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Landfill Cover Design 1. Design considerations Selection of landfill cover components Cost Erosion and slope stability concerns Selection of cover materials,

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Presentation on theme: "Landfill Cover Design 1. Design considerations Selection of landfill cover components Cost Erosion and slope stability concerns Selection of cover materials,"— Presentation transcript:

1 Landfill Cover Design 1

2 Design considerations Selection of landfill cover components Cost Erosion and slope stability concerns Selection of cover materials, slope, and thickness Long-term durability Estimation of runoff quantities onto adjacent lands Estimation of percolation quantities Main design consideration: minimize leachate production during filling of a landfill and after closure of a landfill, and collect landfill gas for beneficial use 2

3 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; d 15 (filter)/d 85 (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 3

4  Difficulty of compacting on a soft foundation  Cracks due to desiccation  Damage by freezing  Cracks due to differential settling  Difficult to repair once damaged  Inability of restricting the movement of landfill gas Solution to problems associated with clay liner  Use one or more geomembranes* 4 * Sheet-like structures, which are commonly used in environmental and water protection applications

5 5 Can be replaced with geotextile

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10 10 Waste Gas venting Clay Geotextile Gravel Cover soil Top soil Geotextile

11 11 Waste Geocomposite for Gas venting Geocomposite for drainage Geomembrane Cover soil Top soil

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13  To operate with a min. postclosure maintenance  To allow the site to be returned to some beneficial use as quickly as possible  To make the site aesthetically acceptable to nearby residents  To accommodate settlement  To prevent the blowing of litter or dust onto adjacent properties  To suppress fire dangers  To contain gases and vapors  To allow placement as each cell is completed  To be water- and erosion-resistant  To be stable against slumping, cracking, and slope failure  To be resistant to cold weather distress and to disruption by animals and plants 13

14 Refuse Interception : by vegetation; function of the season of the year, vegetative species, the age and density of the vegetation, and the character of the storm. Precipitation Infiltration Surface runoff Cover material Change in water content Percolation Interception and evapotranspiration by vegetation 14

15 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 generation Lysimeter: 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) 15

16 16 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 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. 17

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19 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.Soil Conservation Services (SCS) methodology 19 S =potential max. soil moisture retention after runoff begins Ex. 10 inches of sand with 30% voids; rainfall, P of 8 in. S = 10 × 0.3 = 3 in. CN= 1,000/(10+3) = 77 Q (direct runoff) = 5.3 in. S

20 20 Reference:

21 21 (1)Average runoff condition and initial abstraction of 0.2 S (2)Poor: 75% ground cover (3)If CN is < 30, use CN = 30 for runoff computations (4)CN: min. 30 and max. 100 Infiltration rate, in/hr, I c 0.3~0.4 0.15~0.3 0.05~0.15 0~0.05 Transmission rate, cm/hr, r t ≥0.76 0.38~0.76 0.13~0.38 < 0.13

22 USCS: Unified Soil Classification System; G: gravel, S: Sand, M: Silt, C: Clay, O: Organic P: Poorly graded (uniform particle), W: Well graded, H: High plasticity, L: Low plasticity 22 GWSame as GP (except well graded in grain sizes) GPGravel, very gravelly sand (less than 5 percent silt and clay) GMVery gravelly sandy loan, very gravelly loamy sand, very gravelly silt loam, very gravelly loam GCVery gravelly clay loam, very gravelly sandy clay loam, very gravelly silty clay loam, very gravelly silty clay, very gravelly clay SWSame as SP (except well graded in grain sizes) SPSand, gravelly sand (less than 20 percent very fine sand) SMLoamy sand, sandy loam; sand; gravelly loamy sand and gravelly sandy loam SCSandy clay loam, sandy clay; gravelly sandy clay loam and gravelly sandy clay MLSilt, silt loam, loam, sandy loam CLSilty clay loam, clay loam, sandy clay OLMucky silt loam, mucky loam, mucky silty clay loam, mucky clay loam MHSilt, silt loam (highly elastic, micaceous or diatomaceous) CHSilty clay, clay CHMucky silty clay PTMuck, peat

23 * Not recommended because of thinness 23 Layer Type and ThicknessInstalled Cost, dollars/yd 2 Loose soil (2 ft)0.35 Compacted soil (2 ft)0.70 Cement concrete (4 in.)9.00 Asphalt concrete (4 in.)2.50~3.50 Soil-cement (7 in.)1.50 Soil-asphalt1.50 Polyethylene membrane (10 mil) * 1.00~1.50 Polyvinyl chloride membrane (20 mil)1.30~2.00 Chlorinated polyethylene membrane (20-30 mil)2.40~3.20 Hypalon membrane (20 mil)2.50 Neoprene membrane5.00 Ethylene propylene rubber membrane2.70~3.50 Butyl rubber membrane2.70~3.80 Paving asphalt1.20~1.70 Sprayed.asphalt membrane (1/4 in.) and soil cover1.25~1.75 Reinforced asphalt membrane (100 mil) and soil cover1.50~2.00 Bentonite layer (2 in.)1.40 Bentonite admixture (9 lb/yd 2 ) in soil0.75

24 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 (t c ) (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. 24

25 Time of Concentration (t c ) (min) (Kirpich formula) t c = 0.0078 L 0.77 S -0.385 where L = max. length of flow (ft) and S = watershed gradient (ft/ft) Rational Method: widely used for the design of storm sewers Q (ft 3 /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. 25

26 t c = 0.0078 L 0.77 S -0.385 = 0.0078 1000 0.77 0.05 -0.385 = 5.0 min 26 3.3 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.

27 Flow Recurrence Calculation : Estimation of flows associated with specific recurrence intervals requires utilization of t c and the intensity-duration-frequency (IDF) curves specific to a particular region. Chicago, IL Ex. A landfill with t c of 20 min. and a storm with a recurrence interval of 5 yrs. Estimate rainfall. Precipitation intensity = 3.6 in/hr 27

28 Material Selection: Based on permeability; subject to availability and cost; soil or geomembranes Compaction: Effective in reducing infiltration/percolation but additional equipment cost; poor compaction when soil pore water is frozen Soil Layering: Lower layers act to impede percolation while upper layers support vegetation, provide erosion protection, and help retain capillary water in the lower layers Thickness: 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 28

29 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 materials  Volume changes from biological decomposition and chemical reaction  Loss of volume due to dissolution into leachate  Movement of smaller particles into larger voids  Settlement of underlying soil materials beneath the landfill Maintain a 5% slope toward the edge; ave. settlement - 11% of the overall depth (max. 30%); major settlement within a year Discontinuities 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 29

30 Both soil and non-soil materials are used. Example non-soil cover alternatives Bituminous 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. 30

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34  Completed landfills are being developed to include parks, golf courses, nature areas, and bicycle path.  Vegetation: select appropriate species  Top soil: medium-textured soils, e.g., loam soils; test for pH, Mg, Ca, P, NO 3, NH 4, K, Cu, Fe, Zn, Mn, conductivity, particle-size distribution, bulk density, and organic matter  Top soil thickness: grasses - 24~30 in.; shrubs - 36~42 in.  Application of seed and mulch 34

35  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 vegetation  Active or passive collection system  Require a coarse particle-size gradation  Require a min. slope in the collector pipe of 2% (2~5%) Gravel Riser Vented gas Vegetation 35 Drainage layer Final cover Compacted soil Perforated lateral

36  Difficulty of Maintaining Vegetation: high rate of vegetation die-off due to low moisture, elevated CO 2 and CH 4 conc.; gas barrier systems  A soil trench underlain with plastic sheeting over gravel and vented by means of vertical PVC pipe  A 0.9 m soil mound underlain with 30 cm of clay  A 0.9 m soil mound with no clay barrier  Environmental Soil Conditions: freezing/thawing, wetting/drying, root penetration, and burrowing animals  Erosion due to Excessive Velocities: < 0.75 ~ 2.4 m/sec (2.5 ~ 8 ft/sec) depending on type of vegetation or cover  Off-Site Flows: post-development runoff level  predevelopment level 36

37 Universal Soil Loss Equation A (tons/acre/yr) = R K L S C P Rainfall factor, R: function of kinetic energy of a storm and its intensity; Table 8.15 Soil erodibility factor, K: function of physical/chemical properties of soil; Table 8.16 Slope-length factor, L: Figure 8.14 Slope-gradient factor, S: Figure 8.14 Cropping-management factor, C: ratio of soil loss from land cropped under particular conditions to that from continuously fallowed land; Table 8.17 Erosion control practice factor, P: Table 8.18 0 < A < 5: frequently considered as an acceptable loss/yr 5 < A < 20: sedimentation retention required A > 20: design changes required (terraces or slope/depth changes) 37

38  Calculate 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 vegetation Solution R = 77; K = 0.3; LS = 3.3; C = 1 (Initial); C = 0.05 (First yr); P = 1. Initial : A = 77  0.3  3.3  1  1 = 76.23 ton/acre/yr First yr: A = 77  0.3  3.3  0.05  1 = 3.81 ton/acre/yr 38

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41 K = 0.3 41

42 K 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--0.07 Fine Sand0.080.090.06 Fine Sandy Loam0.180.220.17 Heavy Clay0.170.190.15 Loam0.300.340.26 Loamy Fine Sand0.110.150.09 Loamy Sand0.040.050.04 Loamy Very Fine Sand 0.390.440.25 Sand0.020.030.01 Sandy Clay Loam0.20--0.20 Sandy Loam0.130.140.12 Silt Loam0.380.410.37 Silty Clay0.260.270.26 Silty Clay Loam0.320.350.30 Very Fine Sand0.430.460.37 Very Fine Sandy Loam 0.350.410.33 42

43 14% slope and 200 ft long Figure 8.14 3.3 43

44 S = slope steepness, %; L S = slope length, ft; C = constant (72.5 Imperial or 22.1 metric); and NN = see table below 44

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46 46 Erosion Control Practice P value Surface condition with no Cover; compact, smooth,1.30 Scraped Landfill surface1.00 Rough irregular surface; Equipment (tracks in all0.90 directions) Small sediment basins (10.50 Basin for 4 acres)

47 47 Soil Erosion Class Potential Soil Loss (tons/acre/year) Very low (tolerable)< 3 Low3 ~5 Moderate5 ~10 High10 ~ 15 Severe> 15

48 48 Factor Management Strategies Example R Cannot be altered. -- KCannot be altered.-- Terraces may be constructed to Terracing requires additional LS reduce the slope length resulting investment. Investigate other in lower soil losses.Soil conservation practices first. The selection of crop types and Consider cropping systems that C tillage methods that result in the will provide maximum protection lowest possible C factor will for the soil. Use minimum tillage result in less soil erosion.Systems where possible The selection of a support practiceUse support practices such as P that has the lowest possible factor cross slope farming that will cause associated with it will result in deposition of sediment to occur lower soil losses. close to the source.

49  Tilling 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%. 49

50 1.Selection of erosion resistant soils (See Table 8.16) 2.Dispersive clay and dispersants 3.Try to hold overall top slope  5% 4.Drainage features –stabilized with vegetation, additives, linings, or berms 5.External runoff diversion 6.Side slope protection 7.Favorable cropping practices 8.Mulch application to prevent bare soil 9.Smoothing and compaction 10.Additives: most economical treatment – straw-mulch 50

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55 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 55

56 Fraction > 0.84 mm: 20% Knoll slope (from top of knoll): 5% K’ = 100 ton/acre Figure 8.16 Wind Erosion versus Percent Coarse Fraction Erodibility that would occur from a wide, isolated, smooth, unsheltered, bare field having a calculated % of dry aggregate > 0.84 mm in diameter. 56

57 A 1 ’ = 100 × 2.5 = 250 Figure 8.21 57

58 Recommended roughness factor = 2.5~4.5” Soil roughness factor = 4.5 inches T’ = 0.5 A 2 ’ = 250 × 0.5 = 125 Figure 8.19 Soil Ridge Roughness Factor T’ from Actual Soil Ridge Roughness Roughness factor: a measure of the surface roughness other than caused by clods or vegetation H = ridge height, cm; S = ridge spacing, cm 58

59 Figure 8.17 Wind Erosion Climate Factor C’ in Percent C’ = 0.2 A 3 ’ = 125 × 0.2 = 25 59

60 Given: L ’ = 1,000 ft A 2 ’ = 125; A 3 ’ = 25 A 4 ’ = 22 Figure 8.18 60 A 2 ’ = 125 Move this scale to match A 2 ’ = 125 in the main graph with A 3 ’ = 25 in the scale A 3 ’ = 25 L ’ = 1,000

61 Figure 8.20 Relationship between Vegetative Cover Quantity V’ and Type Vegetative Cover 800 lb/acre actual flat residue V ’ = 2,500 lb/acre 61

62 Figure 8.22 A 5 ’ (A) = 7 ton/acre/yr A 4 ’ = 22 62

63 Design Aspects 1.Determine from Figure 8.16 an erodibility increment, A 1 ’ = K’; adjust K’ for knoll configuration as necessary using Figure 8.21 2.Account for the effect of roughness, T, from Figure 8.19 and determine the erodibility increment, A 2 ’ = A 1 ’ · T’ 3.Account for the effect of local wind velocity and surface soil moisture, C’, from Figure 8.17 and find the erodibility increment, A 3 ’ = A 2 ’ · C’ 4.Account for the effect of length of field, L’, and determine A 4 ’ = A 3 ’ · L’ 5.Account for vegetative cover in V’ (Figure 8.22) and determine the annual erosion as A 5 ’ = A 4 ’f(V’), as per Figure 8.20 63

64  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 ponds  Detention basin design steps  Determine 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. 64

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