Landfill Cap Research Articles

Using Fiber Optics to Detect Moisture Intrusion into a Landfill Cap Consisting of a Vegetative Soil Barrier

The intrusion of moisture into landfills can pose a health hazard because of the possibility that the moisture will carry harmful substances into the groundwater. Early detection of moisture anywhere within these landfills is essential if corrective action is to be taken well before an occurrence of this kind. This paper presents the results of a field-scale simulation test of the use of fiber optics to detect the presence of moisture within landfill covers, using a detection method based on the thermal response of soils as a function of their moisture content. By sending electrical current through an embedded stainless-steel tube, soils of varying moisture content were heated and time-dependent temperature measurements were obtained with a fiber-optic distributed temperature sensor system. The optical fiber itself lay within the tube, but its temperature was a function of how rapidly heat was conducted into the surrounding medium. The results of this experiment, which are in agreement with those obtained using more traditional “point” sampling and laboratory analysis, are presented along with the strengths and limitations of the thermal-response method of detecting moisture.

Utilization of Illinois PCC Dry Bottom Ash for Compacted Landfill Barriers

Compacted soil barriers are one of the most important components of municipal waste landfills. The material used to construct a landfill liner and/or cap must prevent the flow of fluids through them. Soils with low values of permeability (such as compacted clays) are often used to construct landfill barriers. Natural sands and other cohesionless materials are used to construct hydraulic barriers by adding admixtures to modify their properties. Several studies have been conducted that dealt with determining geotechnical engineering properties of sand-bentonite mixtures. Pulverized coal combustion (PCC) dry bottom ash is a coal combustion by-product of burning coal to produce electricity. Because of the increasing costs associated with the disposal of bottom ash and the environmental regulations in place, there is a need to develop alternate methods for profitable and environmentally safe uses of this waste material. Most scientists and researchers have concluded that bottom ash has geotechnical characteristics similar to those of sands. However, information on the use of bottom ash, with or without admixtures, in the construction of landfill barriers is limited. Most of the available literature on the engineering properties of bottom ash deals with its use as a fill material. The physical and chemical characteristics of bottom ash depend on several factors including type of coal used and type of boiler and collection system. This paper presents the results of an experimental study conducted to determine the possible use of Illinois PCC dry bottom ash amended with bentonite to construct landfill barriers. Test results presented show that the average value of hydraulic conductivity of Illinois PCC dry bottom ash with 15% bentonite content is close to the acceptable value required for its use as hydraulic barrier. Therefore, it was concluded that Illinois PCC dry bottom ash, modified with 15% or higher bentonite content, is likely to provide adequate hydraulic conductivity for its use to construct landfill barriers.

Vegetative covers for waste containment.

Disposal of municipal and hazardous waste in the United States is primarily accomplished by containment in lined and capped landfills. Evapotranspiration cover systems offer an alternative to conventional landfill cap systems. These covers work on completely different principles than traditional covers do, and that difference may slow understanding and acceptance by site owners, regulators, and stakeholders. This chapter provides an introduction to this alternative technique and explains some of the common concerns regarding its implementation.

Mounding as a Technique for Restoration of Prairie on a Capped Landfill in the Puget Sound Lowlands

AbstractClosed landfills create large open spaces that are often proposed as sites for restored or created ecosystems. Grasslands are probably prescribed most often because of the presumption that grass root systems will not breach the landfill cap. Capped landfills have a number of soil degradation problems, including compaction, decreased permeability, lack of organic material, diminished soil fauna, inappropriate texture, and lack of structure. In this study in the Puget Sound lowlands, Washington, U.S.A., mounding (low sandy‐loam mounds, about 20 cm high and 2 m in diameter), addition of fertilizer, and mulching with yard‐waste compost were applied to landfill sites as treatments in a factorial‐design experiment. Prairie plants (1,344 individuals, 7 species) were planted into 4‐m2 plots (n = 48), and plant growth and survival and the increase in weed biomass were monitored for 3 years. Mulching had no effect on plant survival or growth. Fertilization had a negative effect on Lupinus lepidus, a nitrogen‐fixing species. Mounding had a positive effect on growth and survival of Eriophyllum lanatum, Festuca idahoensis, and Aster curtus. Potentilla pacifica was indifferent to mounding, and Carex inops responded negatively. Mounds should probably be used as one element of a complex of habitats on restored landfills.

Landfill Capping with Woodland Ecosystems

As the field of phytoremediation has evolved over the last 10 years, there has been a growing interest in the use of vegetative covers as alternative landfill caps. Consequently, a number of vegetative caps (grass, shrubs, and trees) have been installed at landfills across the United States, with tree caps being the focus of this paper. The two primary objectives of vegetative caps are to minimize water percolation into landfill waste via a “sponge and pump” mechanism and to prevent surface soil erosion. The soil pores hold precipitation like a sponge until plant roots can access the water; plants take up this water for growth and release it into the atmosphere by transpiration. The erosion control objective is achieved by canopy interception of rainfall, minimizing the “splash effect,” and the growing of a root matrix that interlaces through soil particles. Secondary benefits of vegetative caps include aesthetic appeal for the neighboring community, sequestration of greenhouse gases, reduced construction...

Natural bentonites—influence of the ion exchange and partial desiccation on permeability and self-healing capacity of bentonites used in GCLs

Geosynthetic clay liners (GCLs) are sealing elements, which contain natural sodium bentonite encapsulated between geotextile components. They are used in a variety of sealing applications and are most commonly employed to replace compacted clay liners (CCLs). One important field of application is landfill capping systems. In that geochemical environment an ion exchange of sodium-bentonite to calcium-bentonite occurs over a period of approximately 1–3 years, if in contact with cover-soil seepage. As a result, there is a slight increase in hydraulic conductivity of approximately half to one order of magnitude. This is, however, not alarming, as long as the design engineers take this effect into consideration. Landfill capping systems are characterized by unsaturated conditions, as often found in applications with low confining stress (<15 kN/m 2, less than 0.75 m soil-cover). In such cases, GCLs tend to show desiccation cracks, which cause a significant increase of the permeability. In contrast to CCLs, where self sealing is unlikely to occur under low confining stress, a self-sealing of calcium-bentonite GCls takes place by swelling and plastification of bentonite, if a soil-cover of more than 0.75 m (confining stress >15 kN/m 2) is provided.

The potential for short rotation energy forestry on restored landfill caps

This review examines the potential for producing biomass on restored landfills using willow and poplar species in short rotation energy forestry. In southern England, the potential production may be about 20 t ha −1 of dry stem wood annually. However, actual yields are likely to be constrained by detrimental soil conditions, including shallow depth, compaction, low water holding capacity and poor nutritional status. These factors will affect plant growth by causing drought, waterlogging, poor soil aeration and nutritional deficiencies. Practical solutions to these problems include the correct placement and handling of the agricultural cap material, soil amelioration using tillage and the addition of organic matter (such as sewage sludge), irrigation (possibly using landfill leachate), the installation of drainage and the application of inorganic fertilizers. The correct choice of species and clone, along with good site management are also essential if economically viable yields are to be obtained. Further investigations are required to determine the actual yields that can be obtained on landfill sites using a range of management inputs.

Cost-Effective Landfill Closure: Boston's “Menino Project”

The city of Boston owns a 40-ha (98-acre) site that contains a 34-ha (85-acre) landfill along the banks of the scenic Charles River. Faced with a multimillion dollar state-mandated closure, the city decided that it wanted to utilize this significant piece of real estate (which is bigger than the Boston Gardens and Common combined) for a postclosure use. The mayor's office, Department of Public Works and Parks, Community Advisory Committee, and a consultant developed a postclosure master plan for the site that included landfill closure and constructing a public park on the capped landfill. The total project was initially estimated at >$20,000,000. The city realized early that, to move this ambitious project forward, it would need to be creative in funding. Therefore, during the planning stage, areas were identified for potential cost savings: modification of state closure standards, substituting/manufacturing a portion of the required landfill cap, and utilizing creative/alternative funding sources. Implem...

Comparison of peel bond and shear tensile test methods for needlepunched geosynthetic clay liners

Geosynthetic clay liners (GCLs) are an established product group in the geosynthetics industry for various sealing applications. They can be used in slope applications, such as in landfill caps, covers or base sealing systems, effluent and sludge lagoons, where needlepunching or continuous internal reinforcement of the bentonite is required. The needlepunching of GCLs has resulted in internal shear angles of 45° and higher. It is recognized that the needlepunching influences the internal shear stress. A peel bonding test method based on the the strip width tensile strength method and the grab test method was developed by GCL manufacturers. Meanwhile the ASTM taskgroup D35.04 is developing a standardized peel bonding test method. Recently a shear tensile test has been introduced as a new test method to measure the bonding strength of needlepunched GCLs. These two manufacturing quality control (MQC) tests, the existing peel test and the newly introduced shear tensile test are compared with each other.

Landfill Methane Oxidation Response to Vegetation, Fertilization, and Liming

AbstractThis study was conducted to evaluate the effects of vegetation, N fertilizers, and lime addition on landfill CH4 oxidation. Columns filled with compacted sandy loam and sparged with synthetic landfill gas were used to simulate a landfill cover. Grass‐topped and bare‐soil columns reduced inlet CH4 by 47 and 37%, respectively, at peak uptake; but the rate for both treatments was about 18% at steady slate. Nitrate and NH4 amendments induced a more rapid onset of CH4 oxidation relative to KCl controls. However, at steady state, NH4 inhibited CH4 oxidation in bare columns but not in grassed columns. Nitrate addition produced no inhibitory effects. Lime addition to the soil consistently enhanced CH4 oxidation. In all treatments, CH4 consumption increased to a peak value, then declined to a lower steady‐state value; and all gassed columns developed a pH gradient. Neither nutrient depletion nor protozoan grazing could explain the decline from peak oxidation levels. Ammonium applied to grassed cover soil can cause transient reductions in CH4 uptake, but there is no evidence that the inhibition persists. The ability of vegetation to mitigate NH4 inhibition indicates that results from bare‐soil tests may not always generalize to vegetated landfill caps.

97/00690 Energy consumption in the recovery of argon from purge gas in ammonia synthesis plant

Below-ground growth of four tree species (ash (Fraxinus excelsior L.), alder (Alnus spp.), sycamore (Acer pseudoplatanus L.) and Corsican pine (Pinus nigra var. maritima (Ait.) Melville)) was investigated on a 15-year-old containment landfill in Hertfordshire, UK. The site was restored using a variable but predictable thickness of soil-forming material over the mineral cap and this permitted a study of the relationship between soil thickness and indices of tree root growth. Tree root habit was affected by soil thickness, and the likelihood of root penetration into the cap was significantly reduced when soil cover was greater than 1.25 m. Penetration into the cap appeared to occur because bulk density was often lower than threshold values set to meet permeability criteria necessary for pollution control. The study results suggest that previously published guidance on minimum soil thickness over unprotected landfill caps (1.5 m) should be adhered to, and that risk of root penetration into the cap will be acceptable if it is.

Capping with fiber clay: C. H. Floess, R. F. J. Smith & R. H. Hitchcock, Civil Engineering — ASCE, 65(8), 1995, pp 62–63

Landfill-cap construction is expensive and often results in economic hardship, especially for small towns. Costs for conventional clay or geomembrane caps for municipal solid-waste landfills in the Northeast can approach $100,000 per acre. Three landfills, in Hubbardston, Mass., Corinth, N.Y., and Marlborough, N.H., sought to improve the process and become the first in their respective states to use fiber clay--collected from the waste of paper mills--as barrier-layer material. Now, other municipalities are considering fiber-clay landfill caps and a new project is under way in Montague, Mass. A money-saver for both the mill, which saves disposal costs, and the town, the innovative process serves as a true example of a win-win situation.

Geotextile reinforcement of soft landfill process sludge to facilitate final closure: An instrumented case study

This paper describes the results of an instrumented, geotextile reinforced landfill cap for a process sludge landfill. The landfill is approximately 34,803 m 2 (8·6 acres) in size and filled with sludge to an average depth of 7·6 m (25 ft) . The sludge was ‘zero strength’ material and required a geotextile reinforcement and separation layer to facilitate the installation of a closure system. The geotextile was instrumented with foil strain gauges and the sewn seams instrumented with extensometers. It was the first attempt at measuring seam deformation to the authors' knowledge. The resulting data is presented and analyzed in this paper. In addition, a critique of the analysis of the installation practice as well as the effectiveness of the geotextile is included.

Three end-uses for closed landfills and their impact on the geosynthetic design

Closed landfills do not need to be unused land areas requiring continuous monitoring for protection of the surrounding environment. Closed landfills can provide an important benefit to the surrounding communities and environment. Wildlife habitats, parks and golf courses are a few examples of how these closed facilities can be developed to enhance practical ‘end-uses’ of the land. However, any ‘end-use’ design of the landfill facility must influence the design of the landfill cap. The use of specific geosynthetic materials in the end-use designs may also present construction and long-term design considerations not normally associated with other landfill closures. This paper presents three specific examples of landfill closure designs where their proposed end-use and geosynthetic components had various design impacts on the landfill end-use design.

Flexible membrane liners: C. R. Field & S. D. Stone, Civil Engineering — ASCE, 65(4), 1995, pp 60–61

Unless there is a long-range plan for the postclosure use of a landfill--such as turning it into a golf course or a park--final grades are usually designed to be as high and as steep as possible to provide the most usable space for refuse. Engineers designing such landfill closure systems must maximize space and meet federal, state, and local regulations as they ensure that the structure will be stable. In the past, compacted clay was used as a barrier layer in landfill caps. Some engineers concerned about slope stability still prefer to sue clay instead of flexible membrane liners. However, because Subtitle D regulations of the Resource Conservation and Recovery Act require new landfills to have caps that are no more permeable than the bottom liner, only landfills built before promulgation of Subtitle D can be capped with clay. For those landfills, if clay is locally available, a clay cap may be the most economical alternative. Yet a new generation of flexible membrane liners offers engineers new ways to fight instability on landfill slopes.

Use of geophysical surveys during the planning, construction and remediation of landfills

Abstract There are growing environmental pressures over the development, use and remediation of closed landfill sites throughout the UK, as well as overseas. There is also a reluctance to use direct investigative methods for fear of compromising the integrity of the landfill cap and liner systems. Consequently, there is an increasing role for geophysical surveys to be undertaken that are technically reliable, cost-effective and environmentally benign. Geophysical surveys have been proven to be successful in the assessment of site suitability during the planning stage. For example, evidence of former mining, adverse geological structure, soft zones and areas with anomalous permeability may be detected. Geophysics can be used for mapping the thickness of mineral liners during the construction phase. Closed landfill sites can be investigated geophysically without invasion of the structure to determine the location of margins and the depth of fill, map possible leachate ponding and migration, and to assess the integrity of mineral and artificial liners, etc. It is concluded that with current developments in data acquisition, processing, display and interpretation of geophysical data, the scope of environmental applications where multi-method geophysical surveys can be used is increasing rapidly. The range of geophysical methods currently available provides a powerful suite of investigative tools which complement direct observations.

A Re-Evaluation of Objections To Tree Planting On Containment Landfills

Legislation from developed countries indicates that planting trees on containment landfills is generally forbidden. Concerns centre on the supposition that tree roots can penetrate into and through capping materials, and will thus compromise control of water ingress into waste, and allow the escape of landfill gas. An associated anxiety is that if roots penetrate a clay cap they could cause desiccation and cracking of the clay through excessive moisture abstraction. It is also considered that trees growing on the relatively shallow soil above a landfill cap could be especially prone to uprooting. However, a review of the world literature indicates that maximum depths achieved by tree roots are usually between 1-2 m. Almost 90% of a tree's roots may be found in the upper 0.6 m of soil. Tree roots are highly sensitive to environmental conditions and their downward penetration can be restricted by a number of soil factors including compaction, poor aeration and infertility. A detailed study of these factors indicates that the materials used for capping landfill sites, such as HDPE (high density polyethylene) and compacted clays, can provide an effective barrier to downward root growth. The available information also suggests that tree roots are extremely unlikely to be a primary cause of desiccation cracking in a clay cap owing to their inability to extract more than about one-quarter of the total moisture held in a clay of the density required to ensure a permeability of 1 ×10-9 m s-1. Trees growing on landfill sites with a rootable soil depth of at least 1.5 m should be at no greater risk of windthrow than most forest trees on undisturbed sites. Methods are available to assess the likelihood of windthrow. In any event, windthrow should not cause disruption of a cap, due to the inability of tree roots to penetrate HDPE, or mineral materials compacted to a bulk density of 1.8 g cm-3. © 1995 ISWA

Effects of Pocket Gopher Burrowing on Cesium‐133 Distribution on Engineered Test Plots

AbstractVery low levels of radionuclides exist on soil surfaces from atmospheric fallout following weapons testing or from stack discharges, and from exposure of some of the older waste storage and disposal sites worldwide. Biological factors including vegetation and animal burrowing can influence the fate of these surface contaminants. Animal burrowing introduces variability in radionuclide migration that confounds estimation of nuclide migration pathways, risk assessment, and assessment of waste burial performance. A field study on the surface and subsurface erosional transport of surface‐applied 133Cs as affected by pocket gopher (Thomomys bottae) burrowing was conducted on simulated waste landfill caps at the Los Alamos National Laboratory in north central New Mexico. Surface loss of Cs, adhered to five soil particle size ranges, was measured several times over an 18‐mo period while simulated rainfalls were in progress. Gophers reduced Cs surface loss by significant amounts, 43%. Cesium surface loss on plots with only gophers was 0.8 kg totalled for the study period. This compared with 1.4 kg for control plots, 0.5 kg for vegetated plots, and 0.2 kg for plots with both gophers and vegetation. The change in Cs surface loss over time was significant (P &lt; 0.01). Relatively little subsurface Cs was measured in plots containing only gophers (0.7 g kg−1). Vegetation‐bearing plots had significantly more total subsurface Cs (µ = 1.7 g kg−1) than plots without vegetation (µ = 0.8 g kg−1). An average of 97% of the subsurface Cs in plots with vegetation was located in the upper 15 cm of soil (SDR1 + SDR2) compared with 67% for plots without vegetation. Vegetation moderated the influence of gopher activity on the transport of Cs to soil subsurfaces, and stabilized subsurface Cs by concentrating it in the rhizosphere. Gopher activity may have caused Cs transport to depths below that sampled, 30 cm. The results provide distribution coefficients for models of contaminant migration where animal burrowing occurs.

Waste and waste management: some geotechnical considerations

Waste may be classified into several general types, and there are different systems of disposal. A short consideration of these points is followed by a discussion of the methods of sealed landfill design with reference to some of the preliminary geotechnical factors that need to be taken into account. One example of a sealed landfill in the UK is presented in outline. Toe irnplications of leachate and gas generation, together with the choice of liner and landfill capping system and the problems of leakage, are also discussed. Among the conclusions that are drawn, stress is laid on the irnplementation of construction quality assurance procedures, the need for assessing the potential for groundwater and surface water contamination, and the irnportance of longer term monitoring for leachate and gas emissions.

A re-evaluation of objections to tree planting on containment landfills

Legislation from developed countries indicates that planting trees on containment landfills is generally forbidden. Concerns centre on the supposition that tree roots can penetrate into and through capping materials, and will thus compromise control of water ingress into waste, and allow the escape of landfill gas. An associated anxiety is that if roots penetrate a clay cap they could cause desiccation and cracking of the clay through excessive moisture abstraction. It is also considered that trees growing on the relatively shallow soil above a landfill cap could be especially prone to uprooting. However, a review of the world literature indicates that maximum depths achieved by tree roots are usually between 1–2 m. Almost 90% of a tree's roots may be found in the upper 0.6 m of soil. Tree roots are highly sensitive to environmental conditions and their downward penetration can be restricted by a number of soil factors including compaction, poor aeration and infertility. A detailed study of these factors indicates that the materials used for capping landfill sites, such as HDPE (high density polyethylene) and compacted clays, can provide an effective barrier to downward root growth. The available information also suggests that tree roots are extremely unlikely to be a primary cause of desiccation cracking in a clay cap owing to their inability to extract more than about one-quarter of the total moisture held in a clay of the density required to ensure a permeability of 1 × 10 −9 m s −1 . Trees growing on landfill sites with a rootable soil depth of at least 1.5 m should be at no greater risk of windthrow than most forest trees on undisturbed sites. Methods are available to assess the likelihood of windthrow. In any event, windthrow should not cause disruption of a cap, due to the inability of tree roots to penetrate HDPE, or mineral materials compacted to a bulk density of 1.8 g cm −3 .