Covers With Capillary Barrier Effects Research Articles

Modeling the influence of forest vegetation and climate change on the long-term performance of a cover with capillary barrier effects used to control acid mine drainage: the Lorraine case study

ABSTRACT The Lorraine mine site in Témiscamingue region (Québec, Canada) was reclaimed with a cover with capillary barrier effects (CCBE) to limit acid mine drainage generation. The main objective of this study is to predict the long-term hydrogeological behaviour of the CCBE at Lorraine site, including the influence of vegetation and climate change (CC). Numerical simulations showed that performance of the CCBE is: (1) not affected by forest vegetation and CC in the saturated zone in the north of the Lorraine site and (2) more sensitive to vegetation in the unsaturated zone in the south where the phreatic level is low.

Monitoring moisture dynamics in multi-layer cover systems for mine tailings reclamation using autonomous and remote time-lapse electrical resistivity tomography

The dynamics of moisture content in cover systems constructed on mining wastes were monitored at the pilot scale using 2D autonomous, remote, and non-invasive time-lapse electrical resistivity tomography combined with conventional point sensors. A methodology was proposed to process the daily hydrogeophysical datasets from 23 m-long instrumented sections of covers with capillary barrier effects (CCBEs) designed to act as oxygen barriers, and covers with low saturated hydraulic conductivity layers (LSHCCs) designed to limit the water infiltration rate. Hydrogeophysical monitoring suggested that CCBEs were able to maintain high degrees of saturation in the moisture-retaining layer throughout the one-year monitoring period, which would make it an efficient oxygen barrier. Larger spatio-temporal changes in moisture content were observed in LSHCCs and most of the low hydraulic conductivity layers remained below 85% saturation, which was attributed to the combined effect of low precipitation, rapid vegetation development, and water percolation through the cover. The methodology proposed in this pilot-scale "proof-of-concept" study allowed the hydrogeological behavior of the cover systems to be monitored in the 23 m-long instrumented sections using continuous geoelectrical data, which demonstrated that this innovative monitoring technique could be useful for geochemical and geotechnical monitoring programs in large-scale mining waste storage facilities.

Valorisation of Partially Oxidized Tailings in a Cover System to Reclaim an Old Acid Generating Mine Site

The reclamation of acid-generating mine tailings typically involves building cover systems to limit interactions with water or oxygen. The choice of cover materials is critical to ensure long-term performance, and partly determines the environmental footprint of the reclamation strategy. The objective of this research was to evaluate if tailings pre-oxidized on-site could be used in cover systems. Column experiments were performed to assess the effectiveness of a cover with capillary barrier effects (CCBE), where the moisture retention layer (MRL) was made of pre-oxidized tailings with little to no remaining sulfides (LS tailings). The columns were submitted to regular wetting and drying cycles, and their hydrological and geochemical behaviour was monitored for 510 days. The LS tailings showed satisfying hydrological properties as an MRL and remained saturated throughout the test. The concentrations of Cu in the drainage decreased by more than two orders of magnitude compared to non-covered tailings. In addition, the pH increased by nearly one unit compared to the control column, and Fe and S concentrations decreased by around 50%. Despite these improvements, the leachate water remained acidic and contaminated, indicating that acid drainage continued to be generated despite a hydrologically efficient CCBE.

Evaluation of mine site reclamation performance using physical models: Case of Ity mine (Ivory coast)

The exploitation of the Ity gold mine in Ivory Coast will generate tailings that can be considered as potential generators of acid mine drainage (AMD). Various materials such as marble and clay are available around the mine site that could be used as components of an engineered cover for the reclamation of the reactive tailings impoundment. However, it is important to assess the properties of these potential cover materials and test different reclamation scenarios in the laboratory to select the best scenario for mine site reclamation. For this purpose, physical modeling was performed using experimental instrumented column tests simulating covers with capillary barrier effects (CCBEs) and monolayer covers. The columns were instrumented with sensors for measurement of volumetric water content (VWC) and suction. Leaching tests were performed over many cycles and physical and chemical analyses were carried out on the leachates. The observed hydrogeological behavior shows that either a CCBE or a monolayer cover with clayey silt materials can be used for the Ity mine site reclamation. However, hydrochemical results allow confirming that the monolayer cover made with clayey silt material can be considered as the best scenario for the Ity mine site reclamation.

Preliminary assessment of biosolids in covers with capillary barrier effects

The cost of sourcing material for cover material is often a barrier to implementation of multilayer covers to reclaim mined waste or other disturbed land. This study investigated the use of municipal biosolids blended with stabilizing materials as a low-permeability material in covers with capillary barrier effects (CCBEs) for reactive mine tailings. A blended biosolids referred to as Custom Reclamation Mix or CRM (1:1 volumetric mix of anaerobically digested biosolids and leaf and yard waste) was evaluated as a candidate low-permeability layer in a CCBE using material characterization, physical simulation of the CCBE in columns, and unsaturated flow modelling. The CRM exhibited low saturated hydraulic conductivities (k = 4 × 10 −7 cm/s @ e = 4.0) and an air-entry value of approximately 300 kPa. During laboratory column testing comparing a monolayer biosolids cover to a biosolids CCBEs, biosolids layers within the CCBEs remained highly saturated, acting as a barrier to oxygen diffusion and water flux. Pore-water samples in the tailings of the monolayer cover show some evidence of increased concentrations of nitrate relative to CCBEs columns. No significant change in pH was observed in any of the columns. Numerical models showed reduction in water flux at the tailings surface by 98% and a reduction in oxygen diffusion by up to three orders of magnitude when using biosolids CCBEs relative to uncovered tailings. Cover function was not significantly impacted by the characteristics of the underlying tailings. Whereas substantial cracking occurred in the biosolids monolayer cover, the low-permeability layers in the CCBEs remained sufficiently intact to remain both saturated and apparently retained low hydraulic conductivity. The use of amended biosolids as a low-permeability layer in a CCBE therefore appears promising and should be evaluated at larger scales.

Reactive transport modeling of the geochemical behavior of highly reactive tailings in different environmental conditions

The hydrogeochemical behavior of highly reactive tailings with no or very low neutralizing capacity and their hydrogeochemical response when covered by a cover with capillary barrier effects (CCBE) were evaluated in laboratory columns. The hydrogeochemical response of highly reactive tailings under wetting-drying cycles and saturated conditions was also simulated using the MIN3P code, a finite volume model for coupled groundwater flow, oxygen diffusion and multi-component reactive transport. Laboratory tests were conducted for approximately 18 wetting and drying cycles, over a period of one and a half years. A numerical simulation model was validated using laboratory results of uncovered highly reactive tailings under wetting and drying cycles conditions. The acceptable agreement between the geochemical observations in the laboratory and the numerical simulations shows that MIN3P can simulate the hydrogeochemical response of highly reactive tailings (initially very acidic) under laboratory experimental conditions and can make realistic predictions. The laboratory hydrogeochemical response of highly reactive tailings covered by a CCBE and the numerical modeling results of highly reactive tailings under saturated conditions (an equivalent CCBE oxygen barrier) indicated a decrease in the concentration of sulfate and most metals in the leachate compared to uncovered highly reactive tailings, indicating that both scenarios decreased sulfide oxidation. The simplified approach for geochemical modeling of saturated reactive tailings evaluated in this paper could be used to determine the required oxygen barrier efficiency to reach the target effluent geochemistry as a preliminary stage of reclamation design.

Hydrogeochemical Behavior of Reclaimed Highly Reactive Tailings, Part 2: Laboratory and Field Results of Covers Made with Mine Waste Materials

The possibility of using mine wastes (low-sulfide tailings and waste rocks) as cover components to prevent acid mine drainage (AMD) generation from highly reactive tailings was previously investigated through a laboratory-based characterization of reactive tailings and cover materials (Part 1 of this study). Characterization results showed that the reactive tailings are highly acid-generating, and that the mine waste materials that were used in this study are non-acid-generating and have suitable hydrogeological and geochemical properties to be used in a cover with capillary barrier effects (CCBE). In order to further investigate the use of low-sulfide mining materials in the reclamation of highly reactive tailings, a large laboratory-based column and a field cell simulating a CCBE were constructed. The instrumented field cell used the same configuration and materials as the laboratory column. This paper presents the main findings from 504 days (column test) and three seasons (field test) of monitoring, and compares the hydrogeochemical behavior observed at the two scales. The results show that a CCBE made with low-sulfide mine wastes would be efficient at reducing oxygen fluxes and limiting AMD generation from highly reactive tailings at the laboratory and intermediate scale. However, at these two scales, the concentrations of some contaminants were not reduced to levels of the legally imposed environmental objectives. The results also showed differences in metal and sulfate concentrations in the drainage waters between the laboratory and field scales. The outcomes from this investigation highlight that the previous oxygen flux design targets and the typical configurations of multilayer covers developed for fresh non-oxidized tailings or pre-oxidized tailings may not always be directly applicable for fresh or pre-oxidized highly reactive tailings.

Hydrogeochemical Behavior of Reclaimed Highly Reactive Tailings, Part 1: Characterization of Reclamation Materials

The production of solid mine wastes is an integral part of the extraction and metallurgical processing of ores. The reclamation of highly reactive mine waste, with low neutralizing potential, is still a significant challenge for the mining industry, particularly when natural soils are not available close to the site. Some solid mine wastes present interesting hydro-geotechnical properties which can be taken advantage of, particularly for being used in reclamation covers to control acid mine drainage. The main objective of this research was to evaluate the use of mining materials (i.e., tailings and waste rock) in a cover with capillary barrier effects (CCBE) to prevent acid mine drainage (AMD) from highly reactive tailings. The first part of the project reproduced in this article involves context and laboratory validation of mining materials as suitable for a CCBE, while the companion paper reports laboratory and field results of cover systems made with mining materials. The main conclusions of the Part 1 of this study were that the materials studied (low sulfide tailings and waste rocks) had the appropriate geochemical and hydrogeological properties for use as cover materials in a CCBE. Results also showed that the cover mining materials are not acid-generating and that the LaRonde tailings are highly reactive with pH close to 2, with high concentrations of metals and sulfates.

Oxygen migration through a cover with capillary barrier effects colonized by roots

Covers with capillary barrier effects (CCBEs) are multi-layered oxygen barrier covers used in humid climates to reclaim reactive mine tailings and limit the generation of acid mine drainage. Once constructed, CCBEs are colonized by surrounding plants. Roots modify water storage and respire oxygen. The performance of CCBEs could evolve over time due to root colonization. Twenty-five plots with varying vegetation were investigated at a 17-year-old CCBE in the mixed forest of Quebec, Canada. Geotechnical parameters and root colonization of the moisture-retaining layer (MRL) of the CCBE were characterized. The performance of the MRL to control oxygen migration was assessed using oxygen consumption tests and numerical modeling. Despite root colonization at the surface of the MRL, oxygen fluxes generally complied with the CCBE’s design criteria. Root presence created oxygen consumption in the MRL, which could be expressed with a reactivity coefficient (Kr). A positive correlation (R2 = 0.65) was found between root length density and Kr. Oxygen consumption by root respiration helped to lower oxygen fluxes by 0.5 to 76 g/m2/year, with a mean of 13 g/m2/year. These results will help to better understand the influence of roots on CCBEs’ performance to control oxygen migration.

Influence of climate change on the ability of a cover with capillary barrier effects to control acid generation

Annual precipitation, temperature, and the frequency and duration of drought events are expected to increase in Quebec (Canada) south of the 50th parallel as a result of climate change. Oxygen barriers such as covers with capillary barrier effects (CCBE), which are used to control acid mine drainage (AMD), are sensitive to climate change. Increases in precipitation may have positive effects on maintaining the saturation of a moisture-retaining layer (MRL), which is necessary for reducing oxygen fluxes to reactive tailings; however, drought events could cause temporary desaturation of a MRL and, consequently, increase the potential for acid generation. The Lorraine mine site, in western Quebec, was reclaimed with a CCBE in 1999 and was used as a case study to assess the effects of climate change on reclamation performance. A two-dimensional numerical model was developed and validated using 3 years of meteorological and hydrogeological data. Numerical simulations were performed to assess the CCBE’s performance, considering the influence of: (1) climatic conditions (i.e., precipitation, temperature, relative humidity, wind speed, albedo and solar radiation) by 2100 for three different climate-change scenarios, and (2) extreme drought events. Performance targets were reached for all tested climate-change scenarios, thus indicating that the Lorraine CCBE design is robust with respect to climate change and extreme droughts. More broadly, this study illustrates how numerical simulations can be used to quantify the influence of climate change on the long-term ability of CCBEs to control the generation of AMD.

Acid-Generating Waste Rocks as Capillary Break Layers in Covers with Capillary Barrier Effects for Mine Site Reclamation

Tailings and waste rocks can be used to build covers with capillary barrier effects (CCBE) for the purposes of reclaiming acid-generating waste storage facilities while enhancing the value of the materials available on site. The efficiency of non-acid generating tailings, desulfurized tailings, and non-reactive waste rocks as cover materials was demonstrated in previous laboratory and field studies. However, acid-generating waste rocks are usually not considered for cover construction because of the risk of contamination. Nonetheless, using acid-generating waste rocks as the bottom capillary break layer in a CCBE could have economic and logistical benefits for companies, including helping to reduce the volume of waste rock piles and to valorize material that are generally considered to be problematic. In this study, laboratory column tests were performed to evaluate cover scenarios using acid-generating waste rocks from Westwood-Doyon mine (Quebec, Canada). These waste rocks were placed under a moisture-retaining layer made of desulfurized tailings. A column test with non-acid-generating waste rocks was also performed for comparison purposes. Columns were submitted to eight wetting/drainage cycles. The performance of these systems was assessed by monitoring the volumetric water content in the different layers and by analyzing the water quality of the leachates. Significant reductions in contamination were observed when covers were added on the reactive waste rocks. These results suggest that it could be possible to valorize acid-generating waste rocks in cover systems.

Comparative performance of cover systems to prevent acid mine drainage from pre-oxidized tailings: A numerical hydro-geochemical assessment

The generation of acid mine drainage (AMD) remains a major environmental challenge for the mining industry. The reclamation of old mine sites with pre-oxidized tailings is particularly challenging because of indirect oxidation reactions which can limit the overall effectiveness of an oxygen barrier to prevent AMD. The goal of this project was to quantitatively compare the effectiveness of different cover systems to reclaim two pre-oxidized acid-generating tailings sites, located in Quebec (Canada). Following laboratory column tests, field measurements and observations, coupled hydrogeological and geochemical numerical simulations were conducted to evaluate the effect of various system characteristics. Cover performance was assessed by simulating the evolution of the degree of (water) saturation, pore water pressures, oxygen fluxes and leachate quality. Several reclamation options, including monolayer covers and two- or three-layer covers with capillary barrier effect(s) were simulated. The simulations indicate that because of reduced cover effectiveness with pre-oxidized tailings, the general design targets developed for non-oxidized tailings may not always be directly applicable to already oxidized tailings. The simulations also illustrate how the behaviour and efficiency of a monolayer cover placed over reactive tailings depend on specific factors, including water table position, initial porewater chemistry, and cover materials' hydrogeological properties and thicknesses. The results indicate that under a given set of conditions, a bilayer cover (with a capillary break above the reactive tailings) would not significantly improve cover performance (compared to a monolayer cover) due to water losses by evaporation. The simulations show, however, that a well-designed three-layer cover with capillary barrier effects (CCBE) would be efficient in reducing the oxygen flux and AMD generation, even in the case of highly pre-oxidized tailings. The outcomes from this investigation highlight some of the advantages of carrying out coupled hydrogeological and geochemical simulations to assess the long-term behaviour of reclaimed mining sites.

Hydrogeological Behaviour of Covers with Capillary Barrier Effects Made of Mining Materials

A laboratory study and numerical predictions were conducted to evaluate the effectiveness of a cover with capillary barrier effects (CCBE) made of mining materials in limiting the generation of acid mine drainage. The present study included the installation of three experimental laboratory columns as physical models of CCBEs. Each column was instrumented to monitor the hydrogeological behaviour of the different layers of the cover. In the moisture-retaining layer, which is made of desulphurized tailings, the degree of saturation (Sr) is usually maintained above 85%, while Sr in the two waste rock layers placed above and below the moisture-retaining layer (MRL) is typically between 5 and 15%. Suctions were usually lower than the air entry value for the desuphurized tailings layers, while suctions largely exceeded the water entry value of the mine waste rock in the two capillary break layers (CBLs). The oxygen flux was calculated using numerical modeling at the bottom of the MRL. Results confirm that the oxygen flux is low (between 0.34 and 3 g/m2/year). This study also confirms the potential of mine waste rocks to act as a substitute for natural granular soils in the CBL of a CCBE.

Influence of freeze–thaw cycles on the performance of covers with capillary barrier effects made of crushed rock–bentonite mixtures to control oxygen migration

In temperate climates, covers with capillary barrier effects (CCBEs) are being used successfully to prevent oxygen fluxes from reaching covered potentially acid mine drainage (AMD) generating mine tailings. In northern climates, the more attractive option for mine site reclamation is insulation covers, which are designed to keep reactive materials frozen. This article suggests that CCBEs can simultaneously control oxygen migration and mine waste temperature to inhibit AMD generation. However, in northern conditions, where natural fine-grained materials needed for the CCBE moisture-retaining layer are not always available, soil–bentonite mixtures could be used instead. This laboratory study assessed — using instrumented columns — the effects of freeze–thaw cycles on the performance of three CCBEs made with crushed rock–bentonite mixtures. An oxygen diffusion test was developed to determine the effective diffusion coefficient of oxygen (De) and its sensitivity to freeze–thaw cycles. The results show good initial performance for the saturated CCBEs. However, the tested CCBEs are significantly affected by freeze–thaw cycles and have limited oxygen-limiting ability.

Preliminary geotechnical assessment of the potential use of mixtures of soil and acid mine drainage neutralization sludge as materials for the moisture retention layer of covers with capillary barrier effects

Lime treatment of acid mine drainage (AMD) generates sludge that is commonly stored in ponds for dewatering. The use of soil-aged sludge-based mixtures for mine site rehabilitation can allow the emptying of existing basins, thus extending their storage capacity, reducing the volume of the borrow soil pit required for mine site rehabilitation, and consequently reducing the mine footprint. The authors investigated the geotechnical properties of silty soil–sludge mixtures (SSMs) as possible components of covers with capillary barrier effects (CCBEs) to prevent AMD generation from mine waste. SSMs with β values of 10%, 15%, 20%, and 25% sludge (β = wet sludge mass / wet soil mass) were studied. Two water contents were considered for each of the mixture components: 175% and 200% for the sludge and 7.5% and 12.5% for the soil. Results indicate that saturated hydraulic conductivity (ksat) values were in the range of 10−5cm/s for the soil and SSMs at void ratios ranging from 0.28 to 0.53, with values decreasing slightly when β was increased from 0% to 25%. The air-entry value (AEV) increased from 20 kPa for the soil alone to 35 kPa for the SSM with β = 25%. These values of ksatand AEV are comparable to those of materials used in the moisture retention layers of existing efficient CCBEs. However, the volumetric shrinkage increased from about 2% for the soil alone to values ranging between 24% and 32% for the SSM with β = 25%, depending on the initial water contents of the components. Tools are provided to estimate to which extent the use of sludge in SSMs can reduce the volume of borrow natural soil required for a moisture retention layer of a CCBE.

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.

Bluejoint Is an Effective Bio-Barrier Species on Mine Covers.

Covers with capillary barrier effects (CCBE) are used to prevent acid mine drainage from mine wastes in the short term. However, the long-term efficiency of CCBE can be affected by trees because their roots may reduce the ability of covers to limit oxygen migration and also physically damage the CCBE. Two plant species that are native to boreal Canada, bluejoint () and sheep laurel (, were selected as bio-barrier species (BBS) to test if they reduce the growth and root system architecture of trees established on mine covers (balsam poplar [], willow [ spp], and black spruce []). The experiment was established in 2008 on a mine tailings impoundment located in northwestern Quebec, Canada. Trees were measured for height, diameter, and biomass. Coarse roots were excavated from the plots and digitized in three dimensions. Compared with the control (no BBS), bluejoint strongly decreased tree height and diameter increment, biomass, maximum root depth and radial extension, total root length and volume, and number of second- and third-order tree roots. Height and diameter increment, biomass, maximum root depth and volume, and number of second-order roots of balsam poplar increased with sheep laurel compared with control conditions, whereas willow showed no response to this treatment. Most characteristics of black spruce (except root-to-shoot ratio and number of second-order roots) improved in the presence of sheep laurel compared with the control. Thus, bluejoint was a more efficient BBS than sheep laurel. Bio-barriers comprised of bluejoint can be used as a countermeasure for controlling tree invasion of CCBE.

Numerical Investigation on Gas Emission from Three Landfill Soil Covers under Dry Weather Conditions

The design of existing soil landfill final covers, e.g., a single clay layer cover or a cover with capillary barrier effect (CCBE), aims at limiting water infiltration rather than minimizing landfill gas emissions. We propose an alternative three‐layer capillary barrier landfill cover with an added clay layer underneath a CCBE to improve the effectiveness of limiting gas emission. Finite element analyses of coupled water and gas flow were performed to investigate and compare the performance of these three types of soil covers regarding gas emission under a continuous duration of 34 dry d. Due to evaporation, suction in the single clay layer cover increased larger than its air‐entry value, causing a significant increase in the air coefficient of permeability. For the CCBE, evaporation also led to water loss in the silt layer, which cannot serve as a gas barrier. On the contrary, the bottom clay in the alternative three‐layer capillary barrier cover system was protected by a relatively dry sand layer, which acts as a barrier, minimizing water loss from the clay layer. This is because the dry sand layer has a very low water permeability. As a result, the gas emission rate through the alternative three‐layer capillary barrier cover was the lowest among the three types of covers. This rate was lower than the criterion set by the Carbon Farming Initiative, indicating that the alternative three‐layer capillary barrier cover is a possible barrier to minimize gas emission even under dry weather conditions.

Numerical parametric study of an alternative three-layer capillary barrier cover system

A common practice for disposing of municipal waste is to dump it in landfills, where it is then protected by a cover system. The main purpose of this cover system is to minimise the amount of water percolating into the waste and, hence, to reduce the excessive formation of leachate. To achieve this objective, conventional landfill covers employ low permeability materials, such as composite liners, to meet the permeability design criterion. However, their long-term hydraulic behaviour, cost and shear resistance are not entirely satisfactory. Increasingly, covers with capillary barrier effects (CCBEs) have been considered as an alternative cover system in semi-arid and arid regions. It is questionable, however, whether CCBEs can be successfully applied in humid climate conditions where the annual rainfall often exceeds 2000 mm. In this paper, an alternative three-layer capillary barrier cover system for use in humid climates is proposed, and its feasibility is investigated by a numerical parametric study. This alternative system consists of a fine-grained soil layer overlaying a coarse-grained layer, which in turn overlies a fine-grained soil, such as clay, to minimise water percolation in humid climates. This bottom clay layer is protected by the upper two coarser soil layers. The factors considered in the numerical parametric finite element analyses include the thickness of this additional clay layer, rainfall conditions and degrees of saturation of the municipal waste. It was verified that the middle sand layer serves as a capillary break when the rainfall intensity is light or the duration is short. After the upper two layers are permeated, the bottom clay layer serves as an impeding layer, whereas the sand layer shifts to serve as a lateral drainage layer. It was also found that the amount of percolation increases with an increase in rainfall duration but decreases with saturation of the waste. Based on the six simulated durations of rainfall, the most severe rainfall duration is 1 day, irrespective of the return period.