The Consolidation and Strength Behavior of Mechanically Compressed Fine-Grained Sediments

Transcription

1 The Consolidation and Strength Behavior of Mechanically Compressed Fine-Grained Sediments A Ph.D. Defense by Brendan Casey Thesis Supervisor: Dr. Jack Germaine Committee Chair: Prof. Herbert Einstein Committee Members: Dr. Richard Plumb, Prof. Peter Flemings, Prof. Brian Evans & Prof. Charles Ladd Friday, April 25 th /49

2 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 2 /49

3 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 3 /49

4 Motivation For soils and soft rock, shear strength is complex a function of: composition (w L ) effective stress (σ ) stress history (OCR) This work τ max = f mode of shear (b, α) temperature (T) strain rate (έ) water saturation (S w ) diagenesis 4 /49

5 Majority of previous studies have involved testing intact samples cannot isolate and quantify individual factors influencing behavior disturbance and cost, particularly for deep or offshore samples Resedimentation Technical necessity! Practical advantages Compares well with intact behavior Intact samples Resedimented samples Resedimented samples over wide stress range Best data for resedimented clay behavior from Abdulhadi (2009) tested RBBC for stresses from MPa in triaxial compression Very limited testing of resedimented soil over a wide stress range Bishop et al. (1975); tested London Clay at Imperial College Yassir (1989); tested mud volcano clay at UCL Nüesch (1991); tested unsaturated Opalinus Shale Berre (1992); tested a kaolinite Moum clay mixture at NGI William (2007); tested Bringelly Shale at University of Sydney 5 /49

6 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 6 /49

7 Resedimentation 1. Obtain core material 2. Breakdown into powder and blend 3. Mix dry powder and water into slurry 5. Pour slurry into a consolidometer 4. Vacuum the slurry Comparisons of resedimented vs. intact behavior: Berman 1993 (BBC) Mazzei 2008 (RGoM Ursa) Casey 2011 (BBC) House 2012 (BBC) Betts 2014 (RGoM Eugene Is.) 7 /49

8 Resedimentation 4. Load incrementally Different consolidometers used depending on testing needs Low stress triaxial: σ p = 0.1 MPa Medium stress triaxial: σ p = 2 MPa High stress triaxial: σ p = 10 MPa Time required for resedimentation strongly dependent on soil type (c v ) 5. Swell to OCR = 5 6. Extrude and trim test specimen 8 /49

9 What am I dealing with? Contributing researchers: Grennan (2010) Abdulhadi (2009), Sheahan (1991) Jones (2010) Kontopoulos (2012) Betts (2014), Fahy (2014) 9 /49

10 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 10 /49

11 Permeability 1E-15 1 Vertical Permeability, k (m 2 ) 1E-16 1E-17 1E-18 R. Presumpscot Clay (RPC) 1E-19 R. Ursa Clay (RGoM-Ursa) increasing w L R. London Clay (RLC) 1E Porosity, n 1E-1 1E-2 1E-3 1E-4 1E-5 Vertical Permeability, k (md) 11 /49

12 Permeability Vertical Permeability, k (m 2 ) 1E-15 1E-16 1E-17 k 0.5 log(k) = γ.(n 0.5) + log(k 0.5 ) for 0.20 < n < E-18 R. Presumpscot Clay (RPC) 1E-19 R. Ursa Clay (RGoM-Ursa) R. London Clay (RLC) 1E Porosity, n γ 1 1E-1 1E-2 1E-3 1E-4 1E-5 Vertical Permeability, k (md) 12 /49

13 Permeability Correlations γ log(k) = γ.(n 0.5) + log(k 0.5 ) Cornwall Nankai γ = 0.067(w L ) r 2 = 0.75 log(k 0.5 ) log(k 0.5 )= -7.55log(w L ) 3.4 r 2 = Liquid Limit, w L (%) Liquid Limit, w L (%) 13 /49

14 Permeability Model: Error Analysis 14 /49

15 Permeability: Predicting In situ Behaviour 0 Liquid limit / Porosity Permeability, k (m 2 ) 1E-17 1E-16 1E liquid limit porosity Boston Blue Clay Depth (ft) /49

16 Permeability: Predicting In situ Behaviour 16 /49

17 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 17 /49

18 Typical Triaxial Test Procedure 1. Setup and back-pressure saturation (1 day) 2. K O -consolidation of specimens (3-10 days) Important to mimic field conditions 3. Secondary compression/creep (1 day) 4. K O -swelling (1 2 days) 5. Undrained shear in triaxial compression (1 day) 18 /49

19 low pressure triaxial (σ p < 2 MPa) high pressure triaxial (10 < σ p < 100 MPa) medium pressure triaxial (2 < σ p < 10 MPa) 19 /49

20 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 20 /49

21 Effective Stress Effective Stress: Partial stress which controls changes in deformation and shear resistance of porous materials Conventional Terzaghi (1923) definition for saturated soil: σ = σ u assumes particles are: 1) incompressible, and 2) have a constant yield strength Some have proposed modified definitions, such as: σ = (σ u) + au + (R A) Intergranular stress (Skempton 1960) (a = contact area between particles per unit area) At high stresses the contact area can become significant; can true effective stress deviate from Terzaghi definition?...literature typically assumes no 21 /49

22 Tests of Bishop and Skinner (1977) Most significant testing program to examine effective stress in relation to shear resistance Drained triaxial compression tests involving large changes in backpressure but keeping (σ 3 u b ) constant during shearing Significance of interparticle contact area determined from discontinuities in shear stressstrain curve Tested sand, silt, crushed marble, lead shot for pore pressures up to 40 MPa 22 /49

23 Tests of Bishop and Skinner (1977) Results and conclusions: Terzaghi definition applicable for full range of stresses tested with no observable change in shear resistance Intergranular stress equation not valid Inconclusive re. Skempton s (1960) equation However. No clays were tested Nature of inter-particle contacts is potentially different for clays 23 /49

24 Effective Stress Tests 24/49

25 Outline Motivation and Objectives Resedimentation Permeability Results Triaxial Equipment and Procedures Principle of Effective Stress Shear Strength Behavior Summary and Conclusions 25 /49

26 Stress-Strain Response during Shearing 0.40 Normalized Shear Stress, q/σ' vc s u /σ vc σ vc = 0.2 MPa 1.2 MPa 9.8 MPa 105 MPa R. Ugnu OCR = Axial Strain, ε a (%) 26 /49

27 Undrained OCR = 1 Undrained Strength Ratio, s u /σ' vc s u /σ vc = S 1 (1000σ p [MPa] ) T s u /σ v S 1 T σ p R. Ugnu Clay R. London Clay Preconsolidation Stress, σ' p (MPa) 27 /49

28 Undrained OCR = 1 Undrained Strength Ratio, s u /σ' v Preconsolidation Stress, σ' p (MPa) s u /σ v S 1 T σ p Skibbereen Silt R. Presumpscot Clay R. Boston Blue Clay R. GoM Ursa Clay R. Ugnu Clay R. S.F. Bay Mud R. London Clay R. GoM Eugene Is. 28 /49

29 Undrained Strength - Liquid Limit Correlations S 1 = 0.86log(w L ) r 2 = T = -0.46log(w L ) r 2 = S T Liquid Limit, w L (%) Liquid Limit, w L (%) 29 /49

30 Overconsolidated Behavior OCR = 4 OCR = 8 σ vc = 0.6 MPa σ vc = 40 MPa OCR = 2 OCR = 1 R. Boston Blue Clay 30 /49

31 Increase in Ductility with Stress R. Boston Blue Clay 31 /49

32 Undrained Strength: Overconsolidated Soil 1.6 R. Boston Blue Clay S 1(OC) T Undrained Strength Ratio, s u /σ' vc OCR = 2 OCR = 4 OCR = 8 T is independent of OCR s u /σ vc = 1.701(1000σ p ) s u /σ vc = 1.083(1000σ p ) s u /σ vc = 0.593(1000σ p ) OCR = 1 s u /σ vc = 0.366(1000σ p ) Preconsolidation Stress, σ' p (MPa) 32 /49

33 Undrained Strength: Overconsolidated Soil R. Boston Blue Clay S 1(OC) approx. constant for fine-grained soils S 1(OC) = 0.368(OCR) 0.73, r 2 = /49

34 Summary of Strength Equations Undrained triaxial compressive strength: S 1 = 0.86log(w L ) 1.04 T = -0.46log(w L ) s u /σ vc = S 1 (1000σ p [MPa] ) T (OCR) /49

35 Effect of K O on Undrained OCR=1 σ' V σ' H K O = σ H /σ V 35 /49

36 Friction Angle 40 Critical State Friction Angle, φ cs ( ) φ = A(0.001σ p [MPa] ) B B A σ p φ R. Ugnu Clay R. London Clay Preconsolidation Stress, σ' p (MPa) 36 /49

37 Friction Angle 40 Critical State Friction Angle, φ' cs ( ) φ Preconsolidation Stress, σ' p (MPa) B A σ p Skibbereen Silt R. Presumpscot Clay R. Boston Blue Clay R. GoM Ursa R. Ugnu Clay R. S.F. Bay Mud R. London Clay R. GoM Eugene Is. 37 /49

38 Friction Angle - Liquid Limit Correlations A = -75log(w L ) r 2 = B = -0.39log(w L ) r 2 = A ( ) B Liquid Limit, w L (%) Liquid Limit, w L (%) 38 /49

39 Summary of Strength Equations Undrained triaxial compressive strength: S 1 = 0.86log(w L ) 1.04 T = -0.46log(w L ) s u /σ vc = S 1 (1000σ p [MPa] ) T (OCR) 0.73 Drained triaxial compressive strength: A = -75log(w L ) B = -0.39log(w L ) φ cs = A(0.001σ p [MPa] ) B 39 /49

40 Effect of OCR on φ cs Critical State Friction Angle, φ cs ( ) R. Boston Blue Clay OCR = 1 OCR = 2 OCR = 4 OCR = Preconsolidation Stress, σ' p (MPa) 40 /49

41 Example: Bearing Capacity (assuming drained conditions and no surcharge) a change in friction angle from 40 to 35 reduces bearing capacity by 56 % a change in friction angle from 40 to 30 reduces bearing capacity by 80 %! 41 /49

42 Particle Reorientation Compression Adams (2014), Ph.D. Courtesy of Taylor Nordquist.but failure in triaxial compression occurs at ~ Particle reorientation with stress cannot explain strength behavior /49

43 At very high stresses Porous materials will ultimately reach the friction angle of the solid material, referred to as the intrinsic friction angle ψ (Skempton 1960) Tests on marble, metals, quartz and limestone 400 tests on marble by Von Karman Material Ψ ( ) Limestone 8 Calcite 8 Quartz 16 Clay minerals ~ 5 10 from Skempton (1960) Shear Stress, τ (MPa) Normal Stress, σ v (MPa) 43 /49

44 40 Critical State Friction Angle, φ' cs ( ) (% clay minerals) (51) (23) (16) Skibbereen Silt 20 R. Presumpscot Clay (37) R. Boston Blue Clay (37) R. GoM Ursa R. Ugnu Clay 15 R. S.F. Bay Mud (54) R. London Clay R. GoM Eugene Is. 10 (55) Preconsolidation Stress, σ' p (MPa) 44 /49

45 Yield Surface Evolution R. Boston Blue Clay 45 /49

46 Yield Surface Evolution R. London Clay 0.15 MPa 11.8 MPa 46 /49

47 Conclusions Resedimentation is a technical necessity and practically advantageous to study the behavior of soils systematically Correlations developed from resedimented soil using liquid limit can predict intact permeability, a robust indicator of composition Conventional Terzaghi definition of effetive stress is valid for fine-grained soils at high in situ pore pressures Shear strength properties vary consistently with stress level and are closely linked to composition/plasticity Variations in strength properties with stress reflect an evolving yield surface 47 /49

48 Motivation For soils and soft rock, shear strength is complex a function of: composition (w L ) effective stress (σ ) stress history (OCR) This work τ max = f mode of shear (b, α) temperature (T) strain rate (έ) water saturation (S w ) Future Work diagenesis 48 /49

49 Publications Casey, B. and Germaine, J.T. (2013). The Stress Dependence of Shear Strength in Fine-Grained Soils and Correlations with Liquid Limit, Journal of Geotechnical and Geoenvironmental Engineering, 139 (10), doi: /(ASCE)GT Casey, B., Germaine, J.T., Flemings, P.B., Reece, J.S., Gao, B., and Betts, W. (2013). Liquid Limit as a Predictor of Mudrock Permeability, Journal of Marine and Petroleum Geology, 44, Casey, B. & Germaine, J.T. (2013). Variation of Cohesive Sediment Strength with Stress Level, Advances in Multiphysical Testing of Soils and Shales, Springer Series in Geomechanics Casey, B., Fahy, B.P., Flemings, P.B. & Germaine, J.T. (2014). Shear Strength of Two Gulf of Mexico Mudrocks and a Comparison with Other Sediments, Fourth EAGE Shale Workshop, 6-9 April 2014, Porto Casey, B. & Germaine, J.T. (2014). An Evaluation of Three Triaxial Systems with Results from 0.1 to 100 MPa Geotechnical Testing Journal, in review 49 /49