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Geotechnical ConsiderationsDuring Landfill Closure
Federation of New York Solid Waste AssociationsBolton Landing, NYMay 20, 2014
Walter W. Burke, P.E.Vice President
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Opportunity
• Complete the closure of an existing MSW landfill
• Maximize beneficial re-use of stockpiled andimported processed dredge material (PDM)
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Challenges
• Soft, weak foundation soils
• Insufficient geotechnical data
• Documented existing instability
• Aggressive closure schedule
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Regional Setting
• Meadowlands Area – Bergen County, New Jersey
• Glacial deposits– Alluvial sands
– Peat/organic silt
– Varved glacial lake sediments
– Glacial till
– Bedrock
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Site Location
Site Location
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Landfill• Approximately 100 ft high with a 137 Ac footprint
and a 90 Ac crest• Surrounded by a soil bentonite vertical hydraulic
barrier• Adjacent critical infrastructure
– Railroad tracks– Aqueduct– Electric transmission lines– Major gas mains– Adjacent buildings near the toe
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Geotechnical Considerations
• MSW– Variable density,
shear strength and age
• Organic Deposits– Soft, weak,
discontinuous pockets
• Glacial lake sediments– Dominant geotechnical
feature– 200 ft thick or more– Varved– Anisotropic response
• Hydraulic regimes– Natural soils– Landfill?
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Glacial Geology
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Landfill Development
• Actively received MSW from early 1980s – early1990s
• Up to 25 feet of cumulative settlement
• During several expansions - lateral spreading, deep-seated lateral movements to EL -100 (200 ft deep)and chronic stability concerns
• Additional expansion was halted
• Side slopes incrementally closed and capped
• PDM placed on crest but not properly closed
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Final Closure
• Limited to crest – sideslopes were previouslycapped
• 6” vegetative cover/18” clean soil cover/12”surficial processed dredge material (PDM)/12” lowK (1x10-5 cm/sec) PDM/general grading fill
• Surface water management controls
• Four to 20 feet of new fill
• Nearly 400,000 CYD of new material
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Preliminary Impressions
• MSW is inherently stable
• One groundwater regime
• All failure surfaces passed through the foundation(clay) soils
• Critical failure mode is block
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Design Approach
• Conceptual grading plan
• Compile available regional geotechnical data
• Develop soil strength/response models
• Preliminary stability assessment
• Initiate site investigation at start of capconstruction
• Additional stability analysis
• Develop observational method criteria
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Stability Analysis Approach
• Select appropriate stability analysis
• Select the appropriate soil model– Foundation soils are varved, anisotropic
• Develop a field investigation and testing programto confirm soil properties
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Soil Stability Approach
• Undrained strength analysis (USA) - C. Ladd, 1991
• Determine the in situ effective consolidation stressprofile (consolidation tests and CPT)
• Estimate the existing compressive shear strengthusing SHANSEP
• Estimate the appropriate shear strength model(s)
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Soil Stability AnalysisPSC: Plane Strain CompressionDSS: Direct Simple ShearPSE: Plane Strain Extension
C. Ladd - 1987
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Geotechnical Field Investigation• Borings & instrumentation layout at crest based on
observed or calculated stability performance• Test borings: seven to depths of 190 to 440 feet• Instrumentation
– Inclinometers: seven from 190 to 430 feet
– VW piezometers: seven 190 to 360 feet
– Magnetic extensometers: four from 250 to 440 feet
• Laboratory testing program (consolidation, strength)• Existing test borings, CPTs, instrumentation and
laboratory test data
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Vertical Inclinometer
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Vibrating Wire Piezometer
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Magnetic Extensometer
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Instrumentation Cluster at Landfill Crest
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Shear Strength Plot (typ.)
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Typical Stability Analysis Cross-Section
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Actual Hydrologic Conditions
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Stability Analysis Conclusions
• Per NJDEP minimum factor of safety (Fs) of 1.25during filling. Target Fs was 1.3 (deflection control)
• Insufficient time to deterministically quantify therelationship between Fs and filling rate
• Easily measured field parameters (pore pressure,settlement, lateral movements) were used aspredictors of safe soil response (ObservationalMethod)
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Typical Stability Failure Surface
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Observational Method• A continuous, managed and integrated process of
design, construction control, monitoring and reviewenabling appropriate, previously-definedmodifications to be incorporated duringconstruction
• Selection (in advance) of a course of action ordesign modification for every foreseeablesignificant deviation of the observational findingsfrom those predicted based on the workinghypothesis
R. Peck, 1969 (9th Rankine Lecture)
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Observational Parameters• Ratio of settlement in clay to lateral movements in
clay (extensometers & inclinometers)
• Pore pressure in clay (VW piezometers)• Total and incremental lateral movements
(inclinometers)• Lateral movements at toe
C. Ladd - 1991
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Observational Method• Settlement of clay stratum below crest vs. maximum lateral deformation
at toe (i.e., Slope of vertical strain (extensometer) versus maximumhorizontal strain (inclinometer) at toe should be less than 35% to 40% ).
0
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Fill
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Vertical Displacement
Allowable Limit X vs Y @ 46 ft Fill Height
FILL
LIMITING SLOPE@ 35%
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Observational Method• Increase in pore pressure proportional to loading (if exceeds
80% of applied load then critically review all data).
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Observational Method• Rate of incremental movement from two adjacent 2’ depth intervals should not
exceed 0.04%/month in upper 30 feet and 0.02%/month in lower varved clay.
CLAY
MSW
CLAY
MSW
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Stockpile at Landfill Crest
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Observational Method• Review site conditions for excess movements along the toe.
PEAT/ORGANICS
PEAT/ORGANICS
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Results
• Data interpretation, computer simulations andobservational performance criteria were used tosuccessfully manage fill placement
• Filling and final closure were performed safely andon-time
• Observational method worked, but:– You need to know in advance what your response will
be if the observed behavior is different from expected
– The client needs to buy in
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Preliminary Impressions - Revisited
• MSW is inherently stable– Yes and responds elastically
• One groundwater regime– No, perched leachate in LFL, regional & excess PWP in
clay
• All failure surfaces passed through the foundation(clay) soils
– Yes
• Critical failure mode is block– No, critical failure mode was circular
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Miscellaneous Observations
• 450 ft long inclinometer cable is heavy
• Corrosive and heat resistant instruments(e.g., tungsten piezometers)
• PDM is corrosive, dries rapidly and is problematic
• Data management is harder than data gathering
• Real-time field data interpretation is critical
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Finished Product