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When geosynthetic clay liners (GCLs) are placed over coarse-grained gravel subgrades, the permittivity of the GCLs may increase because of internal erosion. To simulate this condition, geosynthetic clay liners typically are placed over gravel and tested in the laboratory under high hydraulic heads. In this study, a perforated base pedestal was used instead of gravel. The base pedestal was designed to have circular voids to represent the voids of a uniform and rounded gravel subgrade. Results obtained from tests where natural gravel and a perforated base pedestal were used were compared. To verify the effectiveness of the new approach, two different geosynthetic clay liners were tested over two different gravel subgrades. Tests also were conducted using rounded, uniform, coarse-grained gravels to compare to the results of the tests with the perforated base pedestals. The void diameter of the perforated base pedestals was chosen to be approximately the same as the maximum void size between the gravel particles. Test results indicated that a perforated base pedestal with uniform voids simulated a rounded, uniform, coarse-grained gravel subgrade in terms of internal erosion. The hydraulic heads that caused internal erosion were similar when a perforated base pedestal or a rounded gravel subgrade was placed beneath a GCL. When the same GCL was used over a base pedestal or over a gravel subgrade with equivalent void size, the difference in hydraulic heads at failure did not alter more than 5 m, except for one comparison. For most of the tests, the performance of the GCL placed over the gravel subgrade was slightly better than that of the perforated base pedestal in terms of internal erosion. These results indicated that the proposed technique of using perforated subbase to simulate gravel remains conservative for the GCLs and gravel subgrades considered as part of this study.

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Microplastic contamination is a growing threat to marine and freshwater ecosystems, agricultural production, groundwater, plant growth and even human and animal health. Disintegration of plastic products due to mainly biochemical or physical activities leads to the formation and existence of microplastics in significant amounts, not only in marine and freshwater environments but also in soils. There are several valuable studies on microplastics in soils, which have typically focused on environmental, chemical, agricultural and health aspects. However, there is also a need for the geotechnical engineering perspective on microplastic contamination in soils. In this review paper, first, degradation, existence and persistence of microplastics in soils are assessed by considering various studies. Then, the potential role of solid waste disposal facilities as a source for microplastics is discussed by considering their geotechnical design and addressing the risk for the migration of microplastics from landfills to soils and other environments. Even though landfills are considered as one of the main geotechnical structures that contribute to the formation of considerably high amounts of microplastics and their contamination in soils, some other geotechnical engineering applications (i.e., soil improvement with tirechips, forming engineering fills with dredged sediments, soil improvement with synthetic polymer-based fibers, polystyrene based lightweight fill applications), as potential local source for microplastics, are also mentioned. Finally, the importance of geotechnical engineering as a mitigation tool for microplastics is emphasized and several important research topics involving geotechnical engineering are suggested.

Geosynthetic clay liners (GCLs) are often used as lining materials for freshwater reservoirs. To irrigate agricultural land without depleting groundwater, surface water is stored in these artificial ponds. In this study, hydraulic conductivity tests were performed on GCLs placed in flexible-wall permeameters under hydraulic heads of up to 50 m in order to investigate the risk of internal erosion. In these tests, base pedestals made of Plexiglas with uniform circular voids were placed beneath the GCLs instead of a typical gravel subgrade. The voids in the base pedestal represented the voids between uniform rounded gravel particles. Different types of GCLs were tested. GCL-1 was reinforced using needle-punching technology, whereas GCL-2, GCL-3, and GCL-4 were un-reinforced GCLs that were assembled in the laboratory. We investigated the effects on internal erosion of the void size in the subbase; the geotextile component that was in contact with the subbase; the bentonite component; and the manufacturing process of the GCLs. Test results indicated that internal erosion was directly related to the void diameter of the base pedestal. The resistance of the needlepunched GCL to internal erosion was better than that of the un-reinforced GCLs. The degree of internal erosion was also related to the engineering properties of the geotextile in contact with the base pedestal. Higher tensile strength of the GCL reduced the possible potential for internal erosion within it. The type of bentonite did not have a significant effect on internal erosion.157