Experimental and numerical study of wet and dry cycles on an innovative polymer treated clay for geosynthetic clay liners

Geosynthetic clay liners (GCLs) are widely used to isolate waste disposal facilities in order to prevent pollutant migration into the ground. GCLs are factory-manufactured hydraulic barriers containing a thin uniform layer of bentonite sandwiched between two geotextile or glued to a geomembrane. Bentonite is used as barrier material thanks to its low conductivity to water. However, the hydraulic performance may be impaired by contact with aggressive liquids due to cation exchange and highly concentrated solutions. The efficiency of these liners can further deteriorate if hydration with an electrolyte solution is combined with wet and dry cycles, as a result of seasonal changes in temperature, rainfall and groundwater migration. The purpose of this study was to evaluate the effect of wet and dry cycles with seawater on a modified bentonite, HYPER clay. Seawater was selected to simulate conditions where wet-dry cycling is associated with high ionic strength of the inorganic permeant solution, such as a leachate. It represents an aggressive environment for the bentonite clay double layer thickness as it contains a high amount of monovalent and divalent cations. HYPER clay is a polymer amended bentonite with enhanced performance in presence of electrolyte solutions thanks to the irreversible adsorption of the polymer onto the clay. To study the effect of wet and dry cycles a number of tests was performed. The swelling ability was evaluated by means of free one-dimensional swell tests, swell pressure tests, temperature impact tests and CT scanning. Whereas the hydraulic performance is studied through hydraulic conductivity tests with flexible wall permeameters. The influence of drying temperature on the swelling and hydraulic performance was also investigated. Firstly, the thesis includes a description of the main polymer-modified bentonites developed so far. An overview of the hydraulic performance of these materials is provided. The amendment with polymers improved the barrier performance of the bentonite compared to untreated clay. However, the polymer is not always intercalated in the clay structure. Therefore, release of the polymer was experienced in some cases. The hydraulic conductivity and swelling ability of powder HYPER clay subjected to wet dry cycles with seawater was studied. The performances of HYPER clay were compared with those of untreated sodium bentonite. The swelling ability was quantified by means of free one-dimensional swell tests. The treatment with the anionic polymer improved the swelling and sealing ability of the bentonite subjected to six wet and dry cycles with seawater. The swelling of HYPER clay treated with 8% of polymer at the end of the cycles was comparable to the maximum swelling of untreated bentonite in deionized water. In addition, CT analysis demonstrated the better self-healing capacity and the smaller volume of cracks of HYPER clay compared to untreated bentonite. Unlike the untreated clay, HYPER clay maintained low permeability to seawater throughout the cycles. The impact of three different drying temperatures (air, 40°C and 60°C) on the self-healing capacity, swelling ability and water adsorption of HYPER clay and untreated clay was investigated using oedometer cells. These results were then adopted to interpret the hydraulic conductivity of GCL prototypes previously subjected to wet and dry cycles at different drying temperatures (40°C, 60°C and 105°C). In addition, the impact of wet and dry cycles on the GCLs overlap was evaluated by means of the flow box. The flow box allows the measurement of a large scale sample and it is possible to check the permeability in different sections of the sample surface. The swelling ability and water adsorption of HYPER clay always exceeded the values recorded for the untreated clay independently of the drying temperature. The higher swell of HYPER clay suggests a better hydraulic performance compared to untreated clay. Indeed, the hydraulic conductivity to seawater after four wet and dry cycles of the GCL containing the polymer treated bentonite was lower compared to the GCLs containing untreated clay at each drying temperature. These findings demonstrated the persistence of the polymer in the bentonite structure. The HYPER clay treatment intercalates the polymer in the interlayer region of the bentonite, likely inducing a disperse structure of the bentonite. Swell pressure tests were conducted to measure the swelling ability of both untreated clay and HYPER clay through four wet and dry cycles with seawater at a drying temperature of 40°C. Untreated bentonite was also subjected to wet and dry cycles with NaCl solution. HYPER clay confirmed its higher swelling ability even in presence of an aggressive environment, i.e. seawater. These results were interpreted theoretically with the model developed by Dominijanni and Manassero (2012a,b). The theoretical curves were drawn based on the swell pressure of untreated bentonite and HYPER clay 8% to increasing ionic strength. The assumption of constant number of platelets per aggregate independently of the ionic strength was adopted to obtain acceptable estimations of the experiments performed. This first interpretation demonstrated that the polymer treatment increased the net negative charge of the clay and limited the aggregation between clay platelets with a consequent improvement of the swelling ability. The swell pressures values through wet and dry cycles with sodium chloride and seawater were then back analyzed. The results obtained from wet and dry cycles with sodium chloride of untreated clay showed that it might be possible to use the model to simulate the aging process. On the other hand, more investigation is required for the samples subjected to the cycles with seawater due to the lack of information about the final concentration. The back analysis of experimental literature data allowed to define the fixed charge concentration for modified bentonites (BPN and DPH GCL) extending the application of the model to all treated clays. Results of chemico-osmotic tests on BPN and DPH GCL specimens, hydrated with KCl, were used. In general, values of membrane efficiency for modified bentonites were higher than those of conventional bentonite specimens tested with KCl solution. The theoretical curve of the global reflection coefficient versus average concentration represented well the experimental results from both, BPN and DPH GCL. As for untreated bentonite, the membrane efficiency of modified bentonites increased with decreasing the porosity of the specimens. The comparison of the solid skeleton electric charges showed higher value for BPN compared to DPH GCL, probably due to the higher amount of polymer. However, the solid skeleton electric charges of the modified bentonites were higher compared to those of untreated bentonite likely due to the presence of the polymer

Wet and dry effect on the hydraulic conductivity of polymer treated GCL prototype

Geosynthetic clay liners (GCLs) are widely used to isolate pollutants because of their low hydraulic conductivity to water. However, the performance of clay barriers may be impaired by prolonged exposure to electrolytic liquids which may lead to the compression of the diffuse double layer. The consequences are the increase of permeability and the loss of self-healing capacity. Moreover, the efficiency of the liners can further deteriorate by repeated wet and dry cycles, which may lead to desiccation of the bentonite and associated cracking. Modified bentonites have been introduced to improve the resistance of clay barriers to aggressive solutions. This study deals with a polymer-amended clay, HYPER clay. HYPER clay is treated with an anionic polymer and dehydrated and it shows enhanced performance in presence of electrolyte solutions. The effect of wet and dry cycles on the hydraulic conductivity to seawater of needle-punched GCLs prototypes of treated and untreated bentonite was investigated. The prototype samples containing HYPER clay 8% showed lower permeability compared to those containing untreated bentonite. However, the temperature suggested from the standard used in this study is extremely high and it does not represent the temperature in the field

Polymer treated clays subjected to wet-dry cycling with seawater

Free swell in oedometers and hydraulic conductivity tests were conducted to assess how wet-dry cycling affects swell and the hydraulic conductivity of geosynthetic clay liners (GCLs). Similar tests were performed on clays treated with 2% and 8% of polymers to compare their efficiency after wet and dry cycles using deionized water and seawater. Swelling of the untreated clay decreased sharply after two wetting cycles with salt solution and cracks were noticed since the second dry. The amended clays showed good swelling capacity after three cycles, presenting fine cracks only during the third dry. The hydraulic conductivity of the untreated specimens, once permeated with seawater, increased dramatically in respect of the treated bentonite. The permeability increased because cracks formed during desiccation did not fully heal after rehydration

Beneficial impact of polymer treatment of Ca-bentonites on long term hydraulic conductivity

The hydraulic conductivity of geosynthetic clay liners for the confinement of waste disposal facilities mainly depends on the hydraulic conductivity of the core bentonite clay encased between the two geotextiles. Bentonite clay is the most common material for geosynthetic clay liners. Even thought, sodium bentonite has the lowest hydraulic conductivity to water, calcium bentonite is widely used because of low cost and availability. This research concerns the evaluation of the treatment of calcium bentonite with an anionic polymer, sodium carboxy methyl cellulose (Na-CMC) with hyper clay technology. Permeability tests were performed in order to compare hyper clay behaviour to the standard sodium activation treatment of calcium bentonite. Increasing concentrations of calcium dichloride, 5M- 100M - 200 M, were used as hydrating solutions. Test results showed the beneficial effect of the anionic polymer treatment on the hydraulic performance of the clay analysed in the long term

Wet and dry ageing of polymer treated clays using seawater: swell potential and hydraulic conductivity

The efficiency of Geosynthetic clay liners may decrease if they are exposed to aggressive solutions. The performance of the barrier can be further altered if the contact with aggressive solutions is accompanied by wet-dry cycling. Modified clays have been introduced in barrier applications to improve their chemical resistance to aggressive permeants. This study deals with the comparison of a newly developed polymer treated bentonite clay, Hyper clay, and the untreated bentonite clay. Oedometer free swell tests and hydraulic conductivity tests were conducted to assess how wet-dry clycling affects the swell and the permeability of geosynthetic clay liners hydrated with deionized water and seawater. Wet-dry cycling in deionized water had little effect on the swelling of the untreated bentonite. In contrast, its swelling decreased dramatically after two wetting cycles wit seawater. The hydraulic conductivity of the untreated specimens permeated with the salt solution increased considerably after the first rewetting with seawater. The hydraulic conductivity increased because cracks, formed during desiccation, did not fully heal after rehydration with seawater. The treated specimen showed lower hydraulic conductivity and a larger swelling ability compared to the non-amended clay. In contrast to the untreated clay, these beneficial effects were maintained even after wet-dry-cycles with the salt solution

Experimental study to evaluate soil water retention curve of HYPER clay geosynthetic clay liner

Abstract Geosynthetic clay liners are widely used as hydraulic barrier due to their low hydraulic conductivity but bentonite in the liners loses its effectiveness due to significant thermal fluxes by both diurnal and seasonal heating and cooling cycles. Modified sodium carboxy methyl cellulose-based bentonite clay (HYPER clay) has shown better hydraulic performance in both situations. A possible reason for this improved performance of HYPER clay based geosynthetic clay liner is the improvement in the suction under changing thermal conditions. Thus, the relationship between soil suction and moisture content, also called the soil water retention curve, needs to be estimated. Therefore, we investigated the soil-water retention curve of the HYPER clay based geosynthetic clay liner and compared it with the untreated clay based geosynthetic clay liner. The article presents the suction test results on wetting path conducted on geosynthetic clay liner prototypes containing HYPER and untreated clay assessed by the contact filter paper method and the relative humidity sensor. The results showed that the geosynthetic clay liner containing HYPER clay has a high volumetric water content and thus, high water retention compared to untreated bentonite at a given suction value. In other words, the HYPER clay can be considered as a potential alternative to conventional bentonite due to its improved water retention capacity.</jats:p