Fine Coal Refuse 25 Years of Field and Laboratory Testing Data and Correlations October 1, 2018 Blaise E. Genes Gonzalo Castro, Ph.D., P.E. Thomas O. Keller, P. E. Fatma Ciloglu, Ph.D., P. E.
Presentation Outline 1. Introduction 2. Upstream-Constructed Coal Refuse Impoundments and Key Design Aspects 3. Fine Coal Refuse (FCR) In-Situ Field and Laboratory Testing 4. FCR Field and Laboratory Data Application Summary 5. FCR Data Correlations 6. FCR Undrained Strength Analyses Examples
Introduction Evaluated FCR tailings at numerous WV, KY and IL impoundment sites since 1991 10 WV, 2 KY, 3 IL. Amassed a database of field and laboratory data from 15 large, high-hazard upstream-constructed impoundments. CPT-based evaluations and undrained strength analyses performed to evaluate: Material characteristics; Liquefaction triggering; Post-earthquake stability; and Construction loading rate.
Upstream-Constructed Coal Refuse Impoundments Some of the tallest earth structures in the world; 02 02 Delta Unique characteristics; FCR hydraulicallydeposited and used as foundation for subsequent embankment construction; FCR requires sufficient 03 time to settle and excess 03 pore pressure to dissipate; Undrained conditions control upstream pushout and seismic loading; Delta Oldhouse Branch Stage J Embankment
Upstream-Constructed Coal Refuse Impoundments Key design aspects: Developing and implementing risk-appropriate in-situ field and laboratory testing; Evaluating FCR material characteristics, i.e., does FCR behave more sand-like or clay-like; Estimating undrained shear strength and S u /s v ratio; Evaluating if strength loss is triggered due to undrained loading event; Evaluating post-earthquake stability with appropriate S u and corresponding safety factor; and, Evaluating incremental S u required vs. S u gained due to excess pore pressure and consolidation.
Field Testing Methods In-Situ Field Testing Methods: Cone Penetration Testing (CPT); Shear wave velocity measurements; Pore-pressure dissipation and vane shear testing; Fixed-piston undisturbed sampling; and, In-situ/in-tube void ratio.
PS-CPT Field Testing Data FP FP FP FP FP FP
Laboratory Testing Methods Laboratory Testing: Grain-size, Atterberg limits, moisture content, specific gravity; CU triaxial shear strength: Peak, S up and steady-state, S us undrained shear strength; Peak shear strain, e; and, S up /s v. and S us /s v strength-to-effective stress ratios.
Characterizing FCR From Lab Testing Material characterization depends on laboratory data (% passing #200, plasticity, peak strain), which influence behavior. Key Differences Sand-like or Clay-like Behavior: Strain at peak undrained shear strength Abruptness of the drop-off in shearing resistance MSHA. 2009
Characterizing FCR From CPT Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 Site 14 Site 15 After MSHA. 2009 P. K. Robertson and C. E. (Fear) Wride, (1998).
Characterizing FCR From CPT Zone A: Loose sand-like; Liquefaction Possible Depends on EQ M and duration. Zone B: Clay-like; Liquefaction Unlikely Check other criteria, i.e., cyclic stress/strain. Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 Site 14 Site 15 Zone C: Sensitive Clay-like; Liquefaction Possible Depends on plasticity and EQ M. Soils with Ic > 2.6 and F > 1.0% are Likely Non-Liquefiable. P. K. Robertson and C. E. (Fear) Wride, (1998). After MSHA. 2009 Normalized Tip Resistance (Q) and Friction (F) Ratio for FCR Materials/Sites
Peak Undrained Strength vs. Shear Strain Peak Undrained Shear Strength, S up, ksc Peak Undrained Shear Strength vs Shear Strain 10 9 Sand-Like Clay-Like 8 Site 1 7 Site 2 Site 3 Site 4 6 Site 5 Site 6 5 Site 7 Site 8 4 Site 9 Site 10 3 Site 12 Site 13 2 Site 15 1 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Shear Strain at Peak Strength, % Summary of Shear Strain Range for Laboratory Tested FCR Samples
Laboratory-Derived Characterizing Undrained FCR Strength High quality undisturbed samples used to measure S us and S up. S us measured in the laboratory will be higher than insitu. Disturbance accounted for by correcting laboratory S us back to the in-situ S us, which requires: 1. Careful measurement of void ratio during sampling and handling; and, 2. Estimating slope of the Steady-State Line (De/DlogS us ).
Laboratory-Field Undrained Strength MSHA. 2009 Correction of S us from Laboratory to In-Situ Void Ratio
Void Ratio During Shear Laboratory-Derived SSL 1.7 Undrained Steady State Shear Strength vs Void Ratio During Shear Undrained Steady State Shear Strength, S us, ksc 0.01 0.1 1 10 1.8 1.6 De/DS us 1.5 0.11 Sites 1, 2 1.4 0.13 Site 3 1.3 0.14 0.11 Site 4 Site 5 1.2 0.11 Site 6 1.1 0.14 Site 7 1 0.11 Site 8 0.9 0.11 Site 9 0.8 0.11 Site 10 0.7 0.6 0.24 0.11 0.53 Site 11 Site 12 Site 14 0.5 0.4 0.3 0.2 Steady-State Line WV, KY and IL Sites
Applications Characterizing for Testing FCR Data CPT data and S up /s v or S us /s v used in engineering analyses to evaluate: Liquefaction will the undrained loading trigger a strength loss in FCR? Yes Use S us /s v No Use S up /s v Post-earthquake stability factors of safety; and, Pushout strength required for 1.3 safety factor in construction stability.
Post-Earthquake Stability Analyses Upstream Stability s v s v s v Downstream Stability S up /s v or S us /s v s v
Estimated Vertical Effective Stress, ksc Laboratory-Derived FCR S us 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 0 Site 1 1 2 3 4 5 Undrained Steady State Shear Strength In-Situ vs Estimated Vertical Effective Stress Undrained Steady State Shear Strength In-Situ, S us, ksc Site 2 Site 3 Site 4 Site 5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 6 7 8 9 10 S us = 0.16 s' v 11 12 S us min = 0.03 s' v S us max = 0.27 s' v Undrained Steady-State Shear Strength to Effective Stress Strength Ratio
SLaboratory-Derived us /s v Correlations Undrained - References Strength Literature Correlations of S us /s v from Failure Cases MSHA. 2009
Estimated Vertical Effective Stress, ksc Laboratory-Derived FCR S up 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 0 1 2 3 4 5 6 Peak Undrained Shear Strength In-Situ vs Estimated Vertical Effective Stress Peak Undrained Shear Strength In-Situ, S up ksc Site 1 Site 2 Site 3 Site 4 Site 5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 Site 14 Site 15 7 8 S up = 0.24 s' v 9 10 11 12 S up min = 0.19 s' v S up max = 0.35 s' v Peak Undrained Shear Strength to Effective Stress Strength Ratio
S us, ksc Sus, ksc Laboratory-Derived Fines Data Correlations Undrained Strength S us 4 3.5 3 2.5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 S us vs. I c 4 3.5 3 2.5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 S us vs. FC Lab 2 1.5 y = 0.589x - 1.0939 R² = 0.0995 2 1.5 y = 0.0111x R² = -0.328 1 1 0.5 0.5 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 I c, dim 0 0 10 20 30 40 50 60 70 80 90 100 Fines Content, % S us vs. Soil Behavior Index, Ic S us vs. Laboratory Fines Content
S us, ksc S up, ksc N 1,60 Laboratory-Derived Data Data Correlations Undrained S us Strength and S up 4 3.5 3 2.5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 S us vs. N 1,60 4 3.5 3 2.5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 Site 13 S up vs. N 1,60 2 2 1.5 1 y = 0.2715e 0.1003x R² = 0.2872 1.5 1 y = 0.6601e 0.0697x R² = 0.4033 0.5 Lower Bound, GEI 0.5 Lower Bound, GEI 0 0 5 10 15 N 1,60, bpf Lower Bound, Seed & Harder 0 0 5 10 15 N 1,60, bpf Lower Bound, Seed & Harder S us vs. N 1,60 Relationship S up vs. N 1,60 Relationship
Undrained Steady-State Shear Strength, S us, ksc Laboratory-Derived V s Data Correlations Undrained Strength S us 6 Undrained Steady-State Shear Strength vs Shear Wave Velocity 5.5 5 4.5 4 3.5 3 2.5 2 y = 0.0044x R² = -0.049 Site 3 Site 4 Site 6 Site 7 Site 8 Site 9 Site 10 Site 12 1.5 1 0.5 0 0 100 200 300 400 500 600 700 800 900 1000 Shear Wave Velocity, V s, m/s Undrained Steady-State Shear Strength, S us vs. Shear Wave Velocity, V s
S us Laboratory-Derived and S us /s v Correlations Undrained References Strength Literature Correlations of S us with SPT and CPT Data MSHA. 2009
Staged Construction Stability Analysis S u required for stability FS=1.3
Staged Construction Stability Analysis S uu required for stability FS=1.3
Staged Construction Stability Analysis S u required for stability FS=1.3 S u required for stability FS=1.3
Staged Construction Stability Analysis S u required for stability FS=1.3
Staged Construction Stability Analysis S u required for stability FS=1.3
Conclusions Upstream-constructed impoundments must endure high level of scrutiny particularly for seismic and pushout construction undrained loading conditions. 25+ years of consistent field and laboratory testing of FCR yielded a significant volume of high quality data. FCR data and correlations present ranges to evaluate peak and steady-state undrained shear strength in absence of, or for data comparison. Risk-appropriate site-specific testing should be performed to estimate undrained shear strengths. Site-specific FCR strength ultimately control undrained strength analyses.
Questions?? Fine Coal Refuse 25 Years of Field and Laboratory Testing Data and Correlations October 1, 2018 Blaise E. Genes Gonzalo Castro, Ph.D., P.E. Thomas O. Keller, P. E. Fatma Ciloglu, Ph.D., P. E.