BIOREACTOR LANDFILLS: BIOREACTOR LANDFILLS: GEOTECHNICAL ASPECTS OF GEOTECHNICAL ASPECTS OF STABILITY EVALUATION STABILITY EVALUATION Presented by Presented by James Law James Law SCS Engineers SCS Engineers Master Class ISWA Congress 2009 Master Class ISWA Congress 2009 Lisbon Lisbon 10 October 2009 10 October 2009
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BIOREACTOR LANDFILLS: BIOREACTOR LANDFILLS: GEOTECHNICAL ASPECTS OF GEOTECHNICAL ASPECTS OF
STABILITY EVALUATIONSTABILITY EVALUATION
Presented byPresented by
James LawJames LawSCS EngineersSCS Engineers
Master Class ISWA Congress 2009Master Class ISWA Congress 2009LisbonLisbon
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)
Foundation (subgrade)
Veneer StabilityVeneer StabilityDeep Seated Stability Deep Seated Stability (Circular)(Circular)
Deep Seated Stability Deep Seated Stability (Block)(Block)
Final cover
WASTE
Bottom liner
Veneer Stability Models
Infinite Slope Finite Slope
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
Global StabilityTwoTwo--dimensional Limit Equilibrium modelsdimensional Limit Equilibrium models
Computer models based on Spencer, Bishop, Janbu, et alComputer models based on Spencer, Bishop, Janbu, et alMethod of slicesMethod of slices33--D modelsD models
Search for shear surface with lowest Search for shear surface with lowest ““Factor of SafetyFactor of Safety”” (FS)(FS)StaticStaticSeismic (a= xSeismic (a= x·· g)g)
Key Material propertiesKey Material propertiesWasteWaste friction, cohesion & density friction, cohesion & density waste & operation specificwaste & operation specificSoilSoil shear strengths & density shear strengths & density site specificsite specificSoil/Geosynthetic interfaceSoil/Geosynthetic interface strength strength material specificmaterial specificLiquid/Liquid/leachateleachate levels levels
Waste
Soil
The Classical Factor of SafetyActual Shear Strength, Actual Shear Strength, ΤΤactact
Shear Strength for Equilibrium, Shear Strength for Equilibrium, ΤΤeqeq
FS=1.5 means 50% more strength than required for equilibriumFS=1.5 means 50% more strength than required for equilibriumFS=1.2 means 20% FS=1.2 means 20% ““
FS =FS =Sh
ear S
tres
s Peak Strength
Shear Displacement
Residual Strength
[ [
FS FS
(Hiriya Landfill, Tel Aviv, 2002)
MSW STRENGTH BASED ON TESTS
& OBSERVATIONS
Non-Bioreactor LFs
Waste Shear Strength:Assume Mohr – Coulomb Behavior (bi-linear) like a compressible soil
Friction equivalent, ǾCohesion equivalent, CVaries with
Summary of Typical MSW Properties*(non-bioreactor MSW)
InIn--place (field) wet densityplace (field) wet density~1250 to ~1750 pcy (46 to 65 ~1250 to ~1750 pcy (46 to 65 pcfpcf))Higher values reported to 110 Higher values reported to 110 pcfpcfLower values possible below 40 Lower values possible below 40 pcfpcf
Peak shear strengthPeak shear strength –– MohrMohr--CoulombCoulomb behaviorbehaviorFriction (Friction (ǾǾ): ~20): ~20°° to ~36to ~36°°Cohesion (C) : 0 to ~1000 psf Cohesion (C) : 0 to ~1000 psf Residual strength lowerResidual strength lower……post failure post failure
Moisture content (wet weight)Moisture content (wet weight)Range: ~10% to ~60% (wet weight basis)Range: ~10% to ~60% (wet weight basis)Average ~20% to 30%Average ~20% to 30%Field Capacity (Field Capacity (FcFc): ~35% to 55%): ~35% to 55%
PermeabilityPermeability: ~10: ~10--22 to ~10to ~10--66 cm/seccm/secDecreases with overburden pressure and densityDecreases with overburden pressure and density
Disclaimer: *All variable & function of waste type, composition, compaction, daily cover, moisture conditions,age, overburden pressure, etc
•• Assume increase in dry density due to 10% compression*:Assume increase in dry density due to 10% compression*:–– Dry waste density*= 888 Dry waste density*= 888 pcypcy–– Liquid content = 700 Liquid content = 700 pcypcy–– Wet waste density = 1588 Wet waste density = 1588 pcypcy
•• FcFc = = ““moisture contentmoisture content”” that waste will store that waste will store within pores by capillary stress; less than within pores by capillary stress; less than saturationsaturation–– ““one drop in, one drop outone drop in, one drop out””–– FcFc influenced by waste composition, age, density and influenced by waste composition, age, density and
porosityporosity–– Reported Reported FcFc values (volumetric basis): 15% to 44%values (volumetric basis): 15% to 44%
Q: So, what does Q: So, what does FcFc = 40% really mean?= 40% really mean?A: It depends on how you calculate it.A: It depends on how you calculate it.
How Many Gallons Should We Add?•• Assume:Assume:
–– 5 acre cell, 30 feet of waste (average depth)5 acre cell, 30 feet of waste (average depth)•• 242,000 cy waste mass242,000 cy waste mass
–– 200 200 pcypcy initialinitial liquid contentliquid content–– Wet (field) density of 1000 Wet (field) density of 1000 pcypcy–– Calculate liquid needed to achieve Calculate liquid needed to achieve ““40%40%”” for each for each
MC basis*MC basis*
9,670,000 gal (40 9,670,000 gal (40 gpcygpcy))40%40%200 200 pcypcy(20%)(20%)
2B. Wet Weight, 2B. Wet Weight, WWwetwet
3,481,000 gal (14 3,481,000 gal (14 gpcygpcy))40%40%200 200 pcypcy(25%)(25%)
2A. Dry Weight 2A. Dry Weight ,,WWdrydry
13,749,000 gal (57 13,749,000 gal (57 gpcygpcy))40%40%200 200 pcypcy(12%)(12%)
1. Volumetric, 1. Volumetric, WWvolvol
Water To Be AddedWater To Be AddedFinal* Final* Initial Initial MC BasisMC Basis
Points to Ponder•• Read literature carefully; define termsRead literature carefully; define terms•• Numerical differences between moisture content Numerical differences between moisture content
calculation methods are calculation methods are significantsignificant–– More liquid needed (allowed) to reach 40% More liquid needed (allowed) to reach 40% ““wet weightwet weight””
than 40% than 40% ““dry weightdry weight””•• Maintain max. 30 cm hydraulic head on liner per Maintain max. 30 cm hydraulic head on liner per
Subtitle D (check via H.E.L.P. Model), avoid slopes Subtitle D (check via H.E.L.P. Model), avoid slopes and perched zonesand perched zones
••Reference: Reference: ““Retention of Free Liquids in Landfills Retention of Free Liquids in Landfills Undergoing Vertical Expansion,Undergoing Vertical Expansion,”” ZornbergZornberg, et al, , et al, ASCE ASCE GeotechGeotech. Journal, July, 1999). Journal, July, 1999)
In Situ (wet) Waste Density* will increaseIn Situ (wet) Waste Density* will increaseIncreased moisture contentIncreased moisture content……...more on this later...more on this laterCompression or settlement from 5% to 30% (Sowers, 1973)Compression or settlement from 5% to 30% (Sowers, 1973)
Raveling (particle reRaveling (particle re--orientation)orientation)Accelerated decomposition of organic componentsAccelerated decomposition of organic components
Waste shear strength (Waste shear strength (ΤΤ) will change) will changeΤΤ = C + (= C + (NN--μμ))··tan tan ǾǾ
C = C = ““cohesioncohesion”…”…actually itactually it’’s more like internal s more like internal reinforcement reinforcement ǾǾ = internal friction angle= internal friction angleN = normal (overburden) stressN = normal (overburden) stressμμ = pore pressure (if any)= pore pressure (if any)
Key Q: Does Waste get stronger? weaker? the same? Key Q: Does Waste get stronger? weaker? the same?
Bioreactor Waste Property Changes:
In Situ Waste Density:
γwet = = inin--situ density (not airspace utilization)situ density (not airspace utilization)
Increases with depth (overburden)Increases with depth (overburden)++
““ with compaction effortwith compaction effort++
““ with soil daily coverwith soil daily cover++
““ with time and settlementwith time and settlement++
““ with moisture content additionwith moisture content addition
Cumulative effects are significantCumulative effects are significant~40% to ~70% increase possible~40% to ~70% increase possible
• 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. 2005– Conclusion: Shear strength decreases with degradation
• Recommend: Ǿ=20º, C=0 psf, γwet = 100 to 110 pcf(2700 to 2970 pcy)
Key A to Key Q:Based on review of available test data and on the performance ofbioreactor 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 bythe 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 non-bioreacted MSW including highly organic and well decomposed waste, very wet to saturated waste, or waste that is bioreactedwithout 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.
How Sensitive is FS to Shear Strength?
LAYER
BioType: DENSITY
O III FRICTION
O III COHESION
O III Upper (newest)
45 pcf 79 pcf 26º 18º 200 psf 40 psf
Middle (average)
55 pcf 96 pcf 30º 22º 250 psf 50 psf
Lower (oldest)
65 pcf 114 pcf 34º 26º 300 psf 60 psf
Foundation (subgrade)
Bottom liner
Upper (newest)
Middle (average)
Lower (oldest)
31
2%
5%
140 f
eet
Bioreactor “Types” Used on Sensitivity Model
75%
50%
25%
0%
Density Increase
Heavy recirculation; at Fc field capacity
Moderate, controlled recirculation(below field capacity)
Limited or intermittent recirculation
Baseline; non-bioreactor
General Description “TYPE”
III
II
I
0
Summary for Circular Failure
TYPE BASE LINE
Δ∅=2° ΔC=40-60 psf
Δ∅=4° ΔC=80-120 psf
Δ∅=6° ΔC=120-180 psf
Δ∅=8°ΔC=160-240 psf
O 2.88 2.59 2.26 1.95 1.52
I 2.74 2.46 2.17 1.89 1.47
II 2.66 2.38 2.11 1.84 1.43
III 2.59 2.33 2.07 1.78 1.39
Summary for Block Failure(smooth liner: Interface Ǿ=8º)
TYPE BASE LINE
Δ∅=2° ΔC=40-60 psf
Δ∅=4° ΔC=80-120 psf
Δ∅=6° ΔC=120-180 psf
Δ∅=8°ΔC=160-240 psf
O 1.59 1.51 1.43 1.35 1.26
I 1.55 1.48 1.40 1.33 1.24
II 1.52 1.45 1.38 1.31 1.23
III 1.50 1.43 1.38 1.30 1.22
*bioreactor retrofits with smooth liners (low interface friction)have higher potential for instability
Based on all the above….in Design:FS>1.5 is achievable with proper design and operations
FS<1.5 possible for bioreacted wasteselect 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
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)
RECOMMENDATIONS
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