Erosion in Geomechanics Applied to Dams and Levees

Foreword  xiii Jean-Louis BRIAUD Introduction  xv Stéphane BONELLI Chapter 1. State of The Art on the Likelihood of Internal Erosion of Dams and Levees by Means of Testing 1 Robin FELL and Jean-Jacques FRY 1.1. An overview of the internal erosion process as it affects dams and levees 1 1.1.1. A description of the overall process 1 1.1.2. The four mechanisms of initiation and progression of internal erosion 2 1.1.3. Concentrated leak erosion 6 1.1.4. Backward erosion 7 1.1.5. Contact erosion 8 1.1.6. Suffusion 9 1.2. Concentrated leak erosion 11 1.2.1. Situations where concentrated leaks may occur 11 1.2.2. Estimation of crack width and depthc of cracking 23 1.2.3. The mechanics of erosion in concentrated leaks 23 1.2.4. Commentary on the state of the art and the role of laboratory testing in assessing concentrated leak erosion 35 1.3. Backward erosion piping 37 1.3.1. The mechanics of backward erosion piping 37 1.3.2. Soils that are subject to backward erosion piping 40 1.3.3. Methods available for assessing whether backward erosion piping will initiate and progress 43 1.3.4. Some field observations 51 1.3.5. Global backward erosion 52 1.3.6. Commentary on the state of the art and the role of laboratory testing in assessing backward erosion piping and global backward erosion 53 1.4. Suffusion 57 1.4.1. The mechanics of suffusion 57 1.4.2. Methods of identifying soils that are internally unstable and potentially subject to suffusion (geometric criterion) 58 1.4.3. Hydraulic conditions where soils are internally unstable and potentially subject to suffusion 69 1.4.4. Commentary on the state of the art and the role of laboratory testing in assessing suffusion 71 1.5. Contact erosion 74 1.5.1. The mechanics of contact erosion 74 1.5.2. Methods available to assess the likelihood of contact erosion 76 1.5.3. Contact erosion or “scour” at the interface between open joints in rock foundations and the core of dams 82 1.5.4. Commentary on the state of the art and the role of laboratory testing in assessing contact erosion 83 1.6. Bibliography 85 Chapter 2. Contact Erosion 101 Pierre PHILIPPE, Rémi BEGUIN and Yves-Henri FAURE 2.1. Introduction 101 2.2. General presentation 103 2.2.1. Typical conditions of occurrence 103 2.2.2. Specific nature of CE 107 2.3. At sample scale: quantification of the CE threshold and kinetics 110 2.3.1. Influence of geometry on the occurrence of CE 111 2.3.2. Direct configuration 112 2.3.3. Inverse configuration 129 2.3.4. Summary 133 2.4. At pore scale: local hydrodynamics of CE and statistical modeling 134 2.4.1. Experimental characterization of local hydrodynamics 135 2.4.2. Integration at macroscopic scale 143 2.4.3. Contribution made by the local scale study 158 2.5. At hydraulic structure scale: identification of failure scenarios by CE and scale effects 162 2.5.1. Reasons for a study at this scale 162 2.5.2. Description of the experimental rig and instrumentation 163 2.5.3. Test protocol and the results obtained 167 2.5.4. Proposed interpretation and description of the erosion process 172 2.5.5. Scale effect 176 2.5.6. Summary 179 2.6. Conclusion and outlook 179 2.6.1. Description of CE mechanisms 180 2.6.2. Impact on the safety of hydraulic structures 182 2.7. Bibliography 186 Chapter 3. Backward Erosion Piping 193 Vera VAN BEEK, Adam BEZUIJEN and Hans SELLMEIJER 3.1. Introduction 193 3.2. Phases leading to failure due to backward erosion 197 3.2.1. Seepage 198 3.2.2. Backward erosion – initiation and progression 199 3.2.3. Widening 204 3.2.4. Failure 205 3.3. Backward erosion in the laboratory – overview and setup 206 3.3.1. Overview of experimental research 207 3.3.2. Setup 208 3.4. Backward erosion piping in the laboratory – erosion mechanism 214 3.4.1. Single grain transport 215 3.4.2. Sand boiling phase 216 3.4.3. Regressive or equilibrium phase 217 3.4.4. Progressive phase 220 3.4.5. Which process will occur when? 221 3.5. Backward erosion in the laboratory – critical gradient 226 3.5.1. Initiation of backward erosion piping 226 3.5.2. Progression of backward erosion piping 234 3.5.3. Progression of pipes for vertical seepage paths 240 3.6. Analysis tools 245 3.6.1. Initiation of the pipe 246 3.6.2. Progression of the pipe 250 3.6.3. Progression for structures 256 3.6.4. Summary 258 3.7. From laboratory to field – challenges for the future 259 3.7.1. Scale effects 259 3.7.2. Heterogeneity 262 3.7.3. Uncertainties 263 3.8. Conclusion 264 3.9. Bibliography 264 Chapter 4. Concentrated Leak Erosion  271 Stéphane BONELLI, Robin FELL and Nadia BENAHMED 4.1. Introduction 271 4.2. Theoretical background 275 4.2.1. Assumptions 275 4.2.2. The model for pipe flow with erosion 276 4.2.3. The singular head loss factor 278 4.2.4. The momentum loss factor 279 4.2.5. Characteristic values 280 4.2.6. Closed-form solution in the case of a constant pressure drop 281 4.2.7. Closed-form solution in the case of a constant flow rate 282 4.3. The HET: testing procedure 283 4.3.1. The HET apparatus 283 4.3.2. Preparation of the specimen 285 4.3.3. Determination of the final hole diameter 288 4.4. The HET: method of interpretation 289 4.4.1. Determination of the pipe radius and the wall shear stress 289 4.4.2. Determination of the friction coefficient 291 4.4.3. Determination of the head loss coefficient 292 4.4.4. Determination of the parameters of erosion 294 4.4.5. Examples of results 294 4.4.6. Slaking at upstream or downstream faces of sample of HET 296 4.5. Mechanically based relations for time to failure and peak flow 299 4.5.1. A simplified approach 299 4.5.2. Onset of erosion in the pipe 301 4.5.3. Visual detection of the leak 302 4.5.4. Enlargement of the pipe 303 4.6. Dam and levee break modeling 307 4.6.1. Order of magnitude on case studies 307 4.6.2. A model for dam- and levee-break due to concentrated leak erosion 311 4.6.3. Application to the failure of a homogeneous moraine dam by piping 313 4.6.4. Model analysis 317 4.7. Modeling concentrated leak erosion statistically 319 4.7.1. The probabilistic approach 319 4.7.2. The probability density function of the stress ratio 321 4.7.3. Probabilistic description of erosion 323 4.7.4. Order of magnitude of the relative intensity of the shear stress fluctuations 325 4.7.5. Order of magnitude of the coefficient of variation of the soil critical stress 326 4.7.6. A stochastic erosion law for cohesive soils 327 4.8. Comments 329 4.8.1. Comment on the friction coefficient 329 4.8.2. Comment on the linearity of the erosion law 333 4.9. Bibliography 335 Chapter 5. Relationship between the Erosion Properties of Soils and Other Parameters 343 Robin FELL, Gregory HANSON, Gontran HERRIER, Didier MAROT and Tony WAHL 5.1. Introduction 343 5.2. Definitions of soil erosion properties and the relationships between them 344 5.3. Effects of test methods on soil erosion properties 346 5.3.1. Effect of testing methods on erosion rate 346 5.3.2. Effect of testing methods on critical shear stress (τc) 350 5.3.3. Correlation between critical shear stress and erosion rate index 353 5.4. Relationship to field performance 355 5.4.1. JET tests done in the laboratory and in the field 355 5.4.2. Assessment of rates of erosion from JET and large-scale laboratory tests 355 5.5. Effects of the type of soil 358 5.5.1. General trends 358 5.5.2. Relationship to soil classification 359 5.5.3. Effects of soil structure 360 5.6. Effects of compaction parameters 360 5.6.1. Relationship to compaction parameters 360 5.6.2. Relationship to degree of saturation after compaction 364 5.7. Effects of dispersivity and slaking 366 5.7.1. Effects of dispersivity on erosion rate and critical shear stress 366 5.7.2. Effects of slaking on erosion rate and critical shear stress 369 5.8. Modifications of soil erosion properties 371 5.8.1. Modification by lime 371 5.8.2. Modification by cement 376 5.9. Bibliography 376 List of Authors 383 Index 387