|Ph.D. CANDIDATES||M.Sc. STUDENTS|
Effect of protection layers on aging of geomembrane liners under chemical exposure, elevated temperatures, and physical stresses
High Density Polyethylene (HDPE) geomembranes are extensively used as component of the primary and secondary landfill liner systems. With time, geomembranes experience aging and degradation during their service life. This research conducts laboratory experiments using steel pressure vessels (0.6 m diameter, 0.5 m high), simulating real life landfill conditions, involving elaborate leachate circulation, temperature, and pressure control systems. The results of the testing program will be used to derive improved estimates of geomembrane service life and to provide options to improve long-term performance of geomembrane liners. In addition accelerated ageing tests will examine the effect of (1) different chemical constituents found in municipal solid waste leachate, (2) geomembrane thickness, on the second and the third stages of HDPE geomembrane ageing.
A Field Study of Geomembrane Wrinkles
Geomembranes (GM) are used as barrier in landfill liners and covers, often as a composite barrier system with a geosynthetic clay liner (GCL) to prevent the migration of contaminating fluids. During construction, thermal expansion of the geomembrane may occur as a result of solar radiation, causing wrinkles to develop in the geomembrane. The magnitude of this thermal expansion, and the resulting size and number of wrinkles is controlled by the thermal properties of the geomembrane as well as the exposure temperature. The behavior of geomembrane wrinkles in the field is still not fully understood; there are complex factors involved in their development, including material properties of the geomembrane, weather conditions and construction quality control and quality assurance (QA/QC). The wrinkles will be monitored over two seasons at a field site near Godfrey, Ontario, as well as actual landfill installations across Canada. The data will be gathered by taking low altitude air photos from a tethered blimp, and the photos will be digitally analyzed to determine the length, area and connectivity of the wrinkles. Laboratory studies will be conducted to look at geomembrane thermal expansion and wrinkling in detail. Methods of wrinkle reduction will also be tested. (Co-supervised by Drs. R. Brachman and W.A. Take)
Long-term performance of geomembrane liners
This research investigates the long-term performance of High Density Polyethylene (HDPE) geomembrane (GM). The study includes: (a) aging the GM at different temperature in different exposure media such as air, water and synthetic leachate; and (b) aging of GM as a part of composite liner system at different temperatures. In this study, four GM thicknesses are examined 1.0, 1.5, 2.0 and 2.5 mm to investigate the effect of thickness on the long-term performance of the GM. Another component of the study involves examination of the effect of sunlight exposure on geomembrane.
Thermal behavior of landfill lining system
Biodegradation of waste generates heat and it increases liner temperature. Elevated temperature accelerates aging of geomembrane. It also gives rise to thermal gradients from the geomembrane to depth in the surrounding soil. This thermal gradient induces moisture to move away from the upper part of the liner and may result in desiccation of the clay component. Previous studies have focused on measuring/estimating temperature at the top of primary liner and investigating its effect on service life of liner components. A research program is underway to investigate different techniques for controlling the increase in landfill liner temperature and/or minimizing the effect of temperature on long-term performance of liner components (aging of geomembrane and desiccation of clay liner). Appropriate measures during construction and operation of landfill (e.g. cooling liner by means of geothermal cooling pipes, limiting desiccation by proper construction, liner insulation, etc.) are being studied.
Effectiveness of geosynthetic clay liner on arsenic-rich gold mine tailings
Gold mines frequently have high natural arsenic concentrations in tailings and nearby streams and groundwater. In the case of publicly accessible gold mine sites in Nova Scotia, where arsenic is much higher than the recommended soil guideline, placing a geosynthetic clay liner (GCL) as a cover on the tailings may provided a means of minimizing the release of arsenic. GCLs have the potential to provide a low permeability barrier to infiltration of precipitation through the cover and into the waste. However, research has shown that the effectiveness of GCL will depend on the extent to which there is chemical interaction with cations in the surrounding soils. Field and laboratory tests will be conducted to examine the potential uptake of ions by the GCL as it hydrates and as ions diffuse from typical soil in the area to the GCL. Samples of GCL will be placed in contact with soils similar to that expected in the field and it will be allowed to hydrate from these soils while the change in GCL properties will be monitored. Some samples will also be subjected to freeze-thaw cycles after interaction with the adjacent soils to assess any changes in hydraulic conductivity related to combined geochemical interaction and freeze-thaw activity. Consideration will be given to a conventional GCL, a relatively new product with a polymer film bonded to the GCL, and an GCL with an amended bentonite.
This research involves experiments to examine the rate of contaminant leakage through the GM/GCL system considering a hole in the GM when overlying GCL overlaps. Factors being considered include the impact of the GCL type, overlap width, the presence of bentonite powder in the overlap, the hydraulic gradient across the system, and the normal stress acting on the system. Experiments are being conducted with tap water to assess the effects of the physical interaction between the GM and GCL overlaps for the different GCLs, overlap dimensions, etc. on leakage since this will provide an initial assessment of the potential advective migration of emerging contaminates of concern through the overlap. Since there is often potential for the composite liners to be exposed to both cation exchange as well as wetting and drying cycles for considerable periods of time before covering with a protective layer, GCL seams that have been subjected to these cycles over a period of 3-6 years at Queen’s field test site north of Kingston are being exhumed and also subjected to testing for seam effectiveness. The experiments examine different levels of hydration prior to permeation. The system interactions will be examined for two GM conditions: (1) a linear defect in a GM that is in close contact with the GCL immediately adjacent to an overlapped seam, and (2) a hole in a wrinkle located over the seam (this minimizes the vertical stress applied at the seam and may induce some shear deformation that could impact on the seam’s hydraulic performance).
2D and 3D modeling of reinforced embankments on soft soil
Develop a 3-D elasto-viscoplastice finite element model to investigate 3D effects on the performance of reinforced embankments on rate sensitive soil and compare results with those from 2D analyses. Extend the model to consider the combined effects of reinforcement and PVDs based on 3D analyses and compare with results form 2D approximations.
The analysis of a deep excavation in a gassy soil
Develop a finite element model to represent the behavior of gassy soils and to model complex three-dimensional excavations in these soils (with particular reference to two failures that occurred near Sarnia in 1990s which will be used as case histories to verify the predicative capability of the model). This project will involve (a) building a 3D finite element model using Abaqus (b) developing and implementing a 3 phase gassy soil constitutive model which accounts for the behavior and interaction of the soil particles (solid phase), the pore water (liquid phase) and the methane (gas phase); (c) modeling the transition from the initial conditions two phase conditions (with the gas dissolved in the pore water) to the final three phase conditions; (d) modeling the effect of complex three dimensional unloading conditions on the dissolution of gas and the consequent changes in the soils effective stress and strength response.
Taghizadeh Saheli, Pooneh
Transport of contaminants emerging of concern through geomembranes and GCLs
The migration of three emerging contaminants of concern [carbon nanoparticles (CNP), Bisphenol-A (BPA) and polybrominated diphenyl ether (PBDE)] is being studied. Experiments include examination of: (1) diffusion through geomembranes, (2) diffusion through GCLs, and (3) advection through GCLs. Diffusion and sorption coefficients of GCL and attenuation layer for the three EECs will also be evaluated through experiments. Advective transport through a GCL will be evaluated by performing advection-diffusion tests on GCL.
Modeling Leachate Characteristics and Clogging
Based on both theoretical considerations and the findings from both the field and laboratory studies, a model of clogging incorporating combined biological, chemical and physical processes “Bioclog” has been developed to predict the leachate characteristics and clogging properties for the column tests, mesocosm tests under the controlled laboratory conditions, and to model the field collection systems with sand drainage layers. However more work is needed to calibrate the model against measured mesocosm data and develop it, so it is amenable to model field cases with gravel drainage layers and filter-separator layers. The research being undertaking include: (1) adding the capacity to model sedimentation during transport and examining the effects of bacterial sedimentation on the distribution of species concentration and porosity in the leachate collection systems; (2) incorporating a granular filter layer, geotextile filter layer or a combined filter layer in the Bioclog model, and analyze their effect on the service life of leachate collection system; (3) improving the ability to model situations where there is a very wide (many orders of magnitude) variation in hydraulic conductivity of the drainage layer (from very low in the lower portions that are significantly clogged to very high just below the unsaturated zones when the leachate mound rises); (4) using the model to predict field behavior and provide design guidelines.
Performance of a GCL located on a side-slope
Geosynthetic clay liners (GCLs) are often used in conjunction with an impermeable geomembrane for landfills with the purpose of minimizing leakage that may occur through holes in the geomembrane. A GCL is composed of high-swelling clay (commonly sodium bentonite) placed between two geotextiles. This research is focused on examining the potential for changes in hydraulic conductivity of a GCL in an exposed composite liner installed on a side slope.
Geomembrane puncture in heap-leach pads under extreme vertical pressures
The potential for puncture of geomembranes are used to line the base of heap-leach pads (e.g., as used in copper and gold mineral extraction) is being examined in laboratory experiments using steel pressure vessels (0.6 m diameter, 0.5 m high) that simulate real life heap leach conditions by using crushed rock with particle size less than 50 mm, various protection layers, a GM and then either a clay liner or coarser material with some percentage of fines. Applied pressures of up to 3000kPa are initially being examined. Puncture resistance and the short-term tensile strains induced in a geomembrane are being monitored. Further tests will be conducted to examine the effect of the type and thickness of geomembrane and pressures up to 7000kPa.
Transport of PCBs and BTEX through Landfill and settling pond liner systems from Field Sites
Geomembrane liners are commonly used as the primary barrier to advection to contaminant migration in landfill designs. Two field sites are the focus of the study. One is the Pottersburg Creek PCB Storage Facility, a 25 year old PCB landfill that was decommissioned in 2009. On the Pottersburg Site, research is being conducted on the (1) diffusive and sorptive properties of exhumed geomembrane; (2) profiling of the contaminant migration through the liner system; (3) diffusive properties of the exhumed clay line. A second site also being investigated is Resolution Island, Nunavut. There HDPE and PVC geomembrane are being used to line settling ponds in the remediation of a PCB and hydrocarbon contaminated site. Research being conducted on this site involves BTEX (Benzene, Toluene, Ethylbenzene and Xylenes) diffusion and sorption with respect to the exposed and unexposed geomembranes.
An Advisory System for Landfill Barrier Design
This seeks to integrate current knowledge regarding the design of landfill barrier systems into a software program that will act as a decision support system for landfill design. It will cover both the design and operation of municipal solid waste landfills, with consideration of interactions between components and the maximization of long-term system performance. The program will guide the user through the process of designing a landfill while highlighting issues of concern. The choices available for each component will be presented, along with their suitability for different environments. It will help the user to determine the optimal solution for the site in question. Guidance will also be given to advanced users on the required input parameters for using models such as POLLUTE, BioClog and HELP.
|Barrier Systems Project|