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Strength and deformability of fractured rocks
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents a systematic numerical modeling framework to simulate the stress-deformation and coupled stress-deformation-flow processes by performing uniaxial and biaxial compressive tests on fractured rock models with considering the effects of different loading conditions, different loading directions (anisotropy), and coupled hydro-mechanical processes for evaluating strength and deformability behavior of fractured rocks. By using code UDEC of discrete element method (DEM), a series of numerical experiments were conducted on discrete fracture network models (DFN) at an established representative elementary volume (REV), based on realistic geometrical and mechanical data of fracture systems from field mapping at Sellafield, UK. The results were used to estimate the equivalent Young’s modulus and Poisson’s ratio and to fit the Mohr-Coulomb and Hoek-Brown failure criteria, represented by equivalent material properties defining these two criteria.

The results demonstrate that strength and deformation parameters of fractured rocks are dependent on confining pressures, loading directions, water pressure, and mechanical and hydraulic boundary conditions. Fractured rocks behave nonlinearly, represented by their elasto-plastic behavior with a strain hardening trend. Fluid flow analysis in fractured rocks under hydro-mechanical loading conditions show an important impact of water pressure on the strength and deformability parameters of fractured rocks, due to the effective stress phenomenon, but the values of stress and strength reduction may or may not equal to the magnitude of water pressure, due to the influence of fracture system complexity. Stochastic analysis indicates that the strength and deformation properties of fractured rocks have ranges of values instead of fixed values, hence such analyses should be considered especially in cases where there is significant scatter in the rock and fracture parameters. These scientific achievements can improve our understanding of fractured rocks’ hydro-mechanical behavior and are useful for the design of large-scale in-situ experiments with large volumes of fractured rocks, considering coupled stress-deformation-flow processes in engineering practice. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xvi, 97 p.
Series
TRITA-LWR. PHD, ISSN 1650-8602 ; 2014:07
Keyword [en]
Fractured crystalline rocks, Numerical experiments, Discrete element methods (DEM), Discrete fracture network (DFN), Representative elementary volume (REV), Coupled hydro-mechanical processes, Anisotropy, Effective stress, Failure criteria, Stochastic realizations
National Category
Engineering and Technology
Research subject
Land and Water Resources Engineering
Identifiers
URN: urn:nbn:se:kth:diva-155719ISBN: 978-91-7595-324-3 (print)OAI: oai:DiVA.org:kth-155719DiVA: diva2:762222
Public defence
2014-11-25, F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20141111

Available from: 2014-11-11 Created: 2014-11-10 Last updated: 2014-11-11Bibliographically approved
List of papers
1. Numerical evaluation of strength and deformability of fractured rocks
Open this publication in new window or tab >>Numerical evaluation of strength and deformability of fractured rocks
2013 (English)In: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 5, no 6, 419-430 p.Article in journal (Refereed) Published
Abstract [en]

Knowledge of the strength and deformability of fractured rocks is important for design, construction andstability evaluation of slopes, foundations and underground excavations in civil and mining engineering.However, laboratory tests of intact rock samples cannot provide information about the strength anddeformation behaviors of fractured rock masses that include many fractures of varying sizes, orientationsand locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costlyand often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numericalpredicting using discrete element methods (DEM) is a suitable approach for such modeling because of theiradvantages of explicit representations of both fractures system geometry and their constitutive behaviorsof fractures, besides that of intact rock matrix. In this study, to generically determine the compressivestrength of fractured rock masses, a series of numerical experiments were performed on two-dimensionaldiscrete fracture network models based on the realistic geometrical and mechanical data of fracturesystems from field mapping. We used the UDEC code and a numerical servo-controlled program forcontrolling the progressive compressive loading process to avoid sudden violent failure of the models.The two loading conditions applied are similar to the standard laboratory testing for intact rock samplesin order to check possible differences caused by such loading conditions. Numerical results show thatthe strength of fractured rocks increases with the increasing confining pressure, and that deformationbehavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses andstrains obtained from these numerical experiments were used to fit the well-known Mohr-Coulomb (MC)and Hoek-Brown (H-B) failure criteria, represented by equivalent material properties defining thesetwo criteria. The results show that both criteria can provide fair estimates of the compressive strengthsfor all tested numerical models. Parameters of the elastic deformability of fractured models during elasticdeformation stages were also evaluated, and represented as equivalent Young’s modulus and Poisson’sratio as functions of lateral confining pressure. It is the first time that such systematic numerical predictingfor strength of fractured rocks was performed considering different loading conditions, with importantfindings for different behaviors of fractured rock masses, compared with testing intact rock samples undersimilar loading conditions.

Keyword
Strength, Deformability, Fractured rocks, Discrete element methods (DEM), Failure criteria
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-155710 (URN)10.1016/j.jrmge.2013.09.002 (DOI)2-s2.0-84901324491 (Scopus ID)
Note

QC 20141112

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically approved
2. Effects of loading conditions on strength and deformability of fractured rocks - A numerical study
Open this publication in new window or tab >>Effects of loading conditions on strength and deformability of fractured rocks - A numerical study
2014 (English)In: Rock Engineering and Rock Mechanics: Structures in and on Rock Masses - Proceedings of EUROCK 2014, ISRM European Regional Symposium, Taylor & Francis Group, 2014, 365-368 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents a systematic numerical study to evaluate the effects of two different loading conditions, namely the axial stress and axial velocity, on testing compressive strength and deformability properties of fractured rocks. The UDEC code was used to perform a series of numerical tests on two-dimensional fracture network (DFN) models, in the similar ways for the uniaxial and biaxial laboratory testing on intact rock samples. The obtained stresses and strains from these numerical experiments were used to estimate equivalent directional Young's modulus and fit the Mohr-Coulomb and Hoek-Brown failure criteria, represented by equivalent material properties defining these two criteria. The numerical results show that stress-strain behaviors changes by loading conditions with higher averaged axial stress under axial velocity condition than that under axial stress condition. Therefore, the effects of different loading conditions should be carefully considered for designing and interpretation of results for in-situ experiments with large volumes of fractured rocks.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2014
Keyword
Compressive strength, Deformation, Elastic moduli, Elasticity, Experiments, Fracture testing, Rock mechanics, Stress analysis, Structural design, Deformability properties, Equivalent material properties, Hoek-Brown failure, In-situ experiments, Laboratory testing, Numerical experiments, Numerical results, Stress-strain behaviors
National Category
Geology
Identifiers
urn:nbn:se:kth:diva-146696 (URN)10.1201/b16955-60 (DOI)000345985300056 ()2-s2.0-84901344637 (Scopus ID)978-1-138-00149-7 (ISBN)
Conference
2014 ISRM European Regional Symposium on Rock Engineering and Rock Mechanics: Structures in and on Rock Masses, EUROCK 2014, Vigo, Spain, 26 May 2014 through 28 May 2014
Note

QC 20150108

Available from: 2014-06-13 Created: 2014-06-13 Last updated: 2015-01-08Bibliographically approved
3. Anisotropy of strength and deformability of fractured rocks
Open this publication in new window or tab >>Anisotropy of strength and deformability of fractured rocks
2014 (English)In: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 6, no 1, 156-164 p.Article in journal (Refereed) Published
Abstract [en]

Anisotropy of the strength and deformation behaviors of fractured rock masses is a crucial issue fordesign and stability assessments of rock engineering structures, due mainly to the non-uniform and nonregulargeometries of the fracture systems. However, no adequate efforts have been made to study thisissue due to the current practical impossibility of laboratory tests with samples of large volumes containingmany fractures, and the difficulty for controlling reliable initial and boundary conditions forlarge-scale in situ tests. Therefore, a reliable numerical predicting approach for evaluating anisotropy offractured rock masses is needed. The objective of this study is to systematically investigate anisotropy ofstrength and deformability of fractured rocks, which has not been conducted in the past, using a numericalmodeling method. A series of realistic two-dimensional (2D) discrete fracture network (DFN)models were established based on site investigation data, which were then loaded in different directions,using the code UDEC of discrete element method (DEM), with changing confining pressures. Numericalresults show that strength envelopes and elastic deformability parameters of tested numerical modelsare significantly anisotropic, and vary with changing axial loading and confining pressures. The resultsindicate that for design and safety assessments of rock engineering projects, the directional variations ofstrength and deformability of the fractured rock mass concerned must be treated properly with respectto the directions of in situ stresses. Traditional practice for simply positioning axial orientation of tunnelsin association with principal stress directions only may not be adequate for safety requirements.Outstanding issues of the present study and suggestions for future study are also presented.

Keyword
Anisotropy, Strength criterion, Deformation behavior, Numerical experiments, Fractured rock mass, Discrete element method (DEM), Discrete fracture network (DFN)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-155712 (URN)10.1016/j.jrmge.2014.01.009 (DOI)2-s2.0-84925287460 (Scopus ID)
Note

QC 20141111

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically approved
4. Water pressure effects on strength and deformability of fractured rocks under low confining pressures
Open this publication in new window or tab >>Water pressure effects on strength and deformability of fractured rocks under low confining pressures
2015 (English)In: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 48, no 3, 971-985 p.Article in journal (Refereed) Published
Abstract [en]

The effect of groundwater on strength anddeformation behavior of fractured crystalline rocks is one ofthe important issues for design, performance and safetyassessments of surface and subsurface rock engineeringproblems. However, practical difficulties make the directin situ and laboratory measurements of these properties offractured rocks impossible at present, since effects of complexfracture system hidden inside the rock masses cannot beaccurately estimated. Therefore, numerical modeling needs tobe applied. The overall objective of this paper is to deepenour understanding on the validity of the effective stressconcept, and to evaluate the effects of water pressure onstrength and deformation parameters. The approach adopteduses discrete element methods to simulate the coupled stressdeformation-flow processes in a fractured rock mass withmodel dimensions at a representative elementary volume(REV) size and realistic representation of fracture systemgeometry. The obtained numerical results demonstrate thatwater pressure has significant influence on the strength, butwith minor effects on elastic deformation parameters, comparedwith significant influence by the lateral confiningpressure. Also, the classical effective stress concept to fracturedrock can be quite different with that applied in soilmechanics. Therefore, one should be cautious when applyingthe classical effective stress concept to fractured rock media.

Keyword
Coupled hydro-mechanical, Effective stress, Discrete element methods (DEM-DFN), UDEC, Failure criteria, Fractured crystalline rocks
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-155713 (URN)10.1007/s00603-014-0628-3 (DOI)000352905100006 ()2-s2.0-84928376984 (Scopus ID)
Note

Updated from E-publ to published. QC 20150630

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically approved
5. Stochastic analysis of strength and deformability of fracture rocks using multi-fracture system realizations
Open this publication in new window or tab >>Stochastic analysis of strength and deformability of fracture rocks using multi-fracture system realizations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, a systematic numerical framework is presented to predict stochastic variationsof strength and deformation parameters of fracture rocks, using multiple realizations ofstochastic discrete fracture network (DFN) models at established representative elementaryvolume (REV). Fifty 2D square geometrical models, which are generated using the MonteCarlo technique of the fracture system based on the data obtained from a real site, aregenerated for stochastic analysis of results of stress-deformation behaviors from a series of350 compressive numerical experiments, using the discrete element method (DEM). The Chi-Squared goodness-of-fit test was used to frequency and probability and cumulativedistribution functions (PDF-CDF) of the strength and deformability of fracture rocksdistributions. The results show that (i) the Young’s modulus and Poisson’s ratio during elasticdeformation stages have normal and lognormal distributions, respectively, (ii) both thefriction angle and cohesion derived from Mohr-Coulomb (M-C) strength criterion obeynormal distributions, (iii) the m and s parameters of Hoek-Brown (H-B) strength criterionhave lognormal distributions. The results of stochastic analysis show that it is a usefultechnique for evaluating random variations of strength and deformability parameters of thefractured rock, in cases where there is significant scatter in the rock and fracture parameters.

Keyword
Discrete Element Methods (DEM-DFN), Stress-Deformation Analysis, UDEC, Failure Criteria, Numerical Experiment, Stochastic Realization
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-155716 (URN)
Note

QS 2014

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2014-11-11Bibliographically approved

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