Three-layer Landfill Cover System Research Articles

Anti-seepage performance and oxygen barrier performance of the three-layered landfill cover system comprising neutralized slag under extreme climate conditions

For acidic industrial solid wastes, an effective cover system is needed to reduce the rainwater infiltration and oxygen intrusion, thus reducing the generation of acid mine drainage (AMD) from wastes. A three-layered cover using low-permeability neutralized slag (abbreviated as TCNS) at the bottom of the traditional capillary barrier cover is proposed, in line with the novel concept of “waste protecting waste”. This study investigates the anti-seepage performance and oxygen barrier performance of TCNS under extreme climate conditions, through laboratory column test and numerical simulations. The results show that the water content of the neutralized slag (NS) layer remained stable (i.e., changed by less than 0.02) under either extremely wet condition or extremely dry condition. Under extremely wet condition (i.e., heavy rainfall corresponding to a 50-year return period), no water percolation was observed at the cover bottom; under extremely dry conditions, oxygen diffusion was greatly impeded, e.g., after 208 days of column test, the SO42− concentration in the AMD of the exposed (i.e., uncovered) waste rock was 4.6 times higher than that of the waste rock covered by TCNS. Numerical simulations considering two realistic climate conditions (i.e., humid and arid) showed that TCNS were effective in controlling water percolation and oxygen intrusion. The saturated permeability coefficient (ks) and initial saturation degree (Se) of the NS layer have significant effects on the performance of TCNS, i.e., decreasing ks or increasing Se can effectively reduce the water percolation and oxygen flux. In sum, TCNS is an effective barrier for controlling water percolation and oxygen intrusion, even in extreme climate conditions. Consequently, it can effectively minimize AMD leakage and thus reduce geological disasters such as groundwater contamination.

Analysis of water migration in an earthen landfill cover with recycled construction waste

In this study, numerical analysis was conducted to investigate the effects of particle sizes on water migration in a three-layer landfill cover system using recycled concrete aggregates (RCAs). This cover system consists of a top fine recycled concrete (FRC) aggregate layer and a middle coarse recycled concrete aggregate layer overlying a bottom silty refuse soil layer. Numerical simulations were performed to back analyse a flume model test data and further evaluate the particle size effects of RCAs on the effectiveness of the cover system. With the increase in rainfall duration, surface runoff reduced, while the lateral diversion increased pronouncedly. Nonetheless, no significant percolation is observed through the cover system using RCAs during 100-year extreme rainfall with different durations. Based on the results, the d10 of FRC is recommended to be from 0.2 to 0.5 mm.

Probabilistic analysis of a sustainable landfill cover considering stress-dependent water retention model and copula-based random fields

Material properties, crucial design parameters in landfill cover systems, can exhibit spatial variability due to non-uniform particle size and uneven compaction, affecting cover performance. Additionally, cover thickness, another key design parameter, can induce stress variation and influence soil-water retention capability. Previous designs always ignored these uncertainties and stress effects, potentially leading to unreasonable design schemes. This study aims to provide design recommendations for a three-layer landfill cover system under heavy rainfall, considering these factors simultaneously by the random finite element method (RFEM). The stress effects are captured using a stress-dependent soil-water retention curve (SDSWRC) and the hysteresis in the SDSWRC is considered in probabilistic analyses. For practical applications, uncertainties in two easily controllable particle size parameters, D10 and D60, are depicted by copula-based cross-correlated random fields. Based on empirical equations, the spatial variability of SDSWRC and water permeability function (WPF) can be further characterised by D10 and D60 random fields. It is found that considering SDSWRC and its spatial variability (e.g., shape) can overestimate percolation by at least 1.7 times compared to deterministic analysis using SWRC. The spatial variability of SDSWRC shape has a more significant impact than spatially variable saturated water permeability on cover system performance. Among candidate copulas, the CClayton copula (survival copula of Clayton copula) yields the most conservative results due to its upper tail dependence. The particle size and thickness of the bottom layer influence the cover system performance more than the upper two layers. A bottom layer with D10≤ 0.009 mm and D60/D10≥ 7.54, and thickness ≥ 0.8 m is recommended to achieve a satisfactory performance level with a 100-year design life.

Life cycle analysis of common landfill final cover systems focusing on carbon neutrality

Carbon emissions from landfill construction and management have become a global concern. Life cycle analysis (LCA) has been widely used to assess the environmental impacts of engineered infrastructures over their lifetimes. LCA has also been applied to landfill leachate and gas management, but rarely to landfill final cover systems. This paper reports the results of an LCA of the following landfill final cover systems: compacted clay cover, geomembrane cover, cover with capillary effects (CCBE), dual capillary barrier cover, three-layer landfill cover system using natural soils, three-layer cover using recycled concrete aggregate (RCA) and biochar-amended three-layer landfill cover system using RCA. The LCA assessment of landfill cover considers the cost, carbon emissions and carbon sequestration during the production, construction and operation phases. The effects of landfill cover on global warming, freshwater eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity and fossil resource scarcity are also evaluated. In addition, the sensitivities of cost and carbon emission to the use of electric-powered machines and transportation distance are analysed. It is revealed that the three-layer cover system using RCA and biochar has the lowest unit cost and carbon emission of all of the covers, up to 88 % and 66 % lower, respectively, than those of the other six covers. In addition, this cover system has the highest carbon sequestration rate, with a value of 47.9 kg CO2/(y·m2), four times higher than that of the compacted clay cover. Finally, this sustainable cover mitigates global warming and reduces adverse environmental impacts by up to 82 %. Therefore, the biochar amended three-layer cover system using RCA without geomembrane offers the greatest economic benefits, performs effectively in terms of the pursuit of carbon neutrality and promotes sustainable development.

Analysis of a landfill cover without geomembrane using varied particle sizes of recycled concrete

Previous studies have demonstrated the effectiveness of a novel three-layer landfill cover system constructed with recycled concrete aggregates (RCAs) without geomembrane in both laboratory and field. However, no systematic investigation has been carried out to optimize the combination of the particle sizes for fine-grained RCAs (FRC) and coarse-grained RCAs (CRC) that can be used for the three-layer landfill cover system. The aim of this paper is to assist engineers in designing the three-layer landfill cover system under a rainfall of 100-year return period in humid climate conditions using an easily controlled soil parameter D 10 of RCAs. The numerical study reveals that when D 10 of FRC increases from 0.05 mm to 0.16 mm, its saturated permeability increases by 10 times. As a result, a larger amount of rainwater infiltrates into the cover system, causing a higher lateral diversion in both the top FRC and middle CRC layers. No further changes in the lateral diversion are observed when the D 10 value of FRC is larger than 0.16 mm. Both the particle sizes of FRC and CRC layers are shown to have a minor influence on the percolation under the extreme rainfall event. This implies that the selection of particle sizes for the FRC and CRC layers can be based on the availability of materials. Although it is well known that the bottom layer of the cover system should be constructed with very fine-grained soils if possible, this study provides an upper limit to the particle size that can be used in the bottom layer ( D 10 not larger than 0.02 mm). With this limit, the three-layer system can still minimize the water percolation to meet the design criterion (30 mm/yr) even under a 100-year return period of rainfall in humid climates.

WITHDRAWN: Physical and numerical modelling of a vegetated three-layer landfill cover system using recycled aggregates without a geomembrane

WITHDRAWN: Physical and numerical modelling of a vegetated three-layer landfill cover system using recycled aggregates without a geomembrane

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.

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.

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.

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.

Effects of shrub on one-dimensional suction distribution and water infiltration in a three-layer landfill cover system

目的:研究灌木对干燥和湿润过程中三层土质覆盖系统的吸力分布的影响。 创新点:发现对于三层土质覆盖系统,干燥过程中植物蒸腾作用不仅提高了土的吸力,同时也增加了土层之间的吸力梯度(图3)。 方法: 通过土柱试验模拟干燥和湿润过程,并测量吸力的变化。 结论:1. 植物蒸腾作用导致植有灌木的土在干燥过程中所产生的吸力较没有植物的土高60%~200%。2. 植物蒸腾作用提高了植有灌木的土的持水能力;在湿润过程中水分穿透植有灌木的土所需时间比没有植物的土长50%(图3~4)。

Effects of vegetation type on water infiltration in a three-layer cover system using recycled concrete

To promote environmental sustainability, recycled construction concrete is suggested for civil infrastructure works. The aim of this study was to investigate the effects of vegetation type on water infiltration under extreme rainfall conditions in a proposed three-layer landfill cover system containing recycled concrete. Three soil columns, namely bare, covered with shrub (Schefflera arboricola), and covered with grass (Cynodon dactylon), were subjected to ponding tests. Each column was compacted with a bottom layer of silty soil, an intermediate layer of coarse recycled concrete aggregate, and an upper layer of fine recycled concrete aggregate. Water breakthrough occurred only in the bare cover system after 48 h of ponding, equivalent to a rainfall return period of greater than 1000 years in Hong Kong. Under the vegetated covers, suction maintained in the bottom silty soil layer was higher than under the bare cover by 49–52 kPa and hence no percolation was observed after 48 h of ponding. Comparing the two vegetated cover systems, suction maintained under the shrub cover was 2–12 kPa higher (2%–8% lower volumetric water content) than that under the grassed cover in the layers of recycled concrete. This implies that shrub cover can be more effective than grass cover in reducing water infiltration in humid climates.

Water Infiltration into a New Three-Layer Landfill Cover System

AbstractOne of the main purposes of a landfill cover system is to minimize the migration of water into waste, known as percolation, and thereby reduce excessive leachate production. One possible way to achieve this goal is to use a two-layer cover with capillary barrier effects (CCBEs) for arid and semi-arid regions. For a humid climate or prolonged rainfall, the two-layer system with CCBEs is expected to lose its effectiveness for minimizing water percolation. A new three-layer landfill cover system is proposed and investigated for humid climates. This new system adds a fine-grained soil (i.e., clay) underneath a two-layer barrier with CCBE (i.e., a silt layer overlying a gravelly sand layer). The study is conducted by carrying out a one-dimensional (1D) water infiltration test in a soil column. The soil column was instrumented with tensiometers, heat dissipation matric potential sensors, and moisture probes to monitor the variations of pore-water pressure and water content with depth. The amount of wate...

Feasibility study of a new unsaturated three-layer landfill cover system

As an improvement of the two-layer cover with capillary barrier effect (CCBE) (i.e. fine-grained soil overlying a coarse-grained soil), a new three-layer landfill cover system is proposed and investigated for humid climate. This new system is to add a fine-grained soil (i.e., clay) underneath a two-layer CCBE (i.e., a silt overlying a gravelly sand layer). The feasibility of this proposed cover system was investigated by conducting a one-dimensional water infiltration test. In addition, transient seepage simulations were carried out to back-analyse the test results and investigate the importance of hydraulic properties of the CCBE on the proposed cover. Based on the infiltration experiment and numerical back-analysis, it is found that no percolation was observed after 48 hours of ponding, which is equivalent to a rainfall return period of greater than 1000 years. However, the upper two-layer CCBE is only effective for a rainfall return period of about 35 years. This implies that the proposed bottom clay layer is needed for humid climate. Numerical parametric simulations reveal that increasing the saturated permeability of the upper fine-grained soil by two orders of magnitude (1.4x10-6 m/s to 2.1x10-4 m/s), the wetting front is still within the clay layer after 12 hours of constant water ponding (>1000 year rainfall) and no percolation occurred.