
-
Rock mechanics and DFN models in the Swedish Nuclear Waste Disposal Program
- 2021
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Ingår i: IOP Conference Series. - : IOP Publishing.
-
Konferensbidrag (refereegranskat)abstract
- Discrete Fracture Network (DFN) is a modeling framework for fractured rocks. The core element is the description of geological medium as a network of discrete fractures that can be either generated from statistical distributions or imported as deterministic surfaces. It is an alternative to continuum methods with both advantages of easily integrating the statistical properties of fracture networks, and of not assuming any homogenization scale. DFN has been extensively used to describe fracture network flow properties supported by the fact that connectivity, which is a constitutive element of the network organization, is a key element of fluid percolation. Application of the DFN modeling framework to geomechanics is also promising and, conversely, DFN models will benefit from rock mechanics integration. Integration between DFN and rock mechanics modeling is in expansion in many fields and broad contexts. This includes prediction of mechanical effective properties, increased understanding of the fracture scales and indicators that control these properties, distribution of block sizes and shapes for block fall risk analysis, potential wave attenuation effect and fracture shear displacements caused by and within the fracture network induced by an earthquake, or hydromechanical effects for flow and transport predictions. These applications are relevant only if DFN models involve the right complexity and provide a reliable description of the fracture networks. DFN models also benefit from rock mechanics concepts to improve their realism as it is done with genetic models that mimic the growth and arrest of fractures according to stress conditions prevailing at the time of their formation.
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| 14. |
- Jacobsson, Lars, et al.
(författare)
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Experimental program on mechanical properties of large rock fractures
- 2021
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Ingår i: IOP Conference Series. - : IOP Publishing. ; , s. 012015-
-
Konferensbidrag (refereegranskat)abstract
- Predictions of fracture displacements are required to support the safety assessments of a deep geological repository for nuclear spent fuel. Laboratory and in-situ experiments are used to estimate these properties. Despite significant contributions in the last decades, there is a knowledge gap in terms of the impact of high normal stresses on the mechanical properties of large-scale fractures under Constant Normal Stiffness (CNS) boundary conditions. Within the framework of the POST project, a cooperative effort was made by SKB (Sweden), NWMO (Canada), and Posiva from Finland (in phase 1) to study these questions. In the second phase of the POST project, a first of a kind direct shear testing machine was manufactured and calibrated that can accommodate samples up to 400 × 600 mm under normal stresses up to 10 MPa, for both CNS and Constant Normal Load (CNL) conditions, with the ability to shear the sample up to 50 mm. Several best practice procedures were developed for fracture characterization pre-, syn-, and post-shear test which utilize high resolution optical scanning, contact pressure measurements, Digital Image Correlation (DIC) measurements, and acoustic emission measurements during the shear test. Natural and tensile-induced fractures of a granitic rock as well as replicas of the hard rock fractures, at three different fracture sizes of 35×60 mm, 70×100 mm, and 300×500 mm, are now being tested. It is hoped that this program will provide a set of high-quality data which will help reduce the knowledge gap in the understanding of fracture behavior.
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| 15. |
- Jacobsson, Lars, et al.
(författare)
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Experimental program to study scale effects on mechanical properties of large rock fractures
- 2022
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Ingår i: Abstract Volume. ; , s. 102-103
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The scale effect of natural and artifically induced rock fractures and replicas up to a 500 mm length are experimentally studied by direct shear tests in a testing program including a newly manufactured large shear testing equipment. The fractures are chatacterised pre-, syn-, and post-shear test. Combinations of different measurements provide a high-quality dataset enabling deeper understanding of, and constitutive model development for rock fractures used for safety assessment of deep geological respoitories for nuclear waste disposal.
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| 16. |
- Jacobsson, Lars, et al.
(författare)
-
MEKANISKA EGENSKAPER HOS STORABERGSPRICKOR
- 2024
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Experimentella undersökningar av skjuvegenskaperna hos bergsprickor i hårt berg har generellt fokuserat på mindre sprickprover för normalspänningar på upp till 20 MPa, representativa för ett kärnbränsleförvar på ca 400 meters djup. Vid stora sprickprover har skjuvegenskaperna generellt bestämts för låga spänningar, på någon eller några MPa. För att få en förståelse för inverkan av sprickornas storlek för skjuvegenskaperna vid höga normalspänningar är det avgörande att genomföra skjuvförsök på stora bergsprickor (> 200 mm) i hårt berg under höga normalspänningar. I projektet Parameterization of Fractures, POST (2014–2016), studerades skaleffekterna genom in situ-försök, småskaliga skjuvförsök och beräkningssimuleringar. Det konstaterades att laboratorieexperiment under kontrollerade förhållanden och på stora bergprover är nödvändiga för att få tillförlitliga resultat. Det konstaterades också att in-situ-försök är komplexa med stora underliggande osäkerheter och är samtidigt kostsamma. I det pågående projektet POST 2 som startade 2017 har bergsprickor på upp till 500 mm provats i en ny unik laboratorieutrustning, jämte provning av mindre sprickor, vid höga normalspänningar för både CNL och CNS förhållanden och med ny mätteknik. Teknik för att tillverka replikaprover av bergsprickor har utvecklats och provats med syfte att göra lastparameterstudier. Kvaliteten hos geometriavbildningen hos replikaproverna och sprickornas geometrier har uppmätts med högupplöst skanning. I denna artikel presenteras en del av resultaten från projektet. Resultaten från projektet är tillämpbara för andra områden med undermarkskonstruktioner såsom projektering av tunnlar och bergrum för infrastrukturprojekt och gruvor.
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| 17. |
- Javaid, M. A., et al.
(författare)
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A Bayesian regression analysis of in situ stress using overcoring data
- 2023
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Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315.
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Konferensbidrag (refereegranskat)abstract
- Characterising the state of in situ stress at a target depth is crucial for all underground engineering projects. Consequently, on critical projects such as nuclear waste repositories extensive campaigns are implemented with the goal of estimating the in situ stress state. These campaigns often comprise both direct measurement and indirect estimation methods, but the data obtained across a project volume may exhibit significant variability. This poses significant challenges in both quantifying the variability and uncertainty of in situ stress, and determining the stress state to be used for design purposes. It is often assumed that the state of in situ stress increases linearly with depth, and thus linear regression of principal stress magnitude against depth are often found in the literature. As such methods not honouring the tensorial nature of stress are, strictly, incorrect. To show how this limitation may be overcome, here we present a Bayesian regression analysis of in situ stress with depth that uses the Cartesian stress tensor. The analysis is performed using over 100 overcoring data obtained at the SKB Forsmark site in Sweden. A comparison between the customary and Bayesian approaches is presented, which shows the superiority of the tensorial technique.
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| 18. |
- Larsson, Jörgen, et al.
(författare)
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A novel method for geometric quality assurance of rock joint replicas in direct shear testing – Part 1 : Derivation of quality assurance parameters and geometric reproducibility
- 2023
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Ingår i: Journal of Rock Mechanics and Geotechnical Engineering. - : Chinese Academy of Sciences. - 1674-7755. ; 15:9, s. 2193-
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Tidskriftsartikel (refereegranskat)abstract
- Since each rock joint is unique by nature, the utilization of replicas in direct shear testing is required to carry out experimental parameter studies. However, information about the ability of the replicas to simulate the shear mechanical behavior of the rock joint and their dispersion in direct shear testing is lacking. With the aim to facilitate generation of high-quality direct shear test data from replicas, a novel component in the testing procedure is introduced by presenting two parameters for geometric quality assurance. The parameters are derived from surface comparisons of three-dimensional (3D) scanning data of the rock joint and its replicas. The first parameter, σmf, captures morphological deviations between the replica and the rock joint surfaces. σmf is derived as the standard deviation of the deviations between the coordinate points of the replica and the rock joint. Four sources of errors introduced in the replica manufacturing process employed in this study could be identified. These errors could be minimized, yielding replicas with σmf ≤ 0.06 mm. The second parameter is a vector, VHp100, which describes deviations with respect to the shear direction. It is the projection of the 100 mm long normal vector of the best-fit plane of the replica joint surface to the corresponding plane of the rock joint. |VHp100| was found to be less than or equal to 0.36 mm in this study. Application of these two geometric quality assurance parameters demonstrates that it is possible to manufacture replicas with high geometric similarity to the rock joint. In a subsequent paper (part 2), σmf and VHp100 are incorporated in a novel quality assurance method, in which the parameters shall be evaluated prior to direct shear testing. Replicas having parameter values below established thresholds shall have a known and narrow dispersion and imitate the shear mechanical behavior of the rock joint.
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| 19. |
- Larsson, Jörgen, et al.
(författare)
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A novel method for geometric quality assurance of rock joint replicas in direct shear testing : Part 2 : Validation and mechanical replicability
- 2023
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Ingår i: Journal of Rock Mechanics and Geotechnical Engineering. - : Elsevier BV. - 1674-7755. ; 15, s. 2209-
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Tidskriftsartikel (refereegranskat)abstract
- Each rock joint is unique by nature which means that utilization of replicas in direct shear tests is required in experimental parameter studies. However, a method to acquire knowledge about the ability of the replicas to imitate the shear mechanical behavior of the rock joint and their dispersion in direct shear testing is lacking. In this study, a novel method is presented for geometric quality assurance of replicas. The aim is to facilitate generation of high-quality direct shear testing data as a prerequisite for reliable subsequent analyses of the results. In Part 1 of this study, two quality assurance parameters, σmf and VHp100, are derived and their usefulness for evaluation of geometric deviations, i.e. geometric reproducibility, is shown. In Part 2, the parameters are validated by showing a correlation between the parameters and the shear mechanical behavior, which qualifies the parameters for usage in the quality assurance method. Unique results from direct shear tests presenting comparisons between replicas and the rock joint show that replicas fulfilling proposed threshold values of σmf < 0.06 mm and < 0.2 mm have a narrow dispersion and imitate the shear mechanical behavior of the rock joint in all aspects apart from having a slightly lower peak shear strength. The wear in these replicas, which have similar morphology as the rock joint, is in the same areas as in the rock joint. The wear is slightly larger in the rock joint and therefore the discrepancy in peak shear strength derives from differences in material properties, possibly from differences in toughness. It is shown by application of the suggested method that the quality assured replicas manufactured following the process employed in this study phenomenologically capture the shear strength characteristics, which makes them useful in parameter studies.
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| 20. |
- Libby, S., et al.
(författare)
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Dynamic fracture network generation : A new method for growing fractures according to their deformation history
- 2019
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Ingår i: 53rd U.S. Rock Mechanics/Geomechanics Symposium. - : American Rock Mechanics Association (ARMA).
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Konferensbidrag (refereegranskat)abstract
- This paper presents a new method for generation of simulated fractures where fractures ‘grow’ dynamically, permitting interaction during formation. These interactions mimic the natural processes of stress shadowing, termination of fractures on other fractures, linking of fractures, and the varied growth on a single fracture due to contrasting rock properties. By simulating these interactions and providing the user with fine control over them, the new fracture generation method can create simulated fracture networks that match natural fracture networks more closely than other established methods. These behaviours are implemented mechanistically, allowing fracture generation to be achieved without the significant additional computational cost required to explicitly model the stresses in a fracturing rock volume. A suite of test cases is demonstrated, illustrating how different configurations of the dynamic fracture model allows different connectivity characteristics to be modelled.
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| 21. |
- Libby, Simon, et al.
(författare)
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Exploring the impact of fracture interaction on connectivity and flow channelling using grown fracture networks
- 2024
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Ingår i: Quarterly journal of engineering geology and hydrogeology. - : Geological Society of London. - 1470-9236 .- 2041-4803. ; 57:1
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative assessment of the flow properties and mechanical stability of naturally fractured rock is frequently practised across the mining, petroleum, geothermal, geological disposal, construction and environmental remediation industries. These fluid and mechanical behaviours are strongly influenced by the connectivity of the fracture system and the size of the intact rock blocks. However, these are amongst the more difficult fracture system properties to characterize and honour in numerical simulations. Nonetheless, they are still the product of interactions between fractures that can be conceptualized as a series of deformation events following geomechanical principles. Generating numerical models of fracture networks by simulating this deformation with a coupled and evolving rock mass and stress field is a significant undertaking. Instead, large-scale fracture network models can be 'grown' dynamically according to rules that mimic the underlying mechanical processes and deformation history. This paper explores a computationally efficient rules-based method to generate fracture networks, demonstrates how different types of fracture patterns can be simulated, and illustrates how inclusion of fracture interactions can affect flow and mechanical properties. Relative to methods without fracture interaction and in contrast to some other rules-based approaches, the method described here regularizes and increases fracture connectivity and decreases flow channelling.
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| 22. |
- Lisjak, Andrea, et al.
(författare)
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Analysis of thermo-mechanical damage around tunnel and deposition boreholes of an underground nuclear waste disposal facility at the Forsmark site (Sweden) by 3D coupled FDEM simulations
- 2023
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier BV. - 1365-1609 .- 1873-4545. ; 171
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Tidskriftsartikel (refereegranskat)abstract
- This study focuses on rock damage potentially developing in the near-field of a planned underground nuclear waste repository at the Forsmark site (Sweden). In hard, crystalline rocks, mechanical damage in the form of spalling may be induced during construction by overstressing of the excavation periphery. During operation, thermal damage may develop due to additional thermo-elastic stresses forming in response to the increasing rock temperatures induced by the heat-emitting spent nuclear fuel. Prediction of damage occurrence, location, and extent is critical for an effective repository design and long-term safety assessment as it may negatively affect the long-term isolation properties of the host rock. In this paper, the response of underground structures was studied using a novel 3D coupled thermo-mechanical simulator based on the finite-discrete element method (FDEM). It is the first numerical study to date that explicitly captures both mechanical and thermal fracturing processes while using the latest repository design and site-specific geomechanical input data. A sensitivity study is performed to investigate different combinations of rock mechanical properties, in-situ stresses, and deposition tunnel geometry on the host rock behaviour. Rock mass deconfinement is shown to promote the development of tensile damage in the tunnel sidewalls and floor with fracture surfaces growing parallel to the excavation boundaries. The negative effects deriving from the adoption of a relatively narrower tunnel cross-section and from an increase of horizontal in-situ stresses are highlighted. Thermo-mechanical analyses capture the rock mass behaviour following an increase of borehole surface temperature to 100 °C. Numerical results indicate that the temperature evolution is affected by the shape of the underground cavities and their distance from the heated boreholes. The coupled thermal expansion of the rock induces additional stresses which, in turn, promotes further damage. Despite this increase, however, the total amount of induced rock damage at final conditions remains relatively low.
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| 23. |
- Mas Ivars, Diego, 1974-
(författare)
-
Bonded Particle Model for Jointed Rock Mass
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Jointed rock masses are formed of intact rock and joints. There-fore, proper characterization of rock mass behavior has to consid-er the combined behavior of the intact rock blocks and that of the joints. This thesis presents the theoretical background of the Synthetic Rock Mass (SRM) modeling technique along with example applica-tions. The SRM technique is a new approach for simulating the mechanical behavior of jointed rock masses. The technique uses the Bonded Particle Model (BPM) for rock to represent intact ma-terial and the Smooth-Joint Contact Model (SJM) to represent the in situ joint network. In this manner, the macroscopic behaviour of an SRM sample depends on both the creation of new fractures through intact material, and slip/opening of pre-existing joints. SRM samples containing thousands of non-persistent joints can be submitted to standard laboratory tests (UCS, triaxial loading, and direct tension tests) or tested under a non-trivial stress path repre-sentative of the stresses induced during the engineering activity under study. Output from the SRM methodology includes pre-peak properties (modulus, damage threshold, peak strength) and post-peak proper-ties (brittleness, dilation angle, residual strength, fragmentation). Of particular interest is the ability to obtain predictions of rock mass scale effects, anisotropy and brittleness; properties that can-not be obtained using empirical methods of property estimation. Additionally, the nature of yielding and fracturing can be studied as the rock mass fails. This information can improve our understand-ing of rock mass failure mechanisms.
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| 24. |
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| 25. |
- Moon, Seulgi, et al.
(författare)
-
Present-Day Stress Field Influences Bedrock Fracture Openness Deep Into the Subsurface
- 2020
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 47:23
-
Tidskriftsartikel (refereegranskat)abstract
- Fracturing of bedrock promotes water-rock interactions and influences the formation of the life-sustaining layer of soil at Earth's surface. Models predict that present-day stress fields should influence bedrock fracture openness, but testing this prediction has proven difficult because comprehensive fracture data sets are rarely available. We model the three-dimensional present-day stress field beneath the deglaciated, low-relief landscape of Forsmark, Sweden. We account for ambient regional stresses, pore pressure, topography, sediment weight, and seawater loading. We then compare the modeled stresses to a data set of similar to 50,000 fractures reaching depths of 600 m at Forsmark. We show that modeled failure proxies correlate strongly with the fraction of observed open fractures to depths of similar to 500 m. This result implies that the present-day regional stress field, affected by surface conditions and pore pressure, influences fracture openness in bedrock hundreds of meters beneath the surface, thereby preparing the rock for further weathering. Plain Language Summary The "critical zone"-the life-sustaining part of the Earth that extends from the top of the tree canopy to the bottom of permeable bedrock-is essential for ecosystems and agriculture. The opening of bedrock fractures and onset of water-rock interaction are crucial to the formation of the critical zone. Within the bedrock, the intensities of horizontal regional forces and vertical gravitational forces typically increase with depth. These force intensities, or stresses, are modified by surface effects associated with topography, the weight of overlying seawater and sediment, and by groundwater pressure. However, the influence of these surface effects on fractures has been difficult to observe because comprehensive fracture data sets are rare. In this study, we examine whether, and to what depths, bedrock may fracture under the influence of stress associated with surficial conditions. We compare bedrock stress calculations with similar to 50,000 fractures from 18 cores reaching depths of 600 m at Forsmark, Sweden. We find that the present-day stress field influences the opening of fractures to depths of 500 m, contributing to the formation of the critical zone and the preparation of rock for weathering hundreds of meters beneath the surface, much deeper than previously thought.
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| 26. |
- Pérez-Rey, Ignacio, et al.
(författare)
-
Assessment of Direct Tensile Strength Tests in Rock Through a Multi-laboratory Benchmark Experiment
- 2024
-
Ingår i: Rock Mechanics and Rock Engineering. - : Springer Nature. - 0723-2632 .- 1434-453X. ; 57, s. 3617-
-
Tidskriftsartikel (refereegranskat)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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| 27. |
- Perez-Rey, Ignacio, et al.
(författare)
-
Experimental observations and variability assessment of the basic friction angle of rock and concrete saw-cut surfaces under different testing conditions
- 2022
-
Ingår i: Arabian Journal of Geosciences. - : Springer Nature. - 1866-7511 .- 1866-7538. ; 15:3
-
Tidskriftsartikel (refereegranskat)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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| 28. |
- Perez-Rey, Ignacio, et al.
(författare)
-
Influence of Microroughness on the Frictional Behavior and Wear Response of Planar Saw-Cut Rock Surfaces
- 2020
-
Ingår i: International Journal of Geomechanics. - : American Society of Civil Engineers (ASCE). - 1532-3641 .- 1943-5622. ; 20:8
-
Tidskriftsartikel (refereegranskat)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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| 29. |
- Ríos Bayona, Francisco
(författare)
-
Analytical and numerical approaches to estimate peak shear strength of rock joints
- 2019
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In Sweden, there exists a large number of dams. Many of them are founded on rock masses normally affected by the presence of sub-horizontal rock fractures, which makes sliding along rock joints under the dam foundation one of the most critical failure mechanism. Various attempts have been made to relate the peak shear strength of rock joints to measurable parameters. However, the uncertainty in the determination of the shear strength of rock joints is nonetheless still significant.The main aim of this thesis is to investigate, develop and apply analytical and numerical techniques for estimation of peak shear strength of natural and unfilled rock joints. In a first step, the peak shear strength of several natural and unfilled rock joint was calculated by using surface aperture measurements from high-resolution optical scanning and a modified version of the analytical criterion previously developed by Johansson and Stille in 2014. In a second step, PFC2D was utilised to perform numerical shear tests on two-dimensional profiles selected from high-resolution optical scanning on unweathered and perfectly mated tensile induced rock joints.The results from the analytical approach show that the calculated peak shear strengths of the analysed samples are in good agreement compared with the laboratory investigations. Conversely, the obtained results from the numerical approach show lower peak shear strengths in the analysed two-dimensional profiles compared with the conducted laboratory shear tests.The analytical approach together with the advanced techniques to measure surface roughness available today, may be a possible way forward towards a methodology to determine peak shear strength of large-scale natural rock joints in-situ.
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| 30. |
- Ríos Bayona, Francisco, et al.
(författare)
-
Comparison between shear strength based on Barton’s roughness profiles and equivalent synthetic profiles based on fractal theory
- 2018
-
Ingår i: 52nd U.S. Rock Mechanics/Geomechanics Symposium. - : American Rock Mechanics Association (ARMA).
-
Konferensbidrag (refereegranskat)abstract
- A comprehensive understanding of the shear strength and the mechanical behavior of rock joints is to some extent still missing today. Several attempts have been made to develop empirical and analytical shear strength criteria that explain this mechanism. One of the most important parameters governing the shear strength of rock fractures is the surface roughness, which is generally determined using the Joint Roughness Coefficient (JRC). This parameter is often determined subjectively in the field by comparison with 10 predefined roughness profiles. Recent studies indicate that surface roughness can be accurately represented by using fractal analysis. The aim of this study is to perform a first attempt to investigate the mechanical equivalence in terms of the peak shear strength between synthetic rock fractures, where the surface roughness has been generated using fractal theory, and standard roughness profiles from Barton and Choubey, 1977, using the particle flow code PFC2D. The results from the numerical shear tests under constant normal load (CNL) are compared with the predicted peak shear strength using Barton’s criterion and a back-calculation of the JRC value is carried out.
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| 31. |
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| 32. |
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| 33. |
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| 34. |
- Ríos Bayona, Francisco, et al.
(författare)
-
Peak Shear Strength of Natural, Unfilled Rock Joints in the Field Based on Data from Drill Cores – A Conceptual Study Based on Large Laboratory Shear Tests
- 2022
-
Ingår i: Rock Mechanics and Rock Engineering. - : Springer Nature. - 0723-2632 .- 1434-453X. ; 55:8, s. 5083-5106
-
Tidskriftsartikel (refereegranskat)abstract
- Significant uncertainties remain regarding the field assessment of the peak shear strength of rock joints. These uncertainties mainly originate from the lack of a verified methodology that would permit prediction of rock joints’ peak shear strength accounting for their surface area, while using information available from smaller samples. This paper investigates a methodology that uses objective observations of the 3D roughness and joint aperture from drill cores to predict the peak shear strength of large natural, unfilled rock joints in the field. The presented methodology has been tested in the laboratory on two natural, unfilled rock joint samples of granite. The joint surface area of the tested samples was of approximately 500 × 300 mm. In this study, the drill cores utilised to predict the peak shear strength of the rock joint samples are simulated based on a subdivision of their digitised surfaces obtained through high-resolution laser scanning. The peak shear strength of the tested samples based on the digitised surfaces of the simulated drill cores is predicted by applying a peak shear strength criterion that accounts for 3D roughness, matedness, and specimen size. The results of the performed analysis and laboratory experiments show that data from the simulated drill cores contain the necessary information to predict the peak shear strength of the tested rock joint samples. The main benefit of this approach is that it may enable the prediction of the peak shear strength in the field under conditions of difficult access.
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| 35. |
- Ríos Bayona, Francisco
(författare)
-
Peak Shear Strength of Rock Joints – Towards a Methodology for Prediction Based on Field Data
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The rock joint shear strength at field scale is an important design parameter and remains a challenge for rock mechanics engineers. In Sweden, there exist a large number of concrete dams that are founded on rock masses which in many cases contain sub-horizontal rock joints. The action of water pressure and uplift forces makes sliding along these sub-horizontal rock joints one of the most critical failure mechanisms to be considered in a dam’s safety evaluation. Despite the various attempts to develop empirical, analytical, and numerical methods in recent decades, the uncertainty in the prediction of the peak shear strength of rock joints is still significant. None of the existing methods today fully capture the complex interaction between all the relevant parameters.The overall aim of this research project is to develop a methodology for the prediction of the peak shear strength of rock joints in cases where the whole joint surface is not accessible, such as the foundation under an existing concrete dam. To accomplish this, the prediction of rock joint peak shear strength was studied (1) numerically using discrete element method (DEM), (2) analytically by developing a peak shear strength criterion, and (3) experimentally by characterising the surface roughness and aperture of the tested samples based on high-resolution scanning prior to the direct shear tests.The results of the numerical study showed that the shear test environment in PFC2D used in this project has the capability of simulating the peak shear strength of actual rock joints both qualitatively, and quantitatively. However, a 3D approach is needed to overcome the limitations of the 2D approach, and to realistically simulate the interaction between the asperities in contact during shearing.The results of the analytical study showed that the matedness between the contact surfaces of natural, unfilled rock joints needs to be accounted for when predicting their peak shear strength. In this study, the matedness of the tested natural, unfilled rock joints was estimated based on measurements of the aperture between their contact surfaces. The relationship between matedness and joint surface aperture was integrated in a further developed peak shear strength criterion. Furthermore, the performed investigations on two large-size rock joint samples showed that their peak shear strength can be reasonably well predicted based on information from several small-size samples, such as drill cores. In this work, the drill cores were simulated based on the scanning measurements of the joint surfaces at large size. The measured 3D roughness and aperture in each simulated drill core was used to predict their respective peak shear strength by applying the further developed peak shear strength criterion. Each simulated drill core was considered as an independent component of a parallel system. The peak shear strength of the large-size samples was predicted based on the mean value of the predicted peak shear strength of the small-size samples, including the statistical uncertainty due to the number of small-size samples used in the prediction. The main benefit of this approach is that it may enable prediction of the peak shear strength of large natural, unfilled rock joints under conditions of difficult access, such as a sub-horizontal rock joint under a concrete dam. The developed methodology has only been tested on two large-size samples and further research is necessary to verify its applicability.
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| 36. |
- Ríos Bayona, Francisco, et al.
(författare)
-
Prediction of Peak Shear Strength of Natural, Unfilled Rock Joints Accounting for Matedness Based on Measured Aperture
- 2021
-
Ingår i: Rock Mechanics and Rock Engineering. - : Springer. - 0723-2632 .- 1434-453X. ; 54:3, s. 1533-1550
-
Tidskriftsartikel (refereegranskat)abstract
- The mechanical behaviour of natural, unfilled rock joints is influenced by the interaction between surface roughness and matedness of the contact surfaces. In the field, natural rock joints normally exhibit a mismatch between the contact surfaces, mainly due to different geological processes such as weathering or deformations. Various attempts have been made to estimate how matedness of rock joints influences their peak shear strength. However, the proposed methodologies imply certain difficulties since they are intended to estimate the matedness of rock joints based mainly on visual inspection, and by relating an initial shear displacement to the length of the analysed sample or by relating the opening of saw-tooth and two-dimensional joint profiles with the degree of interlocking. Therefore, a tested peak shear strength criterion for natural, unfilled rock joints that realistically accounts for the influence of matedness on their peak shear strength is still lacking. This paper presents a methodology where objective measurements of the average aperture of natural, unfilled rock joints are used to estimate their matedness as a step in the prediction of the peak shear strength. This measured average aperture is based on high-resolution optical scanning of the surface roughness. The proposed relationship between measured average aperture and matedness of natural rock joints has been included in a further developed peak shear strength criterion. The verification against ten natural rock joint samples of coarse-grained granite showed that the revised criterion can predict the peak shear strength considering rock joint matedness.
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| 37. |
- Ríos-Bayona, Francisco, et al.
(författare)
-
SKJUVHÅLLFASTHET HOS NATURLIGA BERGSSPRICKOR – MOT EN METODIK FÖR BESTÄMNING BASERAT PÅ DATA I FÄLT
- 2024
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Att kunna prediktera en bergssprickas skjuvhållfasthet är svårt i de fall där hela sprickytan inte är tillgänglig, såsom exempelvis en bergsspricka belägen under en befintlig betongdamm eller i projekteringsskedet för en tunnel. Denna artikel presenterar en metodik som undersöker möjligheten att utnyttja information från uppmätt sprickvidd och sprickans råhet i 3D med optisk scanning i mindre storlek, såsom borrkärnor, för att därefter prediktera skjuvhållfastheten i större skalor. Den framtagna metodiken har verifierats med två storskaliga skjuvförsök genomförda i laboratorium med konstant normallast med sprickprover tagna vid Krångede kraftstation. Den främsta nyttan med denna metodik är att den kan utgöra en möjlig väg framåt för att prediktera skjuvhållfastheten för bergssprickor i fall där sprickytan inte är helt tillgänglig.
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| 38. |
- Ríos Bayona, Francisco, et al.
(författare)
-
The importance of accounting for matedness when predicting the peak shear strength of rock joints
- 2021
-
Ingår i: IOP Conference Series. - : IOP Publishing. ; , s. 012017-
-
Konferensbidrag (refereegranskat)abstract
- The contribution from both surface roughness and matedness in the peak shear strength of rock joints is not yet well understood. To be able to account for the influence of matedness on the peak shear strength of rock joints, both surface roughness and aperture need to be considered. Technical developments over the past few decades have shown that both surface roughness and aperture can be accurately measured using optical scanning. This technique has been utilized to account for surface roughness parameters in various shear strength criteria that assume a perfect match between joint surfaces. This paper investigates and compares the capabilities of two shear strength criteria to predict the peak shear strength of rock joints with different matedness. The analysis performed shows that both approaches have their strengths and limitations. For instance, accounting for the matedness of unmated rock joints based on their surface aperture gives better predictions of the peak shear strength. On the other hand, accounting for shearing failure mode becomes relevant at high normal loads. A possible way forward to reduce the limitations of these criteria could be to combine their strengths.
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| 39. |
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| 40. |
- Ríos Bayona, Francisco, et al.
(författare)
-
Using PFC2D to simulate the shear behaviour of joints in hard crystalline rock
- 2022
-
Ingår i: Bulletin of Engineering Geology and the Environment. - : Springer Nature. - 1435-9529 .- 1435-9537. ; 81:9
-
Tidskriftsartikel (refereegranskat)abstract
- Significant uncertainties remain regarding the assessment of the peak shear strength of rock joints. In recent years, Particle Flow Code (PFC) has been used to simulate shear tests of rock joints. Although previous studies showed PFC's capability to simulate rock joint shear behaviour, it is uncertain how different parameters in PFC should be combined to realistically capture roughness and strength of asperities in contact of actual rock joints. Under low normal stresses, the shear behaviour of well-mated hard crystalline joints is governed by the interaction between asperities of some tenths of a millimetre. This paper investigates the capability of PFC2D to realistically simulate the peak shear strength of hard crystalline rock joints under different constant normal stress magnitudes. The simulated two-dimensional profiles were selected from the digitised joint surface obtained with optical scanning measurements. To realistically capture surface roughness and asperity strength in PFC2D, different values of joint segment length, particle resolution per segment, and bond strength between particles were studied and calibrated while taking into account the laboratory observations. The results of the numerical simulations in the PFC2D environment show that the simulated peak shear strength using the profile containing the steepest asperity is in good agreement with that measured in the laboratory. The joint profile needs to be represented by both a magnitude of segment length that captures the grain size, and at least two particles per segment. The bond strength calibration needs to account for both asperity size and the number of particles in contact during shearing.
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| 41. |
- Saceanu, M. Cristina, et al.
(författare)
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Fracture growth leading to mechanical spalling around deposition boreholes of an underground nuclear waste repository
- 2022
-
Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier BV. - 1365-1609 .- 1873-4545. ; 152, s. 105038-
-
Tidskriftsartikel (refereegranskat)abstract
- This study presents a three-dimensional numerical analysis of multiple fracture growth leading to spalling around nuclear waste deposition boreholes. Mechanical spalling due to stress amplification after drilling is simulated using a finite element-based fracture growth simulator. Fractures initiate in tension based on a damage criterion and grow by evaluating stress intensity factors at each fracture tip. Tip propagation is multi modal, resulting in final fracture patterns that are representative of both tensile and shear failure. Their geometries are represented by smooth parametric surfaces, which evolve during growth using lofting. The corresponding surface and volumetric meshes are updated at every growth step to accommodate the evolving fracture geometries. The numerical model is validated by comparing simulated fracture patterns against those observed in the AECL Underground Rock Laboratory Mine-By Experiment. It is subsequently calibrated to simulate fracture initiation and growth around boreholes drilled in the Forsmark granodiorite, subjected to a far-field anisotropic triaxial stress that corresponds to the in situ stress model from the Swedish Forsmark site. The deposition tunnel is implicitly simulated by attaching the deposition borehole to a free domain boundary.Several geomechanical cases are investigated, in which fracture growth is numerically evaluated as a function of in situ stress state, tunnel orientation, borehole geometry, total number of boreholes and borehole spacing. Numerical results show that spalling occurs in all cases, given the underground conditions at Forsmark, with borehole geometry, spacing and stresses affecting the extent of fracture nucleation and growth patterns.The uncertainty in underground stress conditions is evaluated through varying stress magnitudes and orientations relative to the tunnel floor. Whereas tunnel orientation influences the relative locations where fractures initiate with respect to the tunnel floor, fracture growth and its final extent is determined by the relative magnitudes of the in situ stresses. Higher stress differential causes higher spalling depths, but in all cases, failure extent is localised to the borehole vicinity, not exceeding one borehole radius. The cylindrical borehole is modified at the top to provide an access ramp for the spent fuel canisters and fracture growth around several deposition boreholes is simulated for borehole tops having cylindrical, conical, and wedge shapes. The enlargement of the borehole top induces higher stress concentrations at the borehole-tunnel junction, increasing the severity of spalling closer to the tunnel floor. Massive failure occurs when a multiple borehole model is considered and the inter-borehole distance is small enough that adjacent "spalled"areas interact. At Forsmark, through-going fractures are predicted to develop when the borehole spacing is less than 4 m. The effect of spalling on the structural integrity of the deposition boreholes is illustrated for each test case and quantified in terms of maximum spalling depth, spalling width and total fractured surface area.
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| 42. |
- Saceanu, M. C., et al.
(författare)
-
Numerical modelling of spalling around a nuclear waste storage deposition borehole using a fracture mechanics approach
- 2020
-
Ingår i: 54th U.S. Rock Mechanics/Geomechanics Symposium. - : American Rock Mechanics Association (ARMA).
-
Konferensbidrag (refereegranskat)abstract
- Fracture growth leading to mechanical spalling around a deposition borehole for disused nuclear fuel waste is modelled numerically. Simulations are conducted using a finite-element-based discrete fracture growth simulator, which computes deformation in the system based on the mechanical properties of the rock. Fractures are grown by computing stress intensity factors at each fracture tip, and the mesh is re-generated to accommodate the changing fracture geometries at every growth step. Several numerical models are created to explore the effect of boundary conditions on the initiation and development of spalling fractures at Forsmark, where the Swedish repository for nuclear waste is planned to be constructed. It is shown that the reported uncertainty in the principal stress magnitudes and orientations will affect the predicted fracture nucleation and growth patterns, and implicitly the final repository design. The potential effect of spalling on the structural integrity of the deposition borehole is illustrated for each stress scenario.
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| 43. |
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| 44. |
- Stigsson, Martin, et al.
(författare)
-
A Novel Conceptual Approach to Objectively Determine JRC Using Fractal Dimension and Asperity Distribution of Mapped Fracture Traces
- 2019
-
Ingår i: Rock Mechanics and Rock Engineering. - : Springer. - 0723-2632 .- 1434-453X. ; 52:4, s. 1041-1054
-
Tidskriftsartikel (refereegranskat)abstract
- The understanding of fractures in hard rock is important for topics such as geomechanics, rock mechanics and groundwater flow and solute transport. One key aspect is the roughness of the fracture, often described as the joint roughness coefficient, JRC. JRC is often subjectively interpreted by one geologist comparing a fracture trace with different type traces. It has been shown that several geologists are needed to get reliable interpretations of JRC. There are numerous attempts in the literature to develop objective methods to estimate JRC from digital traces. Some methods are not applicable to fractures, which give arbitrary results while other methods are sensitive to the resolution of the digitalisation and hence need a new relationship for each resolution. Another way of describing the roughness is by the two parameters fractal dimension and magnitude distribution of the asperities. These parameters can be objectively inferred using algorithms and act as input for a model to estimate JRC. Using several evaluation methods, the uncertainty can be decreased and, hence, more robust results achieved. A multilinear model is developed, JRC = − 4.3 + 54.6σδh(1mm) + 4.3H, that estimates JRC, of the classic ten type curves by Barton and Choubey, with standard deviation ± 1 unit. Despite the simplicity of the model it explains 96.5% of the variance in JRC. The developed model is benchmarked against an ensemble of geologists, using nine synthetic fracture traces. The median difference of JRC is 0.2 units and the model shows 40% smaller spread compared to the geologists.
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| 45. |
- Zou, Liangchao, 1987-, et al.
(författare)
-
Impact of Normal Stress Caused Closure on Fluid Flow and Solute Retention in Rock Fractures
- 2018
-
Konferensbidrag (refereegranskat)abstract
- Modeling of coupled hydro-mechanical and chemical (HMC) processes in fractured rocks is an important topic for many geoengineering projects. Over the past decades, many efforts have been devoted to study the flow and transport in single fractures with consideration of mechanical effects. It is generally known that the mechanical effects, i.e. normal and shear deformation, significantly affect fluid flow and solute transport processes in rough-walled rock fractures since the deformation may largely alter the structure of fracture apertures that directly controls transmissivity. Due to complicated physical processes combined with complexity of geometry structures, many issues remain open questions, such as fracture surface roughness characterization, deformation dependence of transmissivity and advective transport in natural rock fractures. In this work, we attempt to investigate the impact of stress caused closure on fluid flow and solute advective transport in a rough-walled fracture through numerical modeling. A rough-walled fracture model is created based on a laser-scanned rock surface. The Bandis’s model is used to describe the fracture closure subject to normal stress. The flow is modeled by solving Reynolds equation and the advective transport is simulated through Lagrangian particle tracking. The results show that the normal stress caused fracture closure creates asperity contacts and reduces the mean aperture, which significantly reduces transmissivity, and affects the travel time and transport resistance. With increases of normal stress, the specific surface area reduces nonlinearly due to the nonlinear closure. In practice, especially for important hydrogeological projects, e.g. nuclear waste disposal, it is important to consider the coupled HMC processes in design and risk assessment.
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| 46. |
- Zou, Liangchao, 1987-, et al.
(författare)
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Impact of shear displacement on advective transport in a laboratory-scale fracture
- 2022
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Ingår i: Geomechanics for Energy and the Environment. - : Elsevier Ltd. - 2352-3808. ; 31
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Tidskriftsartikel (refereegranskat)abstract
- The impact of shear displacement under different mechanical boundary conditions on fluid flow and advective transport in a single fracture at the laboratory scale is demonstrated in the present study. The shear-induced changes of fracture aperture structures are determined by using the measured normal displacements and digitalized fracture surfaces from laboratory shear tests. Five shear tests on concrete replicas of the same fracture under different mechanical boundary conditions, including constant normal loading (CNL) and constant normal stiffness (CNS), are conducted to analyse the influence of mechanical boundary conditions on the shear-flow-transport processes. Fluid flow in the fracture with different shear displacements are simulated by solving the Reynolds equation. The Lagrangian particle tracking method is applied to model the advective transport in the fracture after shearing. The results generally show that the shear displacements and the normal loading conditions can significantly affect flow patterns and advective travel time distributions in the fracture. For mated fractures, the flow and transport will be enhanced by the increasing shear displacement because of shear dilation. For cases with the same shear displacement, the median advective travel time increases with the increasing boundary normal stiffness. The median advective travel time under the CNS boundary condition is generally longer than that under the CNL boundary condition. The results from this study can help to improve our understanding of stress-dependent solute transport processes in natural rock fractures. © 2021 The Author(s)
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