Performance Of Cover System Research Articles

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.

A machine learning based approach for 5G network security monitoring

Abstract This paper constructs a 5G network security detection system based on the functional requirements of network security detection and the P2DR process model. The structure can be broken down into three layers from the bottom to the top: acquisition layer, analysis layer, and display layer. The design focuses on writing the SVM algorithm into the software of the communication network security vulnerability monitoring system, and in order to solve the defect of the long training time of the model of this machine learning algorithm, the incremental learning vector machine model is used, which is combined into the CSV-KKT-ISVM model. Test datasets that cover system performance, effectiveness, and leakage are used to test the system after it is completed. The test data was analyzed to prove that the system’s memory usage was maintained at 46M, CPU usage was 5% to 10%, and the response time was no later than 1 s. The monitoring accuracy was 98.5% at the highest but decreased with the increase of the percentage of vulnerability data, and the accuracy dropped to 93.9% at 50%, the minimum was not lower than 90%, and the error rate was no less than 0.8%. To achieve the best outcome, the system threshold should be set to 5, and there should be no false alarms or misreporting.

Assessing water dynamics and net percolation rates within engineered cover systems for mine waste rock piles: A long-term field monitoring study

Mine waste rock piles (WRPs) typically contain trace sulfidic minerals that can react with the atmosphere and produce toxic acid mine drainage (AMD) leachate. Water flow through the waste rock affects both the chemistry and transport of the leachate, and a common remedial solution is the installation of engineered cover systems over the WRP to control net percolation (NP) into the waste rock. As NP is mainly controlled by meteorological conditions and the hydrological properties of cover materials, the water balance (WB) method can be employed to investigate NP and water dynamics within cover systems. However, studies on comprehensive WBs at full-scale WRPs have been limited, and most NP studies have relied on controlled, small-scale laboratory and field tests. This unique study contains the long-term field monitoring data at multiple WRPs needed to comprehensively assess NP rates within cover systems. Field monitoring data with high temporal resolution (hourly) was collected from 2012 to 2018 at four covered WRPs located within the Sydney Coalfield (Nova Scotia, Canada), consisting of one simple soil cover and three multi-layer covers lined with the same geomembrane but differing drainage layer materials. Each WB component – precipitation (PPT), evapotranspiration (ET), water/snow storage, surface runoff, and interflow – was determined, with the residual inferred to be NP into the waste rock. The simple soil cover was not very effective, only reducing NP from the pre-cover 34% PPT to 30% PPT. In contrast, the geomembrane-lined cover systems reduced NP to 4% PPT (no drainage layer), 1.6% PPT (thick gravel drainage), and 1.8% PPT (geocomposite drainage net). While ET and water/snow storage changes at all WRPs were similar, the biggest influence on NP was runoff and interflow. This study provides new field information on the performance of cover systems for controlling NP and water dynamics at full-scale WRPs.

Cover systems with synthetic water‐repellent soils

AbstractA cover system is a crucial component of engineered landfills, to minimize water percolation into the underlying waste. Capillary barriers are an alternative cover system, which has been widely used in the arid and semiarid regions as no cohesive, low‐permeability materials are used. However, the performance of capillary barriers in tropical climate has been unsatisfactory (breakthrough observed). In recent years, synthetic water‐repellent granular materials have drawn increasing attention due to their distinctive hydraulic behavior (inhibited water infiltration and high water entry pressure), suggesting they may also be used to improve the performance of cover systems. In this study, flume tests were conducted with inclined model slopes under artificial rainfall. By monitoring the surface runoff, lateral diversion, and basal percolation and conducting water balance analysis, the performance of monolithic cover, conventional capillary barrier, and water‐repellent cover systems were evaluated. The study revealed that (a) the barrier effect and diversion capacity were significantly strengthened by induced water repellency, providing a promising solution to extend the application of capillary barrier covers; and (b) cover systems can be formed using one raw material to decrease the construction cost, by using synthetic water‐repellent soil as the underlying layer.

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.

Evaluation of Gridded Meteorological Data for Calculating Water Balance Cover Storage Requirements

Core Ideas Required storage is a critical design parameter for water balance soil covers. Gridded meteorological data and station data result in consistent required storage. Gridded data result in higher required storage for sites with significant snowfall. Modeled cover system performance based on station and gridded data is consistent. Gridded data tend to result in conservative or higher estimates of required storage. Water balance cover (WBC) systems are attractive for waste management facilities in arid to semiarid environments because they are a lower cost alternative to conventional geotechnical soil or composite barrier‐style covers. The required storage (Sr) for a WBC is dependent on climatic conditions, with the most influence from the seasonal balance of precipitation (P) and potential evapotranspiration (PET). Gridded meteorological datasets provide long‐term, continuous, and accessible sources of P and PET for Sr estimation. Six sites in Nevada were analyzed for the agreement among P, PET, and Sr calculated from station and gridded meteorological data (gridMET). Two sites each at elevations of 0 to 1000 m asl, 1000 to 1500 m asl, and 1500 to 2000 m asl were considered during a 21‐yr period. The gridMET and station‐based P and Sr were generally not significantly different for sites between 0 and 1500 m asl. For sites above 1500 m asl, the differences in P were significant. For the sites analyzed in this study, gridMET typically resulted in equivalent or more conservative estimates of WBC Sr. Percolation rates from hypothetical WBCs were also simulated in the flow model HYDRUS to evaluate WBC performance when using station‐based or gridMET‐based atmospheric model boundary conditions. Simulations indicate that percolation rates are higher when gridMET atmospheric data are used vs. station‐based data for five out of the six sites. This suggests that gridMET‐based model boundary conditions provide a degree of conservatism in calculating WBC Sr. This trend was not ubiquitous for all sites (one out of six did not follow), indicating that further study is required before applying these findings generally.

Five-year performance monitoring of a high-density polyethylene (HDPE) cover system at a reclaimed mine waste rock pile in the Sydney Coalfield (Nova Scotia, Canada).

Cover systems are commonly placed over waste rock piles (WRPs) to limit atmospheric water and oxygen ingress and control the generation and release of acid mine drainage (AMD) to the receiving environment. Although covers containing geomembranes such as high-density polyethylene (HDPE) exhibit the attributes to be highly effective, there are few, if any, published studies monitoring their performance at full-scale WRPs. In 2011, a HDPE cover was installed over the Scotchtown Summit WRP in Nova Scotia, Canada, and extensive field performance monitoring was conducted over the next fiveyears. A range of parameters within the atmosphere, cover, waste rock, groundwater and surface water, were monitored and integrated into a comprehensive hydrogeochemical conceptual model to assess (i) atmospheric ingress to the waste rock, (ii) waste rock acidity and depletion and (iii) evolution of groundwater and surface water quality. Results demonstrate that the cover is effective and meeting site closure objectives. Depletion in oxygen influx resulted in slower sulphide oxidation and AMD generation, while a significant reduction in water influx (i.e. 512 to 50mm/year) resulted in diminished AMD release. Consistent improvements in groundwater quality (decrease in sulphate and metals; increase in pH) beneath and downgradient of the WRP were observed. Protection and/or significant improvement in surface water quality was evident in all surrounding watercourses due to the improved groundwater plume and elimination of contaminated runoff over previously exposed waste rock. A variably saturated flow and contaminant transport model is currently being developed to predict long-term cover system performance.

11 Years of Evapotranspiration Cover Performance at the AA Leach Pad at Barrick Goldstrike Mines

The AA Leach Pad at Barrick Goldstrike Mine was reclaimed using an evapotranspiration (ET) cover designed to limit the infiltration of precipitation into the facility. Water content and matric potential sensor stations were installed in the cover and underlying leach material after cover system placement. Monitoring of the sensor nests continued for 11 years. Data indicates that the cover is performing well, limiting net percolation to less than 1 % of annual precipitation. The AA Leach Pad is the first large-scale closed mine waste facility that has been robustly monitored for a relatively long time in Nevada, USA. The results provide an understanding of ET cover system performance for closure of mine waste facilities and offer guidance for ET cover system requirements in other arid regions.

Lateral Subsurface Flow in a Soil Cover over Waste Rock in a Humid Temperate Environment

The performance of cover systems over waste rock piles in humid, temperate regions (i.e., where annual precipitation > annual potential evapotranspiration) is likely to be defined largely by their ability to shed water laterally within the cover. Lateral flow processes in this context are still poorly understood. Here we present a field and modeling study of the mechanisms that produce lateral subsurface flow and vertical percolation in a 7‐yr‐old cover system at a mining site in Southeast Alaska. The cover consists of a growth medium on top of a coarse drainage layer, underlain by a highly compacted barrier layer. A second coarse drainage layer separates the cover system from the underlying waste rock. We installed a trench to measure lateral subsurface flow in the cover and then successfully modeled this behavior, without calibration, using a two‐dimensional physics‐based model. Our results show that the cover responds rapidly to precipitation, converting approximately two‐thirds of the input to lateral subsurface flow. Lateral subsurface flow is preceded by the development of transient perched water tables at the interface of the coarse drainage layer and compacted barrier layer. Water balance simulations indicate that flow through the barrier layer is driven by a small but permanent vertical pressure head gradient that develops within the barrier layer and results in vertical net percolation of approximately 15% of the precipitation input. These model results correspond well with lysimeter measurements of vertical percolation into the waste rock.

English

Golden Sunlight mine is located northeast of Whitehall, MT. Two cover systems were installed on the west waste rock complex and tailings area in order to minimize water infiltration and acid drainage. The covers were designed to function as a store-release system and are composed of coarse and fine materials being 0.80 m thick on the waste rock and 1.8 m thick at the tailings area. In order to study the influence of thickness, material properties, vegetation and climate on the performance of the cover systems, a laboratory testing program was carried out to identify the properties of the cover at different depths. In addition, thermal conductivity sensors were installed at three field monitoring stations located on the waste rock dump and tailings area in order to evaluate the variation of suction in the cover profiles. These data were used to calibrate a numerical modeling program that assessed infiltration rates through the covers under different vegetation and climate conditions. The simulation results were used to correlate internal variations in grain size and hydraulic conductivity with cover performance in terms of measured suction profiles and infiltration rates. The results show that vegetation plays a critical role in cover performance and becomes most important during wet and very wet years. The simulations and field measurement also revealed that the internal properties of the cover control infiltration patterns, showing upward and downward fluxes at different zones within the cover. It is shown that while increasing layer thickness reduces the dependence of cover performance on vegetation conditions, the establishment and sustainability of vegetation remains most important with respect to satisfactory long-term performance of store-release cover systems. Additional Keywords: Store-release cover, numerical modeling

English

The accurate determination of saturated and unsaturated hydraulic properties of mine waste and cover material is critical for predicting long-term drainage behavior and closure performance. The rock fragments typically found in mine waste and borrow materials complicate laboratory hydraulic property measurements. Many hydraulic testing laboratories address this issue by removing all material greater than 4.75 mm in diameter, repacking the remaining fine-earth sample in small diameter cores, and “correcting” the resulting measurements for the gravel content using published correction factors. In order to evaluate several of the aforementioned gravel correction and hydraulic property prediction methods, a laboratory experiment was designed to test various well-graded gravelly materials. An alluvial material sample was used to fabricate eight soils with various particle size distributions. The primary sample matrix for all testing was chosen to be the fine-earth fraction, less than 4.75 mm in diameter. Additional soil materials were then fabricated in which either part of the fine-earth fraction was removed, or gravel material ranging from 4.75 mm to 19 mm diameter was added. Test results show that saturated hydraulic conductivity (Ksat) decreased with up to 30% gravel content but increased by orders of magnitude at higher gravel contents. Moisture retention characteristic (MRC) data showed that the air entry value decreased with increasing gravel contents, although the amount of retained water only slightly decreased as gravel content increased. Depending on how the MRC data is interpreted, the predicted unsaturated hydraulic conductivities either showed increasing hydraulic conductivity as gravel contents increased, or the values converge. The measured hydraulic property data could not be accurately predicted using published correction factors, or by other prediction methods that use particle size distribution data. Consequently, removing the gravel fraction could result in significant error in the prediction of mine waste drainage behavior and the performance of cover systems. It is recommended that published correction factors not be used unless the sample is similar in gradation and bulk density to the soils tested by the published method. .