Instabilities Modeling in Geomechanics

Instabilities Modeling in Geomechanics
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Instabilities Modeling in Geomechanics describes complex mechanisms which are frequently met in earthquake nucleation, geothermal energy production, nuclear waste disposal and CO2 sequestration. These mechanisms involve systems of non-linear differential equations that express the evolution of the geosystem (e.g. strain localization, temperature runaway, pore pressure build-up, etc.) at different length and time scales. In order to study the evolution of a system and possible instabilities, it is essential to know the mathematical properties of the governing equations. Therefore, questions of the existence, uniqueness and stability of solutions naturally arise. This book particularly explores bifurcation theory and stability analysis, which are robust and rigorous mathematical tools that allow us to study the behavior of complex geosystems, without even explicitly solving the governing equations. The contents are organized into 10 chapters which illustrate the application of these methods in various fields of geomechanics.

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Jean Sulem. Instabilities Modeling in Geomechanics

Table of Contents

List of Tables

List of Illustrations

Guide

Pages

Instabilities Modeling in Geomechanics

Introduction

1. Multiphysics Role in Instabilities in Geomaterials: a Review

1.1. Introduction

1.2. General remarks

1.3. Solid phase material criteria

1.4. Material sample stability: experimental

1.5. Boundary value problems: uniqueness and stability at the field scale. 1.5.1. Landslides

1.5.2. Thermal pressurization problem

1.5.3. Localization during drying of geomaterials

1.6. Conclusion

1.7. References

2. Fundamentals of Bifurcation Theory and Stability Analysis

2.1. Introduction

2.2. Bifurcation and stability of dynamical systems

2.2.1. Definition of stability

2.2.2. Linear systems of ODEs

2.2.3. Nonlinear systems of ODEs

2.2.4. An example of LSA

2.3. Stability of two-dimensional linear dynamical systems

2.3.1. Classification of fixed points

2.3.2. Love mechanics: Romeo and Juliet

2.4. Common types of bifurcations

2.4.1. Saddle-node bifurcation

2.4.2. Transcritical bifurcation

2.4.3. Supercritical and subcritical pitchfork bifurcation

2.4.4. From one to two dimensions – limit cycles

2.4.5. Bifurcations in two dimensions – supercritical and subcritical Hopf bifurcation

2.4.6. Mathematical bifurcations in PDEs

2.5. From ODEs to PDEs

2.5.1. Deformation bands and the acoustic tensor

2.5.2. Deformation bands as an instability problem

2.6. Summary

2.7. Appendix

2.8. References

3. Material Instability and Strain Localization Analysis

3.1. Introduction

3.2. Shear band model

3.2.1. Strain localization criterion

3.2.2. Strain localization, loss of ellipticity and vanishing speed of acceleration waves

3.3. Shear band formation in element tests on rock. 3.3.1. Drucker–Prager model

3.3.2. Non-coaxial plasticity

3.3.3. Cataclastic shear banding

3.3.4. Postlocalization behavior

3.4. Strain localization in fluid-saturated porous media. 3.4.1. Strain localization criterion in fluid-saturated porous media

3.4.2. Stability analysis of undrained shear on a saturated layer

3.5. Conclusion

3.6. References

4. Experimental Investigation of the Emergence of Strain Localization in Geomaterials

4.1. Introduction

4.2. Methods

4.2.1. Digital image correlation

4.2.2. X-ray computed tomography

4.2.3. Experimental devices for in situ full-field measurements. 4.2.3.1. A plane strain apparatus for soils

4.2.3.2. A true triaxial cell for rocks

4.2.3.3. An X-ray transparent triaxial cell

4.3. Selected materials. 4.3.1. Hostun sand

4.3.2. Caicos ooids sand

4.3.3. Vosges sandstone

4.3.4. Callovo–Oxfordian clayey rock

4.4. Strain localization in sands

4.4.1. Plane strain compression by FRS

4.4.2. Triaxial compression by X-ray CT and DIC

4.4.3. Triaxial compression by X-ray CT, the critical void ratio

4.5. Strain localization in porous rocks

4.5.1. Strain localization in Vosges sandstone

4.5.2. Strain localization in a clayey rock

4.6. Conclusion

4.7. References

5. Numerical Modeling of Strain Localization

5.1. Introduction

5.2. Cosserat continuum. 5.2.1. Governing equations

5.2.2. Finite element formulation of Cosserat model

5.2.3. Material parameters

5.2.4. Failure in thick-walled cylinder test

5.2.5. Stability analysis of elliptical shape perforations

5.3. Gradient elastoplasticity. 5.3.1. Governing equations

5.3.2. Finite element formulation

5.3.3. Material model

5.3.4. Modeling of the biaxial test

5.3.5. Modeling cavity expansion

5.4. Conclusion

5.5. Acknowledgments

5.6. References

6. Numerical Modeling of Bifurcation: Applications to Borehole Stability, Multilayer Buckling and Rock Bursting

6.1. Introduction

6.2. Borehole stability

6.2.1. Primary loading path

6.2.2. Hole failure

6.2.3. Simulation of hollow cylinder experiments

6.3. Folding of elastic media as a bifurcation problem

6.3.1. Buckling of a layer under initial stress

6.3.2. Eigen-displacements and tractions at layer boundaries

6.3.3. Buckling of a layer system – the transfer matrix technique

6.3.4. Buckling of layered half-space

6.4. Axial splitting and spalling

6.4.1. Buckling of a half-space with surface parallel cracks

6.5. Conclusion

6.6. Acknowledgments

6.7. References

7. Numerical Modeling of Multiphysics Couplings and Strain Localization

7.1. Introduction

7.2. Experimental evidences of strain localization

7.3. Regularization methods

7.3.1. Enrichment of the constitutive law

7.3.2. Enrichment of the kinematics

7.4. Coupled local second gradient model for microstructure saturated media

7.4.1. Balance equations for microstructure poromechanics. 7.4.1.1. Local second gradient model for a monophasic medium

7.4.1.2. Second gradient constitutive equation

7.4.1.3. Coupled local second gradient model

7.4.2. Coupled finite element formulation. 7.4.2.1. Numerical implementation

7.4.2.2. Linearization of the field equations

7.4.2.3. Spatial discretization

7.4.2.4. Global solution

7.4.3. Two-dimensional specimen under compression

7.4.3.1. Classical medium

7.4.3.2. Microstructure enhanced medium

7.4.3.3. Non-uniqueness of the solution

7.4.3.4. Bifurcation criterion for the second gradient model

7.5. Coupled local second gradient model for an unsaturated medium

7.5.1. Partial saturation conditions

7.5.2. Anisotropy of the intrinsic permeability

7.5.3. Compressibility of the solid grains

7.6. Modeling of a gallery excavation

7.6.1. Numerical model

7.6.2. Influence of stress and permeability anisotropies

7.6.3. Influence of second gradient boundary condition

7.6.4. Influence of Biot’s coefficient

7.6.5. Influence of gallery ventilation

7.7. Conclusion

7.8. References

8. Multiphysics Couplings and Strain Localization in Geomaterials

8.1. Introduction

8.2. Thermo-chemo-chemical couplings and stability of shear zones. 8.2.1. Problem statement

8.2.2. Stability of adiabatic undrained shear

8.2.3. Chemical weakening and earthquake nucleation

8.3. Dissolution weakening and compaction banding. 8.3.1. Multiscale modeling of strong chemo-poro-mechanical coupling

8.3.2. Compaction banding in oedometric compression

8.4. Conclusion

8.5. References

9. On the Thermo-poro-mechanics of Chemically Active Faults

9.1. Introduction

9.2. Time-independent formation of shear zones from solid mechanics

9.2.1. Shear zone thickness at boundary temperature conditions

9.2.2. Shear zone thickness at elevated temperature

9.3. Time-dependent evolution of shear zones

9.3.1. Energy considerations

9.3.2. The Taylor–Quinney coefficient

9.3.3. Chemical reactions

9.4. Postfailure evolution of a shear zone

9.4.1. Analysis of the system’s response

9.4.2. Time scales of the system

9.5. Comparison to field observations

9.6. Application to ETS sequences

9.6.1. Regular sequences – Cascadia ETS sequence

9.7. Discussion

9.8. Appendix: poro-chemical model

9.9. References

10. Analysis of Instabilities in Faults

10.1. Introduction

10.2. Description of the model

10.2.1. Cosserat continuum theory

10.2.2. Constitutive equations for a Cosserat continuum

10.2.3. Mass balance equation

10.2.4. Energy balance equation

10.3. Bifurcation analysis. 10.3.1. LSA for a Cosserat continuum with THM couplings

10.3.2. Localization conditions for a fault zone

10.3.3. Shear band thickness evolution in a fault zone

10.4. Numerical analysis

10.4.1. Regularization of the mesh dependency

10.4.2. Response and shear band thickness of a fault gouge

10.5. Conclusion

10.6. Bibliography

List of Authors

Index. A, B

C

D

E

F, G

H, I

L

M

N, O, P

S

T, U

W, X

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Mechanics, Field Director – Gilles Pijaudier-Cabot

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Sensitivity of basic mechanical properties to chemical processes in the environment has been seen as a critical factor of the stability of slopes and coastal structures due to several processes. These include: cyclic changes in the salinity and acidity of pore water, heat effect due to frictional slip during a creep phase of slope instability, oxidation and dissolution of sandstone, dissolution of calcite, dissolution of silica in aging sediments and dissolution of gypsum in abandoned mines, to mention just a few examples.

The conclusion from the above considerations, in slightly more modern terms, is that a proper approach to a realistic evaluation of FOS would be to view the stability as a scenario composed of several processes coupled between them. These processes would be solid and fluid mechanical as well as geochemical, coupled through both balance equations (including exchanges) and constitutive (phenomenological) coupling, i.e. reactions versus porosity (permeability) versus elasticity or softening. The processes may be either simultaneous or sequential. Hence, the evaluation of the instability of such processes or the differential equations describing them should be approached accordingly, as instability either of the whole process or of any element of the sequence.

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