Landfill Cover System Research Articles

Effects of biochar on soil water retention curves of compacted clay during wetting and drying

Compacted clay possesses a low water permeability and has been widely used in geo-environmental facilities such as landfill cover systems. Recent studies revealed the potential applications of compacted biochar-amended clay (BAC) as an alternative landfill cover soil. However, the effects of biochar on the soil water retention curves (SWRCs) of clay at low suction are not known. This study can help fill this knowledge gap by measuring the wetting and drying SWRCs of clay and BAC (20% biochar). Soils were compacted in instrumented soil columns and subjected to a wetting and drying cycle, and soil water content and suction were measured along with the soil depth. BAC was saturated faster when compared with clay. Upon drying, the soil water content of clay at 65 mm depth dropped to almost zero, while about 5% of water was retained in BAC. It showed that biochar increased the soil water retention capacity of compacted clay upon drying. SWRCs showed that biochar-clay composite possesses a higher soil water retention capacity at a low soil suction range (< 1000 kPa) compared with clay alone. It revealed the benefits of using compacted biochar-clay composite as a hydraulic barrier to minimize desiccation-induced cracks and potentially promote its serviceability, especially in arid and semi-arid regions. The composite is also a carbon sink material that can reduce landfill gas emissions and pollutant leaching.

Nonequilibrium molecular dynamics (NEMD) modeling of nanoscale hydrodynamics of clay‐water system at elevated temperature

AbstractThe engineering problems involving clay under non‐isothermal conditions (e.g., geothermal energy harvest, landfill cover system, and nuclear waste disposal) are multiscale and multiphysics by nature. The nanoscale hydrodynamics of clay at elevated temperature is essential in developing a physics‐based multiscale model for clay under non‐isothermal conditions. The nonequilibrium molecular dynamics (NEMD) is a useful tool to study the nanoscale hydrodyndamics of clay. This article presents an NEMD modeling of hydrodynamics of clay nanopores at elevated temperatures. Water flow confined in pyrophyllite and montmorillonite clay nanopores is investigated. The nonequilibrium state is maintained by uniformly exerting an external force on each water molecule. The NEMD simulations have provided a molecular‐scale perspective of temperature effect on clay‐water density, water flow velocity, shear viscosity, clay‐water slip length, hydraulic conductivity, and clay‐water friction coefficient. The numerical results have shown a strong temperature dependence of fluid flow velocity, shear viscosity, clay‐water slip length, and hydraulic conductivity at the nanoscale. We have validated the applicability of cubic law in determining hydraulic conductivity at the nanopore scale at elevated temperatures. It is found from our numerical results that slip clay‐water boundary condition is an essential factor in properly determining nanoscale fluid flow velocity. By numerical examples, we also study the impact of nanopore size and clay layer thickness on the hydrodynamics of the clay‐water system.

Effects of grass type on hydraulic response of the three-layer landfill cover system.

Soil column tests were conducted to investigate the effects of grass type on water infiltration in a three-layer landfill cover under drying and wetting conditions. Five soil columns were prepared, including one bare, two Bermuda grass-planted and the other two vetiver-planted. During the drying period, the suction of vetiver-planted soil column was the largest, while that of bare case was the lowest. During the wetting period, the infiltration rate shows a bimodal form due to the contrasting hydraulic properties of different soil layers. The infiltration rate of vetiver-planted soil column was the lowest, followed by Bermuda grass-planted and bare cases. Correspondingly, the vetiver-planted soil column retained the maximum suction and the deepest ponding depth during rainfall. This was likely due to the larger leaf area and deeper roots of vetiver than those of Bermuda grass, thus inducing the maximum initial suction by root water uptake before rainfall and reducing the water permeability by root occupations of soil pores. These results show that vetiver is more effective than Bermuda grass to reduce water percolation through the three-layer landfill cover.

Estimation of fluorotelomer alcohol emissions from landfill cover systems

This paper presents analytical solutions for estimating emissions of fluorotelomer alcohol (FTOH), a precursor of per- and polyfluoroalkyl substances, from landfill single and composite cover liners. Single liners comprise a compacted clay liner (CCL), a geosynthetic clay liner (GCL) or a geomembrane barrier (GMB). Composite liners included a GMB underlain by a CCL or a GCL. This paper shows that for a single liner, an intact GMB reduces the FTOH emission flux by three orders of magnitudes compared with a CCL and by two orders of magnitude compared with a GCL. Also, a composite cover liner reduces FTOH emissions by at least one to three orders of magnitude relative to that of a single GMB, depending on the use of either a CCL or a GCL. The analytical solutions presented in this study can be used for the preliminary evaluation of different types of covers for mitigating FTOH emissions from landfills.

A Novel Environmentally Friendly Vegetated Three-Layer Landfill Cover System Using Construction Wastes But Without a Geomembrane

The recycling and reuse of construction waste in landfill covers help reduce environmental pollution and promote sustainability. Further enhancement of the performance of the environmentally friendly landfill covers can be made by using vegetation as it is a low-cost, ecologically pleasant solution. Most related studies of vegetation were carried out in single-layer uniform soil while plant effects on water infiltration in layered soil such as a three-layer landfill cover system remain unclear. This keynote presents an integrated research approach combining laboratory one-dimensional tests as well as a 4-year full-scale field trial located in humid climates to explore the hydrological effects of plants on the performance of a novel three-layer landfill cover system using recycled concrete but without the installation of a geomembrane. Matric suction induced by plants was preserved to a greater extent under a novel vegetated three-layer landfill cover than under a bare cover even after an extreme rainfall event with a return period of greater than 1,000 years in humid regions like Hong Kong. During the 4-year field trial, the measured maximum annual percolation was 26 mm in the three-layer bare cover system, which was less than the US EPA-recommended 30 mm. Due to the presence of grass roots, the measured amount of annual percolation in the vegetated landfill cover was further reduced by up to 22%. Both the laboratory tests and field trial demonstrated that the vegetated three-layer landfill cover system using recycled concrete even without a geomembrane can effectively reduce percolation in humid climates.

Influence of infiltration on soil erosion in green infrastructures

Abstract Rainwater-induced erosion in green geotechnical infrastructures such as a multilayered landfill cover system (MLCS) is a severe concern in the current era. Although vegetation is a proven measure to control erosion in the MLCS, there are other factors such as infiltration rate which influence the control of the phenomenon. Most of the existing studies are limited to understand influence of vegetation on erosion control or infiltration rate alone. In this study, an attempt is made to incorporate infiltration measurements alongside vegetation cover to understand erosion in surface layer of the MLCS. For this purpose, a pilot MLCS was constructed, and erosion of its surface soil was temporally evaluated through soil loss depth of eroded cover surface under the influence of natural as well as simulated rainfall conditions. Alongside erosion, the amount of vegetated cover was evaluated through photographic image analyses and infiltration rate was measured by mini disk infiltrometer. From the observed results, it is understood that soil erosion and infiltration rate depict a contrasting behaviour with growing vegetation. Antecedent moisture contents were observed to show greater influence on such erosion behaviour which was observed during the testing period. Such studies may be helpful to researchers and practicing engineers for understanding performance of various green geotechnical infrastructures and scheduling the maintenance services to increase the longevity of their layered soil systems.

Seepage characteristics of three-layered landfill cover system constituting fly-ash under extreme ponding condition

A multi-layered final cover system is constructed over the landfill after it reaches its full capacity to minimize water ingress into the underlying hazardous waste. Three layered landfill cover are designed for areas experiencing very humid climatic conditions. Under the effects of climate change, the occurrences of extreme rainfall events become more frequent and this has resulted in catastrophic floods and hence extreme ponding. This study investigates the seepage characteristics of three-layered capillary barrier cover systems under an extreme ponding condition of 1.5 m water head, through detailed laboratory column tests and finite-element seepage analysis. Four 1.2 m-tall columns having different configurations (C1–C4) were studied. Fly ash (FA) was used to amend the surface and barrier layers in columns C2 and C4, in line with the novel concept of “waste protect waste”. Spatiotemporal variations of volumetric water content of the four columns were monitored for three years continuously. With FA amendment in the surface layer and an inclusion of a 0.01 m thick geosynthetic clay liner between the drainage and barrier layers, the onset of basal percolation was significantly delayed until 700 days of ponding, compared to 115 days without FA amendment. Capillary flow dominated the gravitational flow and perched water table was formed as waterfront advanced from the drainage to barrier layers. Further seepage analysis considering a realistic humid climate boundary condition showed that all four configurations were successful in preventing basal percolation for 800 days.

The characterisation of geosynthetic interface friction by means of the inclined plane test

The paper focuses on the evaluation of the shear strength in conditions of low normal stress of various geosynthetic-geosynthetic interfaces, which are typical of landfill cover systems, by means of the inclined plane test, with the aim of studying the friction mobilisation in relation to various kinematic behaviours. The results of three different methods to evaluate the angle of friction were analysed, together with the sensitivity of the interfaces in relation to the wear effect and the influence of the state of hydration. The results showed very different responses of the interfaces to the shear stress, which involved three main types of sliding mechanisms, referred to as sudden, gradual and uneven sliding. Another outcome observed was that the shear strength of geosynthetic-geosynthetic interfaces cannot always be properly characterised following the procedure proposed by the European standard for soil-geosynthetic interfaces (EN ISO 12957–2), since the actual mobilised kinematic behaviour should be taken into consideration. In this regard, the paper provides some hints on the choice of the more representative parameter of friction for each type of sliding. A particular focus was given to the case of gradual sliding interfaces, for which the static friction is difficult to detect due to the very slow movements; for practical purposes, the design friction of these interfaces should be evaluated by using an adequate safety factor with respect to the friction evaluated at 1 mm of displacement.

A New Inclined Pullout Device for Laboratory Tests on Geosynthetics

Abstract Geosynthetics are widely used in landfill cover systems for various purposes. The stability of these geosynthetics on slopes of landfill covers depends on the effectiveness of the anchors in which these are embedded at the top of the slope. These geosynthetics are subjected to pullout forces that are in an inclined direction. However, only two studies have been reported so far on the inclined pullout behavior of geosynthetics embedded in various types of anchors. In this study, a new inclined pullout device is developed to conduct pullout tests under different angles of inclinations. The salient features and details of various components of this device are presented in this article. Because geogrids are the preferred choice for veneer reinforcement, inclined pullout tests are conducted on geogrids embedded in three different types of anchor, namely, run-out, I-type, and L-type anchors. The tests are conducted on two different types of sands, i.e., quarry sand and river sand. The results show that the peak pullout force increases significantly as the inclination of the pullout force increases for all three types of anchor and for both the sands. Moreover, the complete pullout force-displacement response is found to be different for the three anchors. Practical implications of the outcome of the study are given.

Experimental investigation on methane advection and diffusion in geosynthetic clay liners

Geosynthetic clay liners (GCLs) are widely used in landfill and heap-leach facility cover system for mitigating rainfall infiltration and gas migration into atmosphere. Laboratory tests were conducted to investigate methane diffusion and advection through GCLs. Gas permeability coefficient of GCL for the case with moisture content = 47.5% is one and two orders of magnitude greater than the cases with moisture content = 68.5% and 80.9%, respectively, when 20 kPa vertical stress was applied. The batch adsorption tests indicated that adsorption of methane onto bentonite is negligible. The concentration variation for the adsorption of methane onto bentonite can be neglected. However, methane concentration decreased by 14.2% for the test of methane adsorption onto GCL during the first 2–3 days. This is because methane was adsorbed by the geotextiles rather than by the bentonite in GCL. The large porosity and surface area of geotextiles provide lots of micropores for methane adsorption. Analytical model was then developed to analyze the performance of GCL-based liners system with respect to methane transport. The results indicate that methane emission fluxes for the case with SL + GCL are 7.8 and 5.1 times less than the cases with SL + CCL when the moisture contents were 25.9% and 35.1%, respectively. The methane emission fluxes for both of the SL + GCL and SL + CCL can be neglected when they are fully saturated. GCL is recommended to be used in arid and semi-arid regions rather than CCL. GCL is recommended to be used in arid and semi-arid areas rather than CCL. Advection plays a more important role in methane migration through SL + GCL and SL + CCL than that of diffusion. With moisture contents = 25.9% and 32%, methane emission flux attributed to advection accounts for more than 90% of the total emission flux for both cases of SL + GCL and SL + CCL. With the increase of moisture content of SL, the effectiveness of SL in reducing methane emission increases. The saved space for using GCL + SL composite cover compared with using a single SL cover is 0.7 m when the moisture content equals 25.9%, which is 0.5 m greater than the case when moisture content equals 32%. GMB plays a dominant role in inhibiting methane migration and reducing methane emission flux. When moisture content equals 25.9%, the methane emission fluxes for SL + GMB + GCL and SL + GMB + CCL are 343 times and 2643 times less than the cases with SL + GCL and SL + CCL, respectively.

A New Approach for the Design, Construction and Control of Compacted Mineral Liners of a MSW Landfill Capping

Design parameters typically used for the mineral liner of landfill cover system (e.g. water content w, dry density ρd and hydraulic conductivity k) are often difficult to obtain on site. After construction, these experimental parameters need to be checked against the predicted values. In this paper, a rigorous procedure for design, construction and control of a compacted mineral liner in a capping multilayer system has been developed. The procedure has been schematically subdivided in three phases. The first phase (design) was based on a wide preliminary set of laboratory tests, composed of classification, compaction and permeability tests. An analytical relationship among degree of saturation Sr, dry density ρd and hydraulic conductivity k is also defined. The second phase (construction) was developed through a suitable compaction of the mineral liner planned after the design phase. The last phase (in-site control) consists of checking the design parameters (w, Sr, ρd, k) actually achieved on site. A comparison between coefficients k, obtained through site permeability tests and an analytical relationship obtained in the design phase, was made successfully. Owing to this new approach, it will no longer be necessary to perform in situ permeability tests, but it will be enough to define the above-mentioned analytical relationship during the design phase. In this way, a dangerous damage to the mineral liner, caused by the execution of invasive permeability tests, will be also avoided.

Multicomponent landfill gas transport in soil cover: column tests and numerical modelling

A soil column test was carried out to investigate the mechanism of multicomponent landfill gas transport in loess. Four gas components, namely methane (CH4), carbon dioxide (CO2), oxygen (O2) and nitrogen (N2), were considered. Based on the dusty-gas model and mass balance equation, a one-dimensional transient model for multicomponent gas transport in landfill layered cover soils was then developed. Methane oxidation in the soil is also considered in the model. The numerical model was solved by the finite-element method-based program Comsol Multiphysics v5.1. The numerical results agree well with those observed from the soil column tests. The parameter analysis shows that ordinary diffusion plays an important role in the transport of multicomponent gases. For methane and carbon dioxide, ordinary diffusion contributes to 97% of the total transport flux in the shallow zone. The effect of ordinary diffusion decreases with the increase in the depth. Advection contributes to 37% of the flux at the bottom of the cover soil. The effect of Knudsen diffusion is relatively weak, which only contributes 0.5–12% to the total flux. This is due to the relatively high gas permeability of the soil. The contribution of advection is comparable to that of diffusion when the gas permeability increases up to 3 × 10−13 m2. The maximum methane oxidation rate is found to increase by a factor of 1.8, when influent flux increases from 1 × 10−5 to 1 × 10−4 (mol/m2)/s. The numerical solution can be used for the analysis of multicomponent landfill gas transport in the soils and the design of landfill cover systems with respect to gas pollution control.

Influence of biochar from animal and plant origin on the compressive strength characteristics of degraded landfill surface soils

Growing awareness of sustainability in the landfill cover system has increased the use of biochar amendment for degraded landfill surface soils. Hydraulic and vegetative benefits of biochar on cover soil have been studied in the past, while ignoring mechanical characteristics, which is important to understand progressive failure of landfill infrastructure. In this study, the mechanical characteristics of four soil–biochar composites were investigated by conducting 81 unconfined compressive strength test. The results based on four in-house produced biochar were used to study the effect of compaction state (density, moisture content) and biochar percentage (5% and 10%) on unconfined compressive strength of soil–biochar. The ductility of soil–biochar was investigated for all the four biochars. Results from this study indicate a contrasting observation of strength gain depending on the type of biochar. The unconfined compressive strength of soil–biochar is potentially influenced by the different surface functional groups of biochar (hydrophilicity/hydrophobicity) and soil-biochar interlocking. It was noted that the peanut shell biochar gave comparable unconfined compressive strength of soil–biochar with that of bare soil for different compaction state. However, a diminution in the unconfined compressive strength was observed for all the other three soil–biochar sourced from water hyacinth, saw dust, and poultry litter. The study indicates that the use of biochar in soils does not ensure an improvement in the strength of soil–biochar. Enhancement in ductility was found for all the four soil–biochar irrespective of compaction state. Improvement in ductility was maximum when the soil–biochar is compacted at the dry state of optimum. Plant-based biochar has higher potency to increase the ductility of soil as compared to the animal-based biochar. Our study identifies peanut shell biochar ideal for landfill cover amendment material, considering its mechanical characteristics and design criterion. Soil biochar composite from water hyacinth, saw dust, and poultry litter can be used for potential application in green-infrastructure.

Evaluation of Capacitance Sensor for Suction Measurement in Silty Clay Loam

Measurement of soil suction is needed to develop soil–water characteristic curve, which is a mandatory input for behavioral modeling of unsaturated soil. One of the limitations of suction measuring instruments is its inability to capture wide range of suction existing in soil from near saturation to a near dry state. A recently available capacitance sensor (CS) is a promising, compact instrument for measuring wide range of soil suction, continuously, which makes it an ideal choice for several field studies. However, there are limited studies that evaluate the appropriateness of this sensor for wide range of suction measurement in different types of soils. This study critically evaluates the usefulness of CS for measuring low to high suction in a silty clay loam soil used as surface layer in a landfill cover system. This study reveals that the CS can precisely measure matric suction up to 1700 kPa for silty clay loam. Beyond 1700 kPa, the sensor measured low suction as compared to the well-established relative humidity technique. An empirical corrective procedure was proposed in this study for CS measurements greater than 1700 kPa. The corrective procedure is soil specific and depends on the reliable range of CS measurements, which need to be explored in detail.

Two-year evaluation of hydraulic properties of biochar-amended vegetated soil for application in landfill cover system

Landfill cover should ideally have a medium with high water retention ability and low hydraulic conductivity to prevent rainfall infiltrating into the hazardous waste layer. Even though biochar amended soil (BAS) is advocated as cover medium, the interactions between biochar and plant, as well as the effects of biochar aging and plant growth on soil hydraulic properties are still not clear. This study aims to investigate the effects of grass (Cynodon dactylon) growth in BAS on soil water retention and saturated hydraulic conductivity (ks) over a two-year period. Four ground conditions were tested, namely bare silty sand with and without biochar, vegetated silty sand with and without biochar. The biochar content was kept at 10% (v/v). During the first 6 months, soil water content corresponding to field capacity (FC) and permanent wilting point (PWP) in grassed soil increased by 17% and 27%, respectively. With biochar inclusion, 43% and 57% additional increases in FC and PWP respectively were observed. Moreover, ks in biochar-amended grassed soil decreased by 50%. Furthermore, grass growth from 6 to 24 months reduced FC by 32%, PWP by 40% but caused 20 times increase in ks of grassed soil. With the presence of biochar, FC and PWP decreased by only 6% and 8%, respectively, and ks increased by 200% due to the enhanced plant growth (specifically root growth) by biochar. After two years, ks of grassed soil with biochar was 16 times smaller than that without biochar. This study demonstrated the effectiveness of biochar in maintaining the enhanced soil water retention ability and reduced ks in vegetated soil over a two-year study period.

Effect of Basic Oxygen Furnace Slag-Infiltrated Water on Methane Oxidation and Community Composition in Biogeochemical Landfill Cover System

AbstractA sustainable biogeochemical cover system consisting of a biochar-amended soil layer overlain by a basic oxygen furnace (BOF) slag layer is being developed to mitigate fugitive emissions of...

A novel vegetated three-layer landfill cover system using recycled construction wastes without geomembrane

To promote environmental protection and sustainability, the use of plants and recycled wastes in geotechnical construction such as landfill covers is recommended. A landfill cover field test was conducted at the Shenzhen Xiaping landfill site, located in a humid climatic region of China. The main objective was to validate the field performance of a novel vegetated three-layer landfill cover system using recycled construction waste without the need of geomembrane. Unsieved completely decomposed granite and coarsely crushed concrete was used for the top and intermediate layers while sieved completely decomposed granite was used as the lowest layer. One section was transplanted with Bermuda grass while the other section was left bare. To assess the landfill cover performance, pore-water pressure, volumetric water content, percolation, and atmospheric parameters were measured for a period of 13 months under natural climatic conditions. The cumulative rainfall depth was about 2950 mm over the entire monitoring period. During rainfall, the presence of grass led to lower pore-water pressure (or higher suction) and volumetric water content in the three-layer landfill cover system. At the end of monitoring, the cumulative percolation was about 27 and 20 mm for the bare and grass-covered landfill covers, respectively. It is evident that the vegetated three-layer landfill cover system using recycled concrete without geomembrane can be effective in minimizing percolation in humid climates.

Impact Assessment of Vegetation Growth on Soil Erosion of a Landfill Cover Surface

Abstract Soil erosion is a very common phenomenon encountered at many sloped earthen geotechnical structures. For instance, the surface soil of an inclined landfill cover system undergoes the erosion due to various adverse atmospheric variants. This is one of the major causes for performance failure in the cover system. However, previous researchers have rarely conducted the study for field assessment of soil erosion in high rainfall tropical regions such as northeast India. The literature advocates the utilization of vegetation for erosion management. This study investigated the impact of vegetation growth on soil erosion of a cover surface layer under both natural and controlled artificial rainfall. The soil erosion was monitored by collecting the soil loss due to rainfall. Vegetation growth was evaluated based on photographic image analyses. The study clearly indicates that the vegetation growth can contribute to reduction of soil erosion from the landfill cover surface.

20 years performance measurements of a landfill cover system with components constructed from pre-treated dredged sediments

The water balance and effectiveness of a landfill cover containing a liner from fine-grained dredged material (‘METHA-material’) and a drainage layer from sandy dredged material (‘METHA-sand’) from the Port of Hamburg (Northern Germany) was investigated for 20 years using two test fields (lysimeters), each 500 m2 in size. The layering within the standard design test field (FS) is identical to the layering of the cover on the landfill, whereas the alternative design test field (FA) was designed to provoke desiccation of the liner. The setup abstained from continuous side walls cutting through the liners in order to allow for material properties representative for the landfill cover, at the cost of possible minor lateral inflows or outflows, which were, however, considered negligible. By example of the lysimeter FA the risk of desiccation-induced failure of a liner from METHA-material under the given climate could be shown. In contrast, the discharges and hydro-chemical parameters measured for the lysimeter FS indicate continuous high efficiency of the liner until today. The average discharge below the liner was 14.9 mm/yr. The entire cover contributes to the good liner performance. Reason are (1) the low hydraulic conductivity of the liner (5 × 10−10 m/s) and its large thickness (1.5 m); (2) the increased stresses on the liner due to the 2.5 m thick overlaying cover; (3) the 1.0 m thick drainage layer from slowly-draining METHA-sand, allowing for a continuous nearly water-saturated fringe above the liner; and (4) the sufficient amount of plant available water in the recultivation layer.

Innovative Biogeochemical Cover to Mitigate Landfill Gas Emissions: Investigation of Controlling Parameters Based on Batch and Column Experiments

Municipal solid waste (MSW) landfills generate different gases mainly methane (CH4) and carbon dioxide (CO2) during the process of waste decomposition. Modern landfills are provided with gas collection systems, however, significant amount of landfill gas (LFG) escapes into the atmosphere, making landfills one of the largest anthropogenic sources of CH4 and CO2 emissions. Several researchers have investigated various alternative landfill cover systems, such as biocovers, in order to mitigate CH4 transport across landfill covers by enhancing microbial CH4 oxidation. In recent years, biochar as an organic amendment has shown promise in enhanced microbial oxidation due to its inert/stable chemical nature, high surface area, high internal porosity, and high moisture holding capacity. However, in all these efforts, little regard is given to the CO2 that still escapes into the atmosphere in undesirable amounts. The current study introduces the concept of biogeochemical cover, which uses steel slag in conjunction with biochar-amended soil to mitigate fugitive emissions from a landfill. The current study compares the CO2 sequestration potential of steel slag, mainly basic oxygen furnace (BOF) slag under various environmental conditions that may prevail in the landfill cover. BOF slag shows significant CO2 sequestration potential under variable conditions including moisture, temperature, gas flow conditions, and BOF slag type and particle size. The results suggest that the use of BOF slag could be a cost effective and green solution to the problems of fugitive LFG emissions.