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Liao, H.J. (Department of Civil Engineering, Xian Jiaotong University) | Han , J. (Department of Civil Engineering, Xian Jiaotong University) | Sugiyam, M. (Department of Civil Engineering, Tokai University) | Akaishi, M. (Department of Civil Engineering, Tokai University)

Most of the available constitutional equations indicating strain softening effect are based on stress space, but it is found that the elastic-plastic theory based on strain space is superior to stress space on solving the problem associate with large strain and softening. In this paper, a diatomaceous soft rock and its constitutive model are studied. A series of triaxial tests have been carried out on this soft rock. The constitutive equation of consolidated undrained triaxial stress state expressed in strain space is derived to simulate the stress-strain relationship of the results. It indicates that the elastic-plastic model based on strain space is applicable to express the strain softening effect of soft rock.

(Equation in full paper)

2.2 Hardening function

The selection of parameter К in the hardening function is very important. К determines the expansion mode of load surface during loading.

Considering the material softening effect, the hardening function is established now. The yield function that satisfies isotropic hardening in the stress space can be expressed as

(Equation in full paper)

Considering the stress-strain relationship curve of some direct shear tests shown in Fig.1, the hardening parameter К increases from point A to B, and then decreases and approaches constant after it reaches its maximum at point B if the material yield function satisfies equation (14).

(Equation in full paper)

(Table in full paper)

ISRM-SINOROCK-2009-088

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

curve, equation, function, plastic, pressure, relationship, rock, shear, space, strain, strain space, strength, stress, stress space, test, test result, triaxial test, yield

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)

Opening of fractures induced by shear dilation can be a significant source of fracture permeability change. In this study, the zones of fracture shear slip were examined through three-dimensional thermo-mechanical analysis of a nuclear waste repository model using the finite element method. Stress evolutions in selected locations revealed the main mechanisms of the generation of thermal stress important for fracture shear slip. The implications are that fractures of different orientations are vulnerable to shear slip at various locations throughout the lifespan of a geological repository. Stress paths obtained from the thermo-mechanical analysis were used as stress boundary conditions in order to investigate the effect of stress change on permeability. DFN-DEM (Discrete Fracture Network - Discrete Element Method) analysis showed that normal deformation dominated fracture closure/opening in four models and shear dilation dominated in the remaining two models. In the latter two models, modest permeability increases up to a factor of four were observed during thermal loading history. Permeability changes caused by shear dilation were not recovered after cooling of the repository, which was in contrast with the recovery of permeability changes for models in which normal fracture closure dominates.

The objectives of the current study are:

- to examine the contribution of thermal stress to the shear slip of fractures in mid- and far-field around the repository,
- to investigate the effect of the evolution of stress states on the permeability of repository settings, and
- to identify the shear slip potential through the entire lifespan of a KBS-3 type of a deep geological repository.

ISRM-SINOROCK-2009-185

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

Industry:

- Water & Waste Management > Solid Waste Management (1.00)
- Energy > Power Industry > Utilities > Nuclear (1.00)
- Energy > Oil & Gas > Upstream (1.00)

SPE Disciplines:

Iwata, N. (Chuden Consultants Co.,Ltd.) | Sasaki, T. (Suncoh Consultants Co.,Ltd.) | Yosida, J. (Suncoh Consultants Co.,Ltd.) | Sasaki, K. (Suncoh Consultants Co.,Ltd.) | Yoshinaka, R. (Saitama University)

This paper describes the validity of the Multiple Yield Model (MYM) based on the comparison between the prediction by MYM analysis and the measurement results of two cases history about the large vertical excavation about 30m in depth and 100m in width for nuclear power plants. MYM is a kind of finite element method constituted the mechanical properties of intact rock and discontinuity systems in rock mass, and can be analyzed the non-linearity of deformation under loading and unloading stress paths. For analyzing, the geometrical model of rock mass were determined from test adit and borehole observations about the discontinuity conditions such as orientation, spacing, persistence, and the physical parameters were determined by laboratory test using core specimens and also considering scale effect. As the results of MYM analysis, both of the deformation mode and displacement were well corresponded to the measurement and we have been confirmed that the actual behavior of discontinuous rocks can estimate by MYM in practical accuracy"

As well known, the mechanical properties of discontinuous rocks are strongly influenced by the geometrical distribution and its mechanical properties of discontinuities which those strength and deformation behavior are non-linear. However, the practical parameters for design are generally setting by performing

(Equation in full paper)

Thus it is assumed that the joints are distributed periodically and the volume of each joint set is ignored in comparison with the volume of intact rock. And it is assumed that the stresses of the intact rock and joints coincide. The stiffness matrix of joint set I in the local coordinate system is transformed for the global coordinate system using the coordinate transformation matrix by equation (2).

ISRM-SINOROCK-2009-102

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

The issue of prescribing the support requirements for stratified roofs of no major discontinuities otherthanhorizontalbeddingplanesisherebyapproached ideallyaswellaspragmatically. Firstly, the unreinforced case is analytically defined; the solution acquired by elementary beam theory for a fixed beam under distributed load is compared to an Airy stress function solution for a fixed beam under its own weight based on Timoshenko beam theory. Finally, a finite difference numerical solution is performed and verified. The model is then used to investigate the behavior of a two-member stratified roof with contact plane governed by the angle of friction and tightened inordertomobilizetheshearingreactionforceatthediscontinuity.Parametric analyses to investigatethepossibleeffectsofelasticparameterssuchasthe modulusofelasticityandthe Poisson’s ratio and also the interbedding friction angle and its effect on the response of the model conclude thissection. The last part involves the numerical implementation of a bolting support system providingthe previously determinedforce andthe prescription ofits characteristics,i.e. length,spacing,diameterandpretensionofbolts.The impact of applying concentrated compressive forces instead of the theoretical distributed support is also outlined.

(Equation in full paper)

The maximum stress components σ

The modeled plane strain beam was of L/t=12.5, zero Poisson’s ratio, 1 m length, density 2400 Mg/m

ISRM-SINOROCK-2009-123

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

This paper presents a new FEM approach and computer code for modeling fully coupled thermo-hydro- mechanical processes associated with underground nuclear waste repositories. The governing equations are based on the theory of mixtures applied to the multiphysics of porous media, considering solid-phase deformation, liquid-phase flow, gas flow, heat transport, thermally-induced water flow, phase change of water, and swelling deformation in buffer materials. For three-dimensional problems, three displacement components, water pressure, gas pressure, vapor pressure and porosity are chosen as the eight primary variables. The code was tested against a benchmark test that was performed in laboratory conditions on vertical cylindrical columns of compacted MX-80 bentonite by the French Commission of Atomic Energy from 2003 to 2004. The comparison with the benchmark tests shows good agreement between the numerical predictions and the measured data, thus providing a partial validation of our new code. Discussion of outstanding issues and conclusions are presented at the end of the paper.

In the field of geological disposal of radioactive wastes, many coupled THM numerical models have been developed (Olivella, 1994; Börgesson, 1996; Nguyen, 1996; Noorishad, 1996; Ohnishi, 1996; Thomas, 1996). Some of them have the capability to model liquid flow in unsaturated media through the use of the Richards equation (Richards, 1931), but the gas phase movement is usually ignored using the assumption of a constant and small gas pressure. Although some THM numerical models and codes can simulate two-phase (gas and liquid) fluid flow with two components (air and water) in partially saturated soil, coupled with heat transport and mechanical responses (Rutqvist, 2001), they usually neglect the advective flow of vapor and cannot consider the transfer of heat between the phases. By making the assumption of spontaneous thermo-dynamic equilibrium between the soil liquid and the water vapor, the vapor pressure becomes a variable that depends on suction and temperature, and the flux of vapor and liquid water can be modelled using a single equation (Khalili, 2001).

In order to increase the capability of modeling coupled THM processes in buffer and rock-buffer interfaces, a fully coupled THM numerical model, along with a computational FEM code ROLG, are developed and presented in this paper. In this model, the gas flow and the vapor flow are described by their mass conservation equations, and moisture movement and phase changes are considered. The developed numerical model and code are verified through a laboratory benchmark test as part of Work Package 4 of the EC sponsored project THERESA.

ISRM-SINOROCK-2009-113

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

This paper presents the results of a laboratory investigation of the thermomechanical behaviour of anisotropic rock. The tests were performed on natural (Tournemire shale) using special triaxial cell able to control and to go to high temperature. The range of temperatures that were investigated is from 20 C° to 250 C°. (20, 100, 150, 200 and 250 C°), and the range of confining pressures is from 0 MPa to 20MPa. (0, 5, 10, and 20 MPa). The influence of temperature on their mechanical behaviour was investigated for drained tests. Anisotropic elastic response and plastic deformation have been investigated. It seems that, the thermomechanical behaviour of the Tournemire shale is anisotropic and strongly depends on confining pressure and loading orientation at the applied temperature. Hydrostatic compressibility tests (in the perpendicular orientation

The group of sedimentary rocks, termed shales, represents a particular interest in oil industry. Experimental investigations are still necessary to have a better understanding of the thermomechanical behaviour of these materials. In the oil industry, the exploitation of heavy oil by the technical injection of vapour at high temperature, the rocks of the reservoir are subjected to coupled thermal and hydromechanical efforts. So it is necessary to study the thermo-mechanical behaviour of these materials subjected to variations of temperature in order to study the mechanical stability of the petroleum reservoirs.

The object of this study consists in carrying out new experimental study of the thermomechanical behaviour of the saturated stiff shales subjected to high temperatures (until 250 C°) and to compressive stresses. The main aim was to carry out extensive laboratory experiments on the thermomechanical behaviour of Tournemire shale. The emphasis is given to investigating thermal effect on the elastic response, plastic flow and failure behaviour of the shale. Experimental results presented here provide a data base for the development of thermoelastoplastic modelling and failure criteria.

ISRM-SINOROCK-2009-140

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

drift, element, excavation, failure, fallout, Figure, fracture, model, result, rock, rock mass, rock mass strength, shear, shear band, strain, strain value, strength, stress

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.91)

Instability analysis about saturated rocks is carried out to understand the initiation and evolution of rock landslides and rock-falling. The spatial development of a single shear band in saturated rocks for anti-plane shear deformation is mainly investigated here. It is shown that if the angle between the proportional loading path and the direction of the localization

The issue of the inception of localized shearing in rocks subjected to undrained deformation has been the object of both theoretical as well as experimental research. Theoretical contributions are related mainly to stability and bifurcation analysis of diffused and localized failure models (Rice, 1975). Typically, the stability problem is formulated by considering small perturbations in field variables ( e.g. displacement and pore pressure). Classical continuum approach leads, in this case, to the ordinary diffusion equation for the perturbation in pore pressure (Rice, 1975). The results indicated that the uniform response is often followed by the onset of a diffused, nonhomogeneous deformation model, after which distinct shear bands form. However, this problem is often discussed under inertia-free undrained conditions ( Rice, 1975; Pietruszczak et al 1993). Anti-shear is a commonly load bear by Rocks in the condition of earthquake, wind load etc.. Nevertheless, study on the failure under this type of load is few.

In viewpoint of above, we will investigate the spatial development of a single shear band for anti-plane shear-plane shear, perturbation analysis. deformation in this paper so as to obtain a comprehensive and precise picture of the instability phenomenon by using of the method of Douglas &Chen [1985].

(Equation in full paper)

ISRM-SINOROCK-2009-090

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.98)

The Punch-Through Shear with Confining Pressure (PTS/CP-) experiment has been proven to be a reliable testing method for the determination of Mode II fracture toughness. The Mode II fracture toughness, KIIC, is a measure of the resistance of a material to the propagation of a shear loaded fracture. Due to a simple but effective specimen geometry, the PTS/CP- experiment is the only available experiment that is able to apply a confining pressure independent from the shear load. In this contribution the influence of temperature on fracture toughness is presented and discussed. Several studies have dealt so far with the influence of temperature on Mode I (tensile) fracture toughness, but there is little to no data available on K

An alternative to the empirical continuum mechanics strength criteria are fracture mechanics based approaches. Linear fracture mechanics in general assumes preexisting discontinuities in a material that act as stress concentrators. The magnitude of the stress concentration governs the brittle fracture process. If pre-existing cracks or flaws are propagated by the stresses and coalesce to form larger discontinuities, the structures may loose integrity and fail. The mechanistic criteria try to mirror the physical origin of the processes and are therefore more exact.

Based on the principles of fracture mechanics, it is possible to not only asses the stability and safety of underground constructions, like caverns, tunnels or boreholes, but also to simulate – based on physical principles – the development of fractures in the vicinity of such openings. From the simulations the geometry of fracture patterns might be derived and used for different aspects, like fluid flow simulations. Some software packages are already available, e.g. Fracod2D, or under development.

Linear fracture mechanics provides the tools to estimate the stress and displacement fields around the tip of a discontinuity. Cracks or fractures are usually subdivided into three basic types, namely Mode I, Mode II and Mode III, based on the crack surface displacement (Lawn 1993; Fig. 1A). In Mode I, the tensile mode, the crack tip is subject to displacements perpendicular to the crack plane. In Mode II the crack faces move relatively to each other in the crack plane.

ISRM-SINOROCK-2009-041

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

Weng, M.C. (Department of Civil and Environmental Engineering, National Kaohsiung University) | Liao, C.Y. (Department of Civil Engineering, National Taiwan University) | Jeng, F.S. (Department of Civil Engineering, National Taiwan University)

Weak sandstones possess deformational behaviors different from hard rocks; these phenomena, including shear dilation and degradation of deformational moduli, are much more significant. Therefore, a model capable of simulating major deformational characteristics of weak sandstones is essentially needed for engineering purposes. An innovative constitutive model is accordingly proposed. The proposed model was formulated based on the linear elastic model, and it accounts for the variations of moduli K and G through different loading conditions. In addition, an anisotropic factor

The proposed model was then incorporated into a finite element program and was used to analyze a squeezing tunnel case. Overall, this model can describe the deformation behavior for weak sandstones, especially on the significant shear dilation prior to the failure state. As a result, the proposed model shows the versatility in its applicability.

The western region of Taiwan is most populous and accompanied with active constructions of the transportation infrastructure. Many tunnel constructions currently in progress or in planning are, or to be, constructed in sedimentary strata of Tertiary Period. Due to this relatively young rock-geneses period, weathering and other factors, these sedimentary strata are mostly weak rocks. In the past, these weak rocks have caused several engineering difficulties such as squeezing of the tunnel under construction due to shear-induced deformations [1]. It was found that some typical weak rocks exhibit problematic characteristics such as substantial wet weakening, shear-dilation as well as creep deformation. Such behavior is often much less significant in hard rocks. In order to realize the deformation characteristics of weak sandstone, a series of laboratory tests including pure-shear triaxial tests and creep tests were performed by Jeng et al. [1], Weng et al. [2] and Tsai et al. [3]. According to the results of these researches, weak rocks typically exhibit the following behaviors:

- In the hydrostatic loading stage, the total strain possesses nonlinear behavior, which indicates that bulk modulus would increase as hydrostatic stress increases.
- In the shear loading stage, the initial shear modulus increases with increasing hydrostatic pressure applied.
- The volumetric strain induced by shear is initially contractive, and then gradually transits to be dilative upon increases of shear stresses.

Since squeezing phenomenon in tunnel constructions is inherently related to the aforementioned shear-induced deformation, proper assessments for the rock mass prone to such behavior is of interest in engineering practice. Therefore, it is needed to develop a constitutive model that can properly describe these deformational characteristics.

Incorporating the characteristics of deformation behavior of sandstone, especially for shear contraction/dilation, the compliance matrix in the principal stress coordinate is proposed accordingly based on Weng et al. [4] and Graham and Houlsby [5] as:

(Equation in full paper)

where K and G are tangent bulk modulus and shear modulus; β is the anisotropic factor;δσ

ISRM-SINOROCK-2009-079

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.56)

The study of rock fracture under triaxial conditions captures the essential features. Conventional triaxialcompressivetestsunderconfiningpressurebeenusedasasimpleandeffectivewayto investigate theprogressivefailureprocessinrockmaterial.However, intermediate principal stresscan’tbeconsideredinconventionaltriaxialcompressiontests.The paper presents a numerical model to analyze three-dimensional rock failure process and fracture mechanism under multi-aixial stressloading.Six conventional triaxial compression tests are undertaken under different confining pressure to investigate the failure process of rock specimens and the confining pressureeffectisdiscussedintermsofstress-straincurves,fracturepatternsaswellaspeak strength.Thenanothersixtruetriaxialcompressiontestsareundertakenundertruetriaxial loadingstresstostudytheintermediateprincipalstresseffect.In conventional triaxial compression tests,the orientation ofthe shearfracture planes to the loading axis increases with the confining pressure and the peak strength increases with the increasing of confining pressure. Itcanbefoundthatthesignificanteffectofintermediateprincipalstressonthepeakhadtwo zonesintruetriaxial compressiontests, andthis phenomenon can be explained withthe Twins Shear Criterion.The numerical results also reveal that intermediate principal stress also influencesthefracturepatternsignificantly, whichhasbeenignoredby manyresearchers.The presented numerical model is approved to be a useful tool to investigate rock failure behaviors.

On one hand, real fracture processes are 3D and not 2D, and the problems encountered in rock mechanics and engineering arealmostallthreedimensionaltosome extent. Rock and rock masses in nature, which consist of the earth’scrust,areunderthree-dimensionalstress condition,andfractureformationinrocks,including crack propagation,interaction and coalescence all shows three-dimensional features. As found by A. Caballero et al (2004), the complexity of crack patterns which emerge fromanevensimplegeometry,withprofusespatial bridging and branching until the final failure mechanisms are defined.

On theotherhand,thetwo-dimensionalanalysis

ISRM-SINOROCK-2009-087

ISRM International Symposium on Rock Mechanics - SINOROCK 2009

SPE Disciplines: