COST AND PERFORMANCE EVALUATION OF ALTERNATIVE FINAL COVERS

Transcription

1 COST AND PERFORMANCE EVALUATION OF ALTERNATIVE FINAL COVERS E. KAVAZANJIAN, JR.* AND J.G. DOBROWOLSKI** *Consulting Engineer, 7241 Seashark Circle, Huntington Beach, California, 92648, USA **GeoSyntec Consultants, Huntington Beach, California, 92648, USA SUMMARY: Alternative final covers evaluated for the Lebec Landfill included evapotranspirative, geomembrane, geosynthetic clay liner, and asphalt cement concrete configurations. These configurations were evaluated with respect to short and long term performance, compatibility with the post-closure use, regulatory and community acceptance, and cost. All of the configurations were deemed acceptable from a performance perspective. Though an evapotranspirative final cover constructed using on-site borrow soil was the most cost effective option, it was unacceptable based upon regulatory approval considerations. Community acceptance concerns due to aesthetic considerations ruled out an exposed geomembrane final cover. Therefore, the geosynthetic clay liner configuration was employed over most of the landfill. However, an asphalt cement concrete final cover was employed in areas designated for Transfer Station operations in the post-closure period 1. INTRODUCTION Due to concerns with respect to the performance of a low permeability soil barrier layer in the semi-arid climate at the Lebec Landfill, the lead regulatory agency suggested that the landfill owner, the Kern County Waste Management Department (KCWMD), consider employing an alternative final cover at the site. In response to this suggestion, KCWMD authorized an evaluation of the cost and performance of alternative final cover systems for the site. Candidate alternative final cover systems evaluated in the study included evapotranspirative, geomembrane, geosynthetic clay liner (GCL), and asphalt cement concrete (ACC) configurations. The Lebec Landfill is an unlined municipal solid waste (MSW) landfill located about 20 km north of Los Angeles. The landfill consists of a 1.1-hectare top deck, 3.7 hectares (projected) of side slope area, and a 0.8-hectare flat area used for transfer station operations at the base of the landfill. The landfill is approximately 30 m high with 3H:1V (Horizontal:Vertical) side slopes. Benches 4.6-wide at approximate intervals of 9 m (vertical) provide erosion control and access for maintenance. The Transfer Station was to be left in place following closure. Because the Transfer Station is on waste, areas around the transfer station, including areas paved to provide an all-weather access, are required to meet final cover performance standards. The base of the landfill is at approximate elevation 1200 m above mean sea level. The average annual precipitation at the site is 311 mm. Almost all of the precipitation falls in the 5-month autumn

2 and winter period from the beginning of November to the end of March. Less than 30 mm of the average annual precipitation falls as snow that rarely persists on the ground for more than one day. The mean daily temperature range is from 0 to 12 degree C in the winter and from 15 to 30 degrees C in the summer. The landfill sits at the back of a small east to west trending canyon and thus is generally sheltered from strong winds. The landfill is located approximately 2 km from the San Andreas fault, resulting in a design earthquake of Moment Magnitude 7.8 inducing a peak horizontal ground acceleration of over 0.8 g at the site. Following receipt of the last shipment of waste in 1991, on-site borrow soils were placed and compacted over the landfill to create an interim soil cover with a minimum thickness of 0.6 m. The interim cover soil is visually classified as sand with appreciable amounts of gravel and nonplastic fines (e.g., silty sand with gravel). Grain size analyses indicate that the existing interim cover soil contains between 13 and 21 percent by weight finer than the number 200 sieve (soil type SM (silty sand) in the Unified Soil Classification System). Constant head tests yielded a saturated hydraulic conductivity of from 2 x 10-4 cm/s to cm/s for the interim cover soil. Depth to groundwater at the site ranges from 14 to 29 m. Groundwater monitoring at the site showed no indication that percolation of leachate had impacted groundwater. Based upon their chemistry and the temporal patterns of their occurrences, groundwater impacts detected intermittently (once every three or four years) at levels below state and federal maximum contaminant levels for drinking water appeared clearly to be associated with landfill gas migration to groundwater and not to percolation of leachate. Vadose zone monitoring indicated that landfill gas migration is within the allowable limits of all applicable regulatory standards. There was no gas collection system in place at the landfill prior to closure. 2. REGULATORY BACKGROUND United States federal and Californ1a state regulations provide both a prescriptive final cover configuration and criteria for design of engineered alternatives to the prescriptive configuration. At the Lebec Landfill, the more stringent California state regulations governed the design of the final cover. The California prescriptive minimum final cover configuration for an unlined MSW landfill, illustrated in Figure 1, includes the following components, from top to bottom: an erosion control layer with a minimum thickness of 0.3 m; a compacted low-permeability soil barrier layer with a minimum thickness of 0.3 m and a maximum saturated hydraulic conductivity of cm/s; and a compacted soil foundation layer with a minimum thickness of 0.6 m. The erosion control layer must resist wind-scour, rainfall impact, and surface water runoff. Erosion resistance may be provided by either a vegetative layer or a mechanically resistant layer. California criteria for approval of engineered alternatives to the prescriptive minimum cover require a demonstration that the prescriptive standard is either unreasonable and unnecessarily burdensome and will cost substantially more than alternatives which meet final cover performance standards or is impractical and will not promote attainment of applicable performance standards. Applicable performance standards for the prescriptive and alternative final covers include: control of, at a minimum, vectors, fire, odor, liter and landfill gas migration. protection against water quality impairment (with an alternative cover shown to provide protection equivalent or superior to that of the prescriptive standard);

3 Figure 1. Prescriptive Final Cover Figure 2. Evapotranspirative Final Cover protection against wind-scour, raindrop impact, and surface water runoff (with an alternative cover shown to provide protection equivalent or superior to that of the prescriptive standard); a minimum of maintenance; and compatibility with post-closure land use. 3. ALTERNATIVE FINAL COVER CONFIGURATIONS 3.1 Evapotranspirative final cover system configuration The evapotranspirative final cover configuration, illustrated in Figure 2, consisted of a monolithic layer of on-site borrow soil capable of sustaining native vegetation. The thickness of the evapotranspirative final cover was established by water balance analyses (described in the next section), subject to a minimum thickness of 1.2 m for erosion control and waste isolation, established based upon the overall thickness of the prescriptive cover system. The use of on-site borrow soil was specified because of the cost of importing off-site soil with significantly better characteristics (i.e., a greater fines content and lower saturated hydraulic conductivity). 3.2 Geomembrane final cover system configurations Two different geomembrane final cover system configurations were considered: a conventional soil-covered geomembrane and an exposed geomembrane. The soil-covered geomembrane configuration, illustrated in Figure 3, consisted of, from top to bottom: a 0.6-m (minimum) layer of soil capable of sustaining vegetation; a geotextile cushion (deck and benches) or a drainage geocomposite (side slopes); a 0.9-mm geomembrane barrier layer; a geotextile cushion layer; and a 0.6-m (minimum) foundation layer of compacted soil.

4 Figure 3. Soil-Covered Geomembrane Cover Figure 4. Exposed Geomembrane Cover This cross section maintains the 1.2-m thickness established for the evapotranspirative cover for waste isolation and erosion protection. The soil-covered geomembrane final cover system was also employed on the top deck and benches for the exposed geomembrane configuration. The exposed geomembrane side slope configuration consisted of, from top to bottom: a 0.9-mm geomembrane barrier layer; and a 0.6-m foundation layer of compacted soil. The thickness of the exposed geomembrane side slope configuration, presented in Figure 4, is less than the 1.2 m thickness of the prescriptive cover. However, because the geomembrane provides a continuous physical barrier to intrusion and is not susceptible to erosion, the reduced thickness was considered adequate with respect to waste isolation and erosion control. 3.4 Geosynthetic clay liner final cover system configuration The GCL final cover system, illustrated in Figure 5, consisted of, from top to bottom: a 0.6 m (minimum) layer of soil capable of sustaining vegetation; a geocomposite drainage layer (on side slopes only); a needle-punched reinforced GCL; and a 0.6 m (minimum) foundation layer of compacted soil. The GCL cover maintains the minimum 1.2-m overall thickness of the prescriptive cover. 3.5 Asphalt cement concrete final cover system configuration The ACC final cover, illustrated in Figure 6, consists of, from top to bottom: a 5-cm thick ACC layer; a 135 g/m 2 nonwoven geotextile interlayer ; a 0.9 liters/m 2 mm asphalt emulsion tack coat ; and a suitable thickness of underlying ACC. ACC can have a low permeability (on the order of 1 x 10-7 cm/s) when placed. However, ACC is very susceptible to cracking. Therefore, low permeability ACC alone was not considered sufficient as a final cover with respect to infiltration control. Geotextile interlayers can inhibit cracking and reduce infiltration through cracks. A geotextile interlayer typically consists of a nonwoven fabric placed on top of an asphalt tack coat. When the hot mix ACC is placed on

5 Figure 5. Geosynthetic Clay Liner Cover Figure 6. Asphalt Cement Concrete Cover the fabric, the heat and pressure reactivates the tack coat, draws it up into the fabric, and bonds the fabric with the ACC. The result is an asphalt-impregnated reinforcing interlayer. Laboratory testing indicates that the asphalt-impregnated interlayer can attains a permeability on the order of cm/s with a tack coat application rate of approximately 0.90 liters/m 2 [Marienfield, 1998]. Therefore, the interlayer helps control infiltration by both inhibiting cracking through reinforcement and serving as a barrier layer. This cover provides less than the 1.2 m thickness of the prescriptive cover but was considered adequate due to the strength and durability of the ACC. 4. EVALUATION OF CANDIDATE ALTERNATIVE FINAL COVER SYSTEMS 4.1 Evapotranspirative cover system evaluation The primary technical issue associated with the evapotranspirative final cover is equivalence (to the prescriptive final cover) with respect to infiltration (i.e. groundwater protection). A water balance evaluation was conducted using an unsaturated flow model to evaluate the infiltration performance of the evapotranspirative final cover compared to that of the prescriptive final cover. Modeling of the prescriptive cover was performed with the conservative assumption that the low permeability barrier layer would not degrade over time due to desiccation or settlement. The unsaturated flow modeling was conducted using UNSAT-H v2.04 (UNSAT-H) [Fayer, et. al., 1997]. UNSAT-H has become the generally accepted standard of practice for modeling infiltration through earthen final covers in the United States [Benson, 1999]. Soil input parameters for UNSAT-H were determined from laboratory testing for the evapotranspirative cover and were assumed for the prescriptive cover based on typical values. The erosion layer of the prescriptive cover was classified as coarse gravel. The barrier layer of the prescriptive cover was classified as low plasticity clay. Precipitation data for modeling was based on rainfall data collected at Lebec from 1948 to Because the Lebec station only recorded precipitation, maximum and minimum daily temperature, wind speed, dew point temperature, and cloud cover were taken from the closest representative stations. Solar radiation data was stochastically generated for Lebec using the computer program HELP [Schroeder, et al., 1994]. Plant parameters for the evapotranspirative final cover were estimated based on the native vegetation. The rooting depth penetration was considered to be 0.9 m and the fraction of bare surface was considered to be The prescriptive final cover mechanically resistant erosion layer (i.e., the gravel) was assumed to be non-vegetated.

6 Evapotranspirative and prescriptive final cover performance was evaluated for the 10-year period from , the second wettest decade on record at Lebec for the period of record, with an average annual precipitation of 396 mm (85 mm/yr more than average). The wettest decade, 8 mm/yr more precipitation (on average), was not used because of missing daily data. Infiltration modeling included 1.2-m and 1.8-m thick evapotranspirative covers and the prescriptive cover (as the benchmark). Figure 7 presents the cumulative amount of water that percolates through these final cover alternatives for the 10-year modeling period. The modeling indicates that both the 1.2-m and 1.8-m evapotranspirative final covers are more effective than the prescriptive cover at controlling infiltration and that the performance of the 1.8-m evapotranspirative cover is only slightly superior to that of the 1.2-m evapotranspirative cover. Based upon the modeling and minimum thickness considerations (for erosion control and waste isolation), the recommended evapotranspirative cover configuration consisted of a 1.2 m of interim cover and on-site borrow soil compacted to achieve a saturated hydraulic conductivity of no greater than cm/s. As the existing interim cover is at least 0.6 m thick, this cover required placement of an additional 0.6 m of on-site borrow soil across the landfill. Because the existing interim cover appears to be adequately controlling landfill gas migration, no landfill gas control improvements were included in the evapotranspirative final cover configuration. Maintenance issues for the evapotranspirative final cover system include periodic placement of additional cover soil and re-grading due to landfill settlement and soil loss. Re-seeding and installation of silt fences to control erosion until the vegetation has been established were assumed to be part of the maintenance program. The anticipated level of effort for post-closure maintenance and monitoring of the evapotranspirative final cover was taken as the baseline for the alternative final cover systems evaluation. Only the difference between this baseline cost and the post-closure costs for the other alternatives (i.e., the incremental maintenance cost) was considered in the cost comparison among the alternatives. The construction cost estimate for the recommended 1.2-m thick evapotranspirative final cover was approximately $500,000, or $87,600 per hectare. This cost estimate includes excavation, hauling, placement, and compaction of the on-site borrow soil, hydroseeding the finished surface, and the installation of temporary silt fences during the vegetation establishment Figure 7. Infiltration Performance of the Evapotranspirative and Prescriptive Covers

7 period. Regulatory acceptance was a major issue for the evapotranspirative cover. At the time of this study, evapotranspirative covers had only been granted conditional approval in southern California, subject to two to five years of infiltration monitoring prior to consideration for final approval. An incremental post-closure cost of $200,000 was assigned to infiltration monitoring of the evapotranspirative final cover, including installation, monitoring, and reporting. 4.2 Geomembrane cover system evaluation Due to cost considerations, the soil-covered geomembrane final cover alternative was eliminated in preliminary screening, as it was more expensive than either the GCL or exposed geomembrane cover system and was not perceived to offer any significant advantages to these alternative systems. For the exposed geomembrane final cover system, the primary technical issues are: (i) wind uplift; (ii) puncture protection, and (iii) the need for landfill gas control measures. The performance of a geomembrane as an infiltration barrier was presumed to be acceptable without a formal demonstration. Wind uplift was evaluated using methods presented in Giroud et al. [1995] and Zornberg and Giroud [1997]. The analyses indicated that wind loading would not exceed the geomembrane tensile strength and that standard anchor trenches (0.6-m wide by 0.3-m deep) were adequate. The exposed geomembrane requires protection from puncture from hail, vehicular traffic, animals, and other external loads. Puncture due to hail was evaluated and found not to be a concern. Puncture protection was provided in areas of vehicular traffic by the cushion geotextiles. Perimeter fencing was assumed to provide adequate protection against vandals as well as animals. The stability of the exposed geomembrane is superior to that of alternatives with soil cover on the side slopes. While maintenance will be required for the soil cover on the deck and benches, the absence of a side slope soil veneer significantly reduces post-closure maintenance costs. Some repair and re-grading on the decks and benches and in areas of local slumping beneath the geomembrane were considered likely following the design earthquake. Therefore, the postearthquake repair cost was considered similar to that for the baseline evapotranspirative cover system. It was assumed that the side slope geomembrane would be replaced completely once over the 30-year post-closure period. Peak surface water runoff for the exposed geomembrane will be greater than for the evapotranspirative and GCL covers due to the concentrated run-off from the exposed cover on the side slopes. Therefore, some of the surface water management structures required for the exposed geomembrane cover will need to be larger than required for the evapotranspirative and GCL covers. The potential for landfill gas migration may be enhanced by placement of the geomembrane. Therefore, a landfill gas collection system may be required as part of closure with a geomembrane final cover. For the purpose of this alternatives study, a passive gas collection/venting system was assumed to be adequate for landfill gas control beneath the geomembrane cover. However, it was stipulated that the passive system be designed for easy conversion to active collection and the cost of this conversion was carried forward as a contingency cost with this (and some of the other) alternatives. Unquantifiable concerns with the exposed geomembrane cover include community acceptance and regulatory approval. To improve the aesthetics of an exposed geomembrane, the geomembrane can be colored to blend into the surrounding environment as much as possible. However, even if it were colored, an exposed geomembrane cover was deemed likely to be objectionable to some members of the community. Regulatory acceptance was an issue because an exposed geomembrane cover had never been permitted as a final cover for a California MSW

8 landfill. However, precedents in other states and discussions with regulatory officials led to a high level of confidence that regulatory approval could be obtained for an exposed geomembrane final cover. The estimated cost for the exposed geomembrane cover system is approximately $625,000 or $111,000 per hectare. This cost includes excavation, hauling, placement, and compaction of the on-site borrow soil for the 0.6 m of soil on the benches and top deck, installing the geomembrane, placing the cushion geotextiles on the top deck and benches, and excavating and backfilling anchor trenches. This estimate also includes erosion control and the perimeter fencing. The incremental cost for constructing surface water management structures was estimated at $40,000. The cost for installing a passive landfill gas control system was estimated as $50,000, with a contingent cost of $120,000 for conversion to active collection, including construction of an enclosed flare station. A $30,000 credit was applied to maintenance costs over the 30-year post-closure period to account for the absence of a side slope soil veneer. Replacement of the side slope geomembrane was estimated at $300,000 in year 15, discounted to $125,000 in current dollars. 4.3 Geosynthetic clay liner cover system evaluation The primary technical issues associated with the GCL final cover system are: (i) infiltration equivalence, (ii) stability of the landfill side slopes; (iii) damage from cycles of wetting and drying or freezing and thawing; (iv) chemical compatibility with final cover soils, and (v) the need for landfill gas control measures. Assuming a saturated hydraulic conductivity of cm/s for the GCL at an overburden pressure of 12 kpa, infiltration through the GCL is four times less than through the prescriptive cover for a head of 0.6 m, satisfying infiltration equivalence requirements. The stability analysis yielded a minimum static factor of safety (FS) of 1.7, assuming a saturated GCL, exceeding the FS of 1.5 commonly accepted for static conditions. Seismic displacements of the cover system in the design event are inevitable. In addition to re-grading of cover soil (also required for the evapotranspirative cover), it may be also necessary to replace the GCL over portions of the landfill. Therefore, the GCL final cover system was assigned an incremental post-closure cost of $100,000 (in current dollars) for post-closure earthquake repair. The maximum depth of frost penetration at the site is approximately 0.3 m, while the GCL will be covered by 0.6 m of soil. KCWMD recently conducted a field study of GCL durability at another landfill with similar climate conditions in which a GCL exhumed from shallow burial after 4 years of exposure showed no degradation in hydraulic properties (Mansour, 2001). Based upon these data, it was concluded that the resistance of the GCL cover to freeze/thaw and wetting/drying cycles was at least equal to, if not superior to, that of the prescriptive cover, satisfying the regulatory equivalence requirement. Soil mineralogy indicated insignificant amounts of calcium and magnesium in the on-site soils. Therefore, chemical compatibility was not considered a significant issue for the GCL cover system at this site. It was assumed that a passive landfill gas control system would be required as part of the GCL final cover, with a contingency for conversion to active gas control if necessary. The maintenance cost for re-grading and erosion control for the GCL cover (not including earthquake damage) was assumed to be the same as that associated with the evapotranspirative final cover. The construction cost estimate for the GCL cover system was approximately $680,000 or $119,700 per hectare. This cost includes the excavation, hauling, placement, and compaction of on-site borrow soil, installing the GCL and geocomposite drainage layer, placing and compacting the vegetative cover soil, and excavating and backfilling anchor trenches. The costs

9 for constructing surface water management structures were assumed to be the same as for the evapotranspirative cover system. The estimated cost for installing a passive landfill gas control system at the site was approximately $50,000, with a contingency for conversion to active gas control of $120, Asphalt cement concrete cover system evaluation The primary technical issue associated with the ACC cover is long-term performance. Annual sealing of cracks and/or applications of an ACC overlay or slurry coat sealer were assumed to be required maintenance. As the ACC cover system will increase peak run-off from the decks and benches as well as the side slope, it will result in even larger peak flows than the exposed geomembrane cover system, and hence require even larger surface water conveyance structures. The installed cost of the ACC final cover system was estimated at $816,000, or $145,000 per hectare, the most expensive of all the alternatives under consideration. However, in the areas around the Transfer Station that already have ACC pavement, the cost of the ACC cover reduces to $84,700 per hectare. The maintenance cost for applications of the ACC overlay or a suitable slurry coat sealer was considered similar to maintenance costs for the evapotranspirative cover. The cost for increasing the capacity of the surface water control system was set at $75, SELECTION OF THE PREFERRED ALTERNATIVE Table 1 summarizes the costs associated with the candidate alternative final cover systems. Based upon costs summarized in Table 1, the 1.2-m evapotranspirative cover constructed with on-site borrow soils was initially recommended as the preferred final cover system for closure of the Lebec Landfill, except for paved areas around the Transfer Station. In the paved areas around the Transfer Station, the asphalt cement concrete final cover system was the recommended alternative. However, even after an external peer review commissioned by the lead regulatory agency supported the validity of the water balance analysis, the agency remained skeptical of the ability of a cover composed of 1.2 m of soil with a saturated hydraulic conductivity of 3.3 x 10-4 cm/s to control infiltration, despite the fact that the existing 0.6-m interim cover appeared to be adequately controlling infiltration. The exposed geomembrane cover system was the next cheapest alternative, but was rejected due to concerns over community acceptance. Therefore, KCWMD decided to employ the GCL final cover as the primary final cover system at the Lebec Landfill, employing it everywhere except in the paved areas around the Transfer Station. 6. SUMMARY Evapotranspirative, exposed geomembrane, geosynthetic clay liner, and asphalt cement concrete cover systems were evaluated for use in final closure of the Lebec Landfill. While evaluation of cost and technical factors indicated an evapotranspirative final cover composed of on-site borrow soil was the most economical final cover system among these alternative for most of the landfill, regulatory agency skepticism that a sandy soil with a saturated hydraulic conductivity of 3.3 x 10-4 cm/s could effectively control infiltration led to the selection of the GCL cover system as the primary cover system for final closure. In the areas around the Transfer Station that required paving, the ACC cover system was the selected alternative cover system.

10 Table 1 - Cost comparison of alternative final cover systems Cover Type Constructio n Cost Maintenanc e Increment Comments Ranking Cost Evapotranspirative $500,000 $200,000 2 yr infiltration monitoring $700,000 Exposed Geomembrane $625,000 $185,000 Replace membrane once Aesthetic concerns $810,000 GCL $680,000 $150,000 Increased earthquake repairs ACC $815,000 $125,000 Aesthetic concerns Use where pavement exists $830,000 $940,000 ACKNOWLEDGEMENTS The authors wish to acknowledge the Mr. Ramzi Mansour and his staff in the Closure Construction Division of the Kern County Waste Management Department for their contributions to this study. REFERENCES Benson, C.H., (1999) Designing and Implementing Alternative Earthen Final Covers for Waste Containment Facilities, Continuing Education Class, University of Wisconsin-Madison Department of Engineering Professional Development, Los Angeles, CA, February 8-9. Fayer, M.J., and Gee, G.W., (1997) Hydrologic Model Tests for Landfill Covers Using Field Data, Proceedings, Landfill Capping in the Semi-arid West: Problems, Perspectives, and Solutions, Environmental Science and Research Foundation, Idaho Falls, ID. Giroud, J.P., Pelte, T., and Bathurst, R.J. (1995) Uplift of Geomembranes by Wind, Geosynthetics International, Vol. 2, No. 6, pp Marienfield, M.L., and Baker, T.L. (1998) Paving Fabric Interlayer System As A Pavement Moisture Barrier, 77th Annual Meeting of the Transportation Research Board, January 1998 Mansour, R. (2001) GCL Performance in Semi Arid Climate Conditions, Proceedings, 8th International Waste Management and Landfill Symposium, Cagliari, Italy, October Schroeder, P., Lloyd, C., and Zappi, P. (1994) The Hydrogeologic Evaluation of Landfill Performance (HELP) Model, User s Guide for Version 3.0, United States Environmental Protection Agency, Cincinnati, OH. Zornberg, J.G. and Giroud, J.P. (1997) Uplift of Geomembranes by Wind - Extension of Equations, Geosynthetics International, Vol. 4, No. 2, pp