BIOREACTOR LANDFILLS: GEOTECHNICAL ASPECTS OF STABILITY EVALUATION

Transcription

1 BIOREACTOR LANDFILLS: GEOTECHNICAL ASPECTS OF STABILITY EVALUATION Presented by James Law SCS Engineers Master Class ISWA Congress 2009 Lisbon 10 October 2009

2 PRESENTATION OVERVIEW Landfill Slope Stability Overview Typical MSW Geotechnics & Shear strengths Bioreactor Landfill MSW Material Properties MSW Moisture Content Impact on Stability Recommendations and Conclusions

3 LANDFILL OPERATION & SLOPE STABILITY NEW WASTE CELL PAGE CO. VA (2006)

4 Principal Stability Considerations for Modern Sanitary Landfills Analyze critical sideslopes for stability at 3 stages: (1) construction, (2) operations and (3) final load conditions 1. Excavation slopes 2. Interim waste slopes 3. Exterior (Final) slopes a. Deep seated b. Veneer Waste settlement 3b. Veneer Stability 3a. Deep Seated Stability Final cover 1. Excavation 2. Interim Slopes Foundation (subgrade) Bottom liner

5 Critical Shear Surfaces Veneer Stability Deep Seated Stability (Circular) Final cover Deep Seated Stability (Block) Bottom liner WASTE Foundation (subgrade) FACTORS OF SAFETY: FS > 1.5 for Static final (peak) FS > 1.3 for Static interim FS > 1.0 for Seismic (peak) Or, deformation analysis (e.g., Newmark s) STABILITY MODELING: Limit equilibrium models (e.g., PCSTABL, UTEXAS3, XSTABL, etc.) Drained and Undrained conditions (pore pressures) Other Loadings (equipment)

6 Veneer Stability Models Infinite Slope Finite Slope

7 Veneer Instability During Closure Following Heavy Rain (aka. Ernesto) Leachate Recirculation Facility 24 inches of soil cover Soil erosion LFG buildup under liner.gas bubbles

8 Global Stability Two-dimensional Limit Equilibrium models Computer models based on Spencer, Bishop, Janbu, et al Method of slices 3-D D models Search for shear surface with lowest Factor of Safety (FS) Static Seismic (a= x x g) Key Material properties Waste Soil Waste friction, cohesion & density waste & operation specific Soil shear strengths & density site specific Soil/Geosynthetic interface strength material specific Liquid/leachate levels

9 FS FS = The Classical Factor of Safety [ Actual Shear Strength, Τ act Shear Strength for Equilibrium, Τ eq = [C act + (N-μ) tan( tan(ø act )] [C eq + (N-μ) tan(ø eq Ø=friction angle C=cohesion (equivalent) N=normal stress and μ=pore pressure eq )] FS=1.5 means 50% more strength than required for equilibrium FS=1.2 means 20% [ Shear Stress Peak Strength Shear Displacement Residual Strength

10 MSW STRENGTH BASED ON TESTS & OBSERVATIONS Non-Bioreactor LFs (Hiriya Landfill, Tel Aviv, 2002)

11 Waste Shear Strength: Assume Mohr Coulomb Behavior (bi-linear) like a compressible soil Friction equivalent, Ǿ Cohesion equivalent, C Varies with Waste type Compaction Liquids additions Daily cover Density Moisture content Age, time-dependent Heterogeneous, anisotropic, changes with time Τ=Shear Strength C N = Normal Stress Ǿ

12 Summary of Typical MSW Properties* (non-bioreactor MSW) In-place (field) wet density ~1250 to ~1750 pcy (46 to 65 pcf) Higher values reported to 110 pcf Disclaimer: *All Lower values possible below 40 pcf variable & function Peak shear strength Mohr-Coulomb behavior of waste type, composition, Friction (Ǿ):( ~20 to ~36 compaction, daily Cohesion (C) : 0 to ~1000 psf cover, moisture Residual strength lower post failure conditions, Moisture content (wet weight) age, overburden pressure, etc Range: ~10% to ~60% (wet weight basis) Average ~20% to 30% Field Capacity (Fc( Fc): ~35% to 55% Permeability: : ~10-2 to ~10-6 cm/sec Decreases with overburden pressure and density

13 Relatively Dry, Partially Decomposed Non-Bioreactor MSW DRY TO MOIST WASTE (~1 m) WET WASTE (~5 m)

14 WET TO SATURATED WASTE (NEAR LEACHATE LEVELS) Relatively Wet, Well Decomposed Non-Bioreactor MSW

15 MSW Shear Strength Envelope (Singh & Murphy, 1990) Rumpke LF Slope Failure

16 MSW is strong and can stand on steep slopes FS~ * *5% to 10% more shear strength than needed for stability

17 ..until there s too much water. Hiriya Landfill Slope Failure (1997) Waste Mass Slippage Good Reference: Koerner & Soong, Stability Assessment of Ten Large Landfill Failures, 2000

18 MSW Strength Back Calculation using PCSTABL PCSTABL5M MODEL SECTION AA - HIRIYA BACK-CALCULATED MSW PEAK SHEAR STRENGTH: Ǿ=33 C=167 psf Circular Shear Surface

19 Hiriya MSW vs. Recommended Range Hiriya, 2002

20 Geotechnics of Bioreactor Landfills

21 Leachate Recirculation System - Phase 2 RECIRCULATION PIPING LFG COLLECTION SYSTEM EXTRACTION WELLS HORIZONTAL COLLECTORS STORAGE TANK/PUM P

22 RECIRCULATION PIPING Landfill Sections HORIZONTAL COLLECTORS Phase 1 and 2 Installation Phase 3 and 4 Installation

23 Definitions of Moisture Content 1. Volumetric = W vol = Vol. Liquid Vol. Waste liquid balance models (e.g., HELP) 2. Gravimetric = Wt. Liquid Wt. Waste* 2A. W dry = Dry Weight Basis* geotechnical applications; constitutive relationship in soil mechanics OR 2B. W wet = Wet Weight Basis* Waste water applications Default for bioreactors

24 Compare the Methods Initial Assumptions: Dry waste density = 800 pcy Liquid content = 400 pcy Wet waste density = = 1200 pcy 1 Cubic Yard: Air (voids) Liquid Solid Waste Moisture Content Basis 1. Volumetric, W vol 2A. Dry Weight,W, dry 2B. Wet Weight, W wet Calculation =400/(62.4*27) =400/800 =400/1200 Result 23.7% 50.0% 33.0%

25 Now, add some liquid: Add 300 pcy liquid (~36 gallons/cy) Assuming no change in dry density: Dry waste density = 800 pcy Liquid content = 700 pcy Wet waste density = = 1500 pcy 1 Cubic Yard: Air (voids) Liquid Solid Waste MC Basis 1. Volumetric, W vol 2A. Dry Weight,W dry 2B. Wet Weight, W wet Calculation =700/(62.4*27) =700/800 =700/1500 Result 41.5% 87.5% 46.7%

26 and some waste compression: Assume increase in dry density due to 10% compression*: Dry waste density*= 888 pcy Liquid content = 700 pcy Air (voids) Wet waste density = 1588 pcy 1 Cubic Yard: Liquid Solid Waste MC Basis 1. Volumetric, W vol 2A. Dry Weight,W, dry 2B. Wet Weight, W wet Calculation =700/(62.4*27) =700/888 =700/1588 Result 41.5% 78.8% 44.1%

27 Moisture Content by Wet Wt and Volume 90% 80% 70% 60% 50% 40% 30% 20% 10% Moisture Content - Wet Basis % Moisture Content - Volume Basis % Wet Waste density = 1400 pcy MOISTURE CONTENT COMPARISON CHARTS W wet = 40% W dry = 66.6% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Moisture Content by Dry Wt. W vol = 55.3% 70% 60% 50% Moisture Content - Wet Basis % Moisture Content - Volume Basis % Wet Waste density = 1000 pcy Moisture Content by Wet Wt and Volume 40% 30% 20% W wet = 40% 10% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Moisture Content by Dry Wt. W dry = 66.6% W vol = 39.5%

28 Field Capacity (Fc) of MSW Fc = moisture content that waste will store within pores by capillary stress; less than saturation one drop in, one drop out Fc influenced by waste composition, age, density and porosity Reported Fc values (volumetric basis): 15% to 44% Q: So, what does Fc = 40% really mean? A: It depends on how you calculate it.

29 How Many Gallons Should We Add? Assume: 5 acre cell, 30 feet of waste (average depth) 242,000 cy waste mass 200 pcy initial liquid content Wet (field) density of 1000 pcy Calculate liquid needed to achieve 40% 40% for each MC basis* MC Basis Initial Final* Water To Be Added 1. Volumetric, W vol 200 pcy (12%) 40% 13,749,000 gal (57 gpcy) 2A. Dry Weight,W dry 200 pcy (25%) 40% 3,481,000 gal (14 gpcy) 2B. Wet Weight, W wet 200 pcy (20%) 40% 9,670,000 gal (40 gpcy)

30 Points to Ponder Read literature carefully; define terms Numerical differences between moisture content calculation methods are significant More liquid needed (allowed) to reach 40% wet weight than 40% dry weight Maintain max. 30 cm hydraulic head on liner per Subtitle D (check via H.E.L.P. Model), avoid slopes and perched zones Reference: Retention of Free Liquids in Landfills Undergoing Vertical Expansion, Zornberg,, et al, ASCE Geotech.. Journal, July, 1999)

31 Bioreactor Waste Property Changes: In Situ (wet) Waste Density* will increase Increased moisture content...more on this later Compression or settlement from 5% to 30% (Sowers, 1973) Raveling (particle re-orientation) Accelerated decomposition of organic components Waste shear strength (Τ)( ) will change Τ = C + (N-μ) tan( Ǿ C C = cohesion actually it s s more like internal reinforcement Ǿ = internal friction angle N N = normal (overburden) stress μ = pore pressure (if any) Key Q: Does Waste get stronger? weaker? the same?

32 In Situ Waste Density: γ wet = in-situ density (not airspace utilization) Increases with depth (overburden) + with compaction effort + with soil daily cover + with time and settlement + with moisture content addition Cumulative effects are significant ~40% to ~70% increase possible

33 Example: Moisture + Settlement + Decomposition Initial In-Place Condition: γwet = 1000 W wet =25% Moisture Addition: To achieve W wet =40%=> add 250# water/cy (30 gal) New γwet = =1250 pcy (assumes no by-pass) Settlement (compression, decomposition) = 20% New γwet = (1250 pcy)/(0.80) = 1562 pcy Net Density Increase = ( )/(1000) 1000)/(1000) => 56.2%

34 Bioreactor Waste Shear Strength : Testing evolving Laboratory tests on processes samples Large Triaxial cells difficult and expensive Direct simple shear boxes (6 x6 to 12 x12 ) Waste particles are large compared to testing devices Field tests few reported; site specific Vane shear, plate loading Penetration testing (SPT and Cone) Forensic stability analysis Back-calculation of Ø and C From slope failures (known conditions similar to bioreacted waste)

35 Recent Bioreactor Waste Strength Results: Forensic: Hiriya LF slope stability modeling Ø = 33, C=167 psf Conclusion: wet, dense waste still strong Lab (2003): Direct shear tests on decomposed waste <1 inch particles Drained Ǿ = 27º to 32º at C=0 psf Undrained Ǿ = 29º to 36º Conclusion: not much change Lab (2005): North Carolina State U. Study Reported in Waste Age, Oct Conclusion: Shear strength decreases with degradation Recommend: Ǿ=20º, C=0 psf, γ wet = 100 to 110 pcf (2700 to 2970 pcy)

36 Key A to Key Q: Based on review of available test data and on the performance of bioreactor landfills, it is likely that controlled bioreacted waste maintains a similar shear strength to non-bioreacted waste. The shear strength gained from increased density (lower void ratio, higher internal friction, and improved packing) may be offset by the increase in moisture content and decomposition of organic components that would tend to lower shear strength. Under some circumstances bioreacted MSW may become weaker than nonbioreacted MSW including highly organic and well decomposed waste, very wet to saturated waste, or waste that is bioreacted without proper controls. Predicting a significant shear strength increase would not be considered conservative without substantial evidence, while predicting a significant decrease would be potentially over-conservative. The designer should select MSW strength values based on specific waste composition, placement and operation methods and considering the margin for error defined by Factor of Safety.

37 How Sensitive is FS to Shear Strength? LAYER BioType: Upper (newest) Middle (average) Lower (oldest) DENSITY O III FRICTION O III COHESION O III 45 pcf 79 pcf 26º 18º 200 psf 40 psf 55 pcf 96 pcf 30º 22º 250 psf 50 psf 65 pcf 114 pcf 34º 26º 300 psf 60 psf 5% 1 3 2% Upper (newest) Middle (average) Lower (oldest) Bottom liner 140 feet Foundation (subgrade)

38 Bioreactor Types Used on Sensitivity Model Density Increase General Description TYPE 75% Heavy recirculation; at Fc field capacity III 50% Moderate, controlled recirculation (below field capacity) II 25% Limited or intermittent recirculation I 0% Baseline; non-bioreactor 0

39 Summary for Circular Failure TYPE BASE LINE Δ =2 ΔC=40-60 psf Δ =4 ΔC= psf Δ =6 ΔC= psf Δ =8 ΔC= psf O I II III

40 Summary for Block Failure (smooth liner: Interface Ǿ=8º) TYPE BASE LINE Δ =2 ΔC=40-60 psf Δ =4 ΔC= psf Δ =6 ΔC= psf Δ =8 ΔC= psf O I II III *bioreactor retrofits with smooth liners (low interface friction) have higher potential for instability

41 RECOMMENDATIONS Based on all the above.in Design: FS>1.5 is achievable with proper design and operations FS<1.5 possible for bioreacted waste select FS values based on risk and sensitivity Consider Block and Circular failure modes Waste shear strength is the most critical parameter and will change over time Waste density increases are significant (40% to 70% or more), but have limited impact on FS compared to shear strength Calculate liquids additions carefully and limit to below Fc and prevent pore pressure build-up

42 RECOMMENDATIONS Based on all the above.in Operations: Develop and follow an operations plan based on design criteria and monitor liquids, sludges or other additions continuously Keep liquids injection away from slopes (outside shear surfaces)

43 CONCLUSIONS Bioreactor landfill slope stability is controlled by Waste shear strength (C and Ǿ) Liner interface strength (geomembranes, geonets, etc.) Final slopes Operations (liquid and gas management) Waste density Bioreactor landfills can and should be engineered, constructed and operated to be stable (Factor of Safety >1.5) Operations are critical to maintaining stability and conditions ideal for waste decomposition

44 QUESTIONS AND COMMENTS