Presentation on theme: "Landfill Cover Design Jae K. (Jim) Park, Professor Dept. of Civil and Environmental Engineering University of Wisconsin-Madison."— Presentation transcript:
1 Landfill Cover DesignJae K. (Jim) Park, Professor Dept. of Civil and Environmental Engineering University of Wisconsin-Madison
2 Landfill Cover Design Design considerations Cost Estimation of percolationquantitiesErosion and slopestability concernsSelection oflandfill covercomponentsSelection of covermaterials, slope,and thicknessEstimation ofrunoff quantitiesonto adjacent landsLong-termdurabilityMain design consideration: minimize leachate production during filling of a landfill and after closure of a landfill, and collect landfill gas for beneficial use
3 Role of Cover Components Vegetative soil cover: reduces infiltration and wind erosion, and provides rootzone and temporary moisture retention Filter layer: prevents sifting of overlying cover soil into the drainage layer; cohesionless soil or geotextiles; d15 (filter)/d85 (drainage layer) < 4~5 Drainage layer: provides a lateral path for water to exit rapidly Clay layer: minimizes infiltration through the cover Gas collection layer: collect landfill gas Cover layer: cover refuse
4 Problems with Clay Liner Difficulty of compacting on a soft foundationCracks due to desiccationDamage by freezingCracks due to differential settlingDifficult to repair once damagedInability of restricting the movement of landfill gasSolution to problems associated with clay linerUse one or more geomembranes** Sheet-like structures, which are commonly used in environmental and water protection applications
5 Landfiil Cover Type ICan be replaced with geotextile
13 Landfill Cover Objectives To operate with a min. postclosure maintenanceTo allow the site to be returned to some beneficial use as quickly as possibleTo make the site aesthetically acceptable to nearby residentsTo accommodate settlementTo prevent the blowing of litter or dust onto adjacent propertiesTo suppress fire dangersTo contain gases and vaporsTo allow placement as each cell is completedTo be water- and erosion-resistantTo be stable against slumping, cracking, and slope failureTo be resistant to cold weather distress and to disruption by animals and plants
14 Components of the Hydrologic Cycle (1) Interception andevapotranspirationby vegetationPrecipitationSurfacerunoffInfiltrationCover materialChange inwater contentRefusePercolationInterception: by vegetation; function of the season of the year, vegetative species, the age and density of the vegetation, and the character of the storm.
15 Components of the Hydrologic Cycle (2) Evapotranspiration: determined based on empirical data or pan-evaporation data; function of solar radiation, differences in vapor pressure between a water surface and the overlying air, temperature, wind, atmospheric pressure, and type of vegetation; major factor in determining leachate generationLysimeter: consists of an upright vegetation soil cell so designed that input water and water percolating out the bottom can be accurately measured. The difference is evapotranspiration.Adjusted pan-evaporation: evaporation from a typical pan 4 ft diameter and 10 inch deep times a pan coefficient (0.7)Thornthwaite method: determine the annual heat index, use a table to determine daily potential evapotranspiration (PE), and adjust the PE for month and day lengths with correction factors (Chapter 7 Water Balance)
16 Components of the Hydrologic Cycle (3) Infiltration: penetrates the ground surface and clarify into the soil by gravitational and capillary forces; function of porosity (soil density, grain size and shape, organic content, etc.), initial moisture content, slope and degree of surface deformation, raindrop size and impact velocity, rainfall intensity and duration, inwash of fine materials, vegetation, and temperature
17 Infiltrometer (ASTM D 5093 - 90) The infiltration rate of water through soil is measured using a double-ring infiltrometer with a sealed or covered inner ring. The infiltrometer consists of an open outer and a sealed inner ring. The rings are embedded and sealed in trenches excavated in the soil. Both rings are filled with water such that the inner ring is submerged.The rate of flow is measured by connecting a flexible bag filled with a known weight of water to a port on the inner ring. As water infiltrates into the ground from the inner ring, an equal amount of water flows into the inner ring from the flexible bag. After a known interval of time, the flexible bag is removed and weighed. The weight loss, converted to volume, is equal to the amount of water that has infiltrated into the ground. An infiltration rate is then determined from this volume of water, the area of the inner ring, and the interval of time. This process is repeated and a plot of infiltration rate versus time is constructed. The test if continued until the infiltration rate becomes steady or until it becomes equal to or less than a specified value.
19 Components of the Hydrologic Cycle (4) *07/16/96Components of the Hydrologic Cycle (4)Surface Runoff: develops after the initial demand of interception, infiltration, and surface storage have been satisfied; function of surface slope, antecedent moisture conditions, and vegetative cover; Soil Conservation Services (SCS) methodology is frequently used.S = potential max. soil moisture retention after runoff beginsSEx. 10 inches of sand with 30% voids; rainfall, P of 8 in.S = 10 × 0.3 = 3 in.CN = 1,000/(10+3)= 77Q (direct runoff)= 5.3 in.*
21 Soil Groups Average runoff condition and initial abstraction of 0.2 S Infiltration rate, in/hr, Ic0.3~0.40.15~0.30.05~0.150~0.05Transmission rate, cm/hr, rt≥0.760.38~0.760.13~0.38< 0.13Average runoff condition and initial abstraction of 0.2 SPoor: < 50% ground cover; Fair: 50~75% ground cover; Good: > 75% ground coverIf CN is < 30, use CN = 30 for runoff computationsCN: min. 30 and max. 100
22 USDA Textural Types Corresponding to USCS Soil Designations GW Same as GP (except well graded in grain sizes)GP Gravel, very gravelly sand (less than 5 percent silt and clay)GM Very gravelly sandy loan, very gravelly loamy sand, very gravelly silt loam, very gravelly loamGC Very gravelly clay loam, very gravelly sandy clay loam, very gravelly silty clay loam, very gravelly silty clay, very gravelly claySW Same as SP (except well graded in grain sizes)SP Sand, gravelly sand (less than 20 percent very fine sand)SM Loamy sand, sandy loam; sand; gravelly loamy sand and gravelly sandy loamSC Sandy clay loam, sandy clay; gravelly sandy clay loam and gravelly sandy clayML Silt, silt loam, loam, sandy loamCL Silty clay loam, clay loam, sandy clayOL Mucky silt loam, mucky loam, mucky silty clay loam, mucky clay loamMH Silt, silt loam (highly elastic, micaceous or diatomaceous)CH Silty clay, clayCH Mucky silty clayPT Muck, peatUSCS: Unified Soil Classification System; G: gravel, S: Sand, M: Silt, C: Clay, O: OrganicP: Poorly graded (uniform particle), W: Well graded, H: High plasticity, L: Low plasticity
23 Estimated Unit Costs for Various Cover Layers (Source: RMT, Inc.) Layer Type and Thickness Installed Cost, dollars/yd2Loose soil (2 ft) 0.35Compacted soil (2 ft) 0.70Cement concrete (4 in.) 9.00Asphalt concrete (4 in.) 2.50~3.50Soil-cement (7 in.) 1.50Soil-asphalt 1.50Polyethylene membrane (10 mil)* 1.00~1.50Polyvinyl chloride membrane (20 mil) 1.30~2.00Chlorinated polyethylene membrane (20-30 mil) 2.40~3.20Hypalon membrane (20 mil) 2.50Neoprene membrane 5.00Ethylene propylene rubber membrane 2.70~3.50Butyl rubber membrane 2.70~3.80Paving asphalt 1.20~1.70Sprayed.asphalt membrane (1/4 in.) and soil cover 1.25~1.75Reinforced asphalt membrane (100 mil) and soil cover 1.50~2.00Bentonite layer (2 in.) 1.40Bentonite admixture (9 lb/yd2) in soil 0.75* Not recommended because of thinness
24 Components of the Hydrologic Cycle (5) Caution: Since a daily time increment is used in the hydrologic water balance, a short-duration intensive storm is uniformly distributed over the 24-hr period, resulting in decreased rainfall intensity → more infiltration and less runoff Subsurface Lateral Flow: occurs when infiltrated rainfall meets an underground zone of low transmission (the clay layer) and is diverted laterally travels to either (1) external surface of the landfill or (2) the drainage collection tiles. Runoff Velocity and Time of Concentration (tc) (time required for the most remote location in the drainage catchment to contribute to a point of interest, such as a culvert under a road near the landfill site): estimated for different slope and surface covers, using Manning’s equation.
25 Components of the Hydrologic Cycle (6) Time of Concentration (tc) (min) (Kirpich formula) tc = L0.77 S where L = max. length of flow (ft) and S = watershed gradient (ft/ft) Rational Method: widely used for the design of storm sewers Q (ft3/sec) = CiA where C = dimensionless runoff coefficient [0.1 (sandy loam) ~ 0.6 (tight clay)], i = rainfall intensity (in/hr), and A = contributing drainage area (in acres) Assumptions: 1. The rainfall is assumed to occur at a uniform intensity over the entire watershed. 2. The rainfall occurs at a uniform intensity for a duration equal to the time of concentration. 3. The frequency of the runoff equals that of the rainfall used in the eq. 4. The runoff coefficient is the same for all storm events.
26 Average Velocities for Estimating Travel Time for Overland Sheet Flow Ex. Determine the runoff velocity in a landfill cover at a 5% slope over a grassy surface and the travel time for a distance of 1,000 ft.tc = L0.77 S-0.385== 5.0 min3.3
27 Components of the Hydrologic Cycle (7) Flow Recurrence Calculation: Estimation of flows associated with specific recurrence intervals requires utilization of tc and the intensity-duration-frequency (IDF) curves specific to a particular region.Chicago, ILEx. A landfill with tc of 20 min. and a storm with a recurrence interval of 5 yrs. Estimate rainfall.Precipitation intensity = 3.6 in/hr
28 Design Factors in Infiltration/Percolation Control (1) Material Selection: Based on permeability; subject to availability and cost; soil or geomembranesCompaction: Effective in reducing infiltration/percolation but additional equipment cost; poor compaction when soil pore water is frozenSoil Layering: Lower layers act to impede percolation while upper layers support vegetation, provide erosion protection, and help retain capillary water in the lower layersThickness: Increases water storage capacity and reduces detrimental effects of cracks and settlement; min. cover thickness > 2 R, where R is relief, defined as the vertical distance from the high point to the low point of irregularities on the top surface of the solid waste; governed by coverage, gas migration, infiltration, trafficability and support requirements, and freeze/thaw or dry/soak effects
29 Design Factors in Infiltration/Percolation Control (2) Differential Settlement: Creates depressions detaining runoff, keep bulky objects away from upper part of waste; settlement arises because of the following factors:A reduction in void space and compression of loose materialsVolume changes from biological decomposition and chemical reactionLoss of volume due to dissolution into leachateMovement of smaller particles into larger voidsSettlement of underlying soil materials beneath the landfillMaintain a 5% slope toward the edge; ave. settlement - 11% of the overall depth (max. 30%); major settlement within a yearDiscontinuities and Surface Slope: If the refuse depth is variable, cracks and surface ponding will occur. A steeper slope increases surface runoff. At < 3%, surface irregularities act as traps. 5% is best. At steep slopes, an assessment of slope stability is necessary.Surface Drainage: Assist in conveying the water off the landfill
30 Final and Daily Cover Materials Both soil and non-soil materials are used.Example non-soil cover alternativesBituminous concrete or mortar, bitumen-sulfur concrete, sprayed bituminous membranes, sprayed sulfur membrane, polyurethane foam, pre-fabricated bituminous membrane, plastic and rubber membrane, fly ash, bottom ash and slag, incinerator residue, mill tailings, plant sludges, mulched leaves, asphalt, industrial wastes (foundry sand and paper mill sludge) etc.Geomembranes are typically recommended at 40 to 60 mil thickness. A soil buffer or geotextile must be provided above and below the geomembrane for protection.
34 Vegetative Growth and Surface Preparation Completed landfills are being developed to include parks, golf courses, nature areas, and bicycle path.Vegetation: select appropriate speciesTop soil: medium-textured soils, e.g., loam soils; test for pH, Mg, Ca, P, NO3, NH4, K, Cu, Fe, Zn, Mn, conductivity, particle-size distribution, bulk density, and organic matterTop soil thickness: grasses - 24~30 in.; shrubs - 36~42 in.Application of seed and mulch
35 Gas Control A gas collection layer is required. A low permeable layer of 2 ft of clay and a 20 mil flexible membrane liner, a surface water drainage layer, and a cover layer capable of supporting vegetationActive or passive collection systemRequire a coarse particle-size gradationRequire a min. slope in the collector pipe of 2% (2~5%)Vented gasRiserVegetationFinal coverDrainage layerCompacted soilPerforated lateralGravel
36 Maintenance ProblemsDifficulty of Maintaining Vegetation: high rate of vegetation die-off due to low moisture, elevated CO2 and CH4 conc.; gas barrier systemsA soil trench underlain with plastic sheeting over gravel and vented by means of vertical PVC pipeA 0.9 m soil mound underlain with 30 cm of clayA 0.9 m soil mound with no clay barrierEnvironmental Soil Conditions: freezing/thawing, wetting/drying, root penetration, and burrowing animalsErosion due to Excessive Velocities: < 0.75 ~ 2.4 m/sec (2.5 ~ 8 ft/sec) depending on type of vegetation or coverOff-Site Flows: post-development runoff level predevelopment level
37 A (tons/acre/yr) = R K L S C P Soil Loss EstimationUniversal Soil Loss EquationA (tons/acre/yr) = R K L S C PRainfall factor, R: function of kinetic energy of a storm and its intensity; Table 8.15Soil erodibility factor, K: function of physical/chemical properties of soil; Table 8.16Slope-length factor, L: Figure 8.14Slope-gradient factor, S: Figure 8.14Cropping-management factor, C: ratio of soil loss from land cropped under particular conditions to that from continuously fallowed land; Table 8.17Erosion control practice factor, P: Table 8.180 < A < 5: frequently considered as an acceptable loss/yr5 < A < 20: sedimentation retention requiredA > 20: design changes required (terraces or slope/depth changes)
38 ExampleCalculate the soil loss for Madison, Wisconsin of 5 acres. Design slope = 14% & 200 ft long; 65% silt & 35% sand with slow to moderate permeability; bare of vegetationSolutionR = 77; K = 0.3; LS = 3.3; C = 1 (Initial); C = (First yr); P = 1.Initial :A = 77 0.3 3.3 1 1 = ton/acre/yrFirst yr:A = 77 0.3 3.3 0.05 1 = 3.81 ton/acre/yr
39 Determination of Rainfall Factor R from Table 8.15 *07/16/96Determination of Rainfall Factor R from Table 8.1510 year Storm for Madison WisconsinR = 77*
41 Determination of Soil Erodibility Factor K from Table 8.16 65% silt and 35% sand with slow to moderate permeabilityK = 0.3
42 Organic Matter Content K FactorK Factor Data Organic Matter Content Textural Class Average Less than 2 % More than 2 % Clay0.220.240.21 Clay Loam0.300.330.28 Coarse Sandy Loam0.07-- Fine Sand0.080.090.06 Fine Sandy Loam0.180.17 Heavy Clay0.190.15 Loam0.340.26 Loamy Fine Sand0.11 Loamy Sand0.040.05 Loamy Very Fine Sand0.390.440.25 Sand0.020.030.01 Sandy Clay Loam0.20 Sandy Loam0.130.140.12 Silt Loam0.380.410.37 Silty Clay0.27 Silty Clay Loam0.320.35 Very Fine Sand0.430.46 Very Fine Sandy Loam
43 Determination of L (Slope-Length Factor) S (Slope-Gradient Factor) 14% slope and 200 ft long3.3Figure 8.14
44 Equation for Calculation of LS S = slope steepness, %;LS = slope length, ft;C = constant (72.5 Imperial or 22.1 metric); andNN = see table below
45 Determination of Cropping-Management Factor C from Table 8.17 Initial =1.0Newly seeded 1st year= 0.05
46 Determination of Erosion Control Practice Factor P from Table 8.18 Erosion Control Practice P valueSurface condition with noCover; compact, smooth, 1.30ScrapedLandfill surface 1.00Rough irregular surface;Equipment (tracks in all 0.90directions)Small sediment basins (1 0.50Basin for 4 acres)
47 Soil Loss Tolerance Rates Soil Erosion Class Potential Soil Loss (tons/acre/year) Very low (tolerable) < 3 Low 3 ~5 Moderate 5 ~10 High 10 ~ 15 Severe > 15
48 Management Strategies to Reduce Soil Losses Factor Management Strategies ExampleR Cannot be altered. --K Cannot be altered. --Terraces may be constructed to Terracing requires additionalLS reduce the slope length resulting investment. Investigate otherin lower soil losses. Soil conservation practices first.The selection of crop types and Consider cropping systems thatC tillage methods that result in the will provide maximum protectionlowest possible C factor will for the soil. Use minimum tillageresult in less soil erosion. Systems where possibleThe selection of a support practice Use support practices such asP that has the lowest possible factor cross slope farming that will causeassociated with it will result in deposition of sediment to occurlower soil losses. close to the source.
49 Cross Slope FarmingTilling and planting across the natural slope creates a series of dams which redirect and slow runoff.This allows water to soak into the ground or to flow gently between the rows to grassed headlands at the edge of the field or to grassed waterways within the field.On short slopes, cross-slope farming can reduce erosion by up to 50%.
50 Designing for Water Erosion Control *07/16/96Designing for Water Erosion ControlSelection of erosion resistant soils (See Table 8.16)Dispersive clay and dispersantsTry to hold overall top slope 5%Drainage features –stabilized with vegetation, additives, linings, or bermsExternal runoff diversionSide slope protectionFavorable cropping practicesMulch application to prevent bare soilSmoothing and compactionAdditives: most economical treatment – straw-mulchWhen the ratio of sodium to other ions at these exchange sites is high, clay particles are less tightly bound to each other and the soil aggregates easily disperse when the soil becomes wet.*
55 Wind Erosion Effects (1) Erosion Loss Model A’ (tons/acre/yr) = f(K’,C’,L’,T’,V’) Soil erodibility, K’: reflects the nature of the soil and an adjustment for knoll or hill configuration; determined from the product of the soil erodibility factor from Figure 8.16 Climate factor, C’: combines wind velocity and near-surface water content; Figure 8.17 Field length factor, L’: unsheltered distance along the direction of prevailing wind erosion; Figure 8.18 Soil ridge roughness factor, T’: reflects surface roughness beyond what is caused by clods or vegetation; determined by making amplitude measurements from crest to trough on the ground, as indicated in Figure 8.19 Vegetative cover quantity, V’: combines type and orientation effects with the tons per acre of vegetation cover; Figure 8.20
56 Determine Soil Erodibility Erodibility that would occur from a wide, isolated, smooth, unsheltered, bare field having a calculated % of dry aggregate > 0.84 mm in diameter.Fraction > 0.84 mm: 20%Knoll slope (from top of knoll): 5%K’ = 100 ton/acreFigure 8.16 Wind Erosion versus Percent Coarse Fraction
57 Knoll Adjustment (a) from Top of Knoll and (b) from Upper Third of Slope Figure 8.21
58 Determine Soil Ridge Roughness Recommended roughness factor = 2.5~4.5”Soil roughness factor = 4.5 inchesRoughness factor: a measure of the surface roughness other than caused by clods or vegetationT’ = 0.5A2’ = 250 × 0.5 = 125H = ridge height, cm;S = ridge spacing, cmFigure Soil Ridge Roughness Factor T’ from Actual Soil Ridge Roughness
59 Determine Climate Factor in March, Madison, Wisconsin Figure Wind Erosion Climate Factor C’ in Percent
60 4. Determine Field Length Effect A3’ = 25Given:L’ = 1,000 ftA2’ = 125; A3’ = 25A2’ = 125A4’ = 22Move this scale to match A2’ = 125 in the main graph with A3’ = 25 in the scaleL’ = 1,000Figure 8.18
61 5. Determine Vegetative Cover and Annual Erosion 800 lb/acre actual flat residueV’ = 2,500 lb/acreFigure 8.20 Relationship between Vegetative Cover Quantity V’ and Type Vegetative Cover
63 Wind Erosion Effect (2)Design Aspects 1. Determine from Figure 8.16 an erodibility increment, A1’ = K’; adjust K’ for knoll configuration as necessary using Figure Account for the effect of roughness, T, from Figure 8.19 and determine the erodibility increment, A2’ = A1’ · T’ 3. Account for the effect of local wind velocity and surface soil moisture, C’, from Figure 8.17 and find the erodibility increment, A3’ = A2’·C’ 4. Account for the effect of length of field, L’, and determine A4’ = A3’ · L’ 5. Account for vegetative cover in V’ (Figure 8.22) and determine the annual erosion as A5’ = A4’f(V’), as per Figure 8.20
64 Off-Site Runoff Control Construction of a landfill results in large increases in off-site flows and sediments.Types of off-site flow control:Swales and storm-water recharge pondsDetention basin design stepsDetermine the peak flow for pre-existing conditions for a specified recurrence interval.Construct a hydrograph for the design storm for the completed landfill site arriving at the detention pond.Calculate the storage needed for major storms such as the 25-year and 100-year storms.Check the performance of the basin during the more frequent storms, such as the 5- and 10-year storms.