A Laboratory Examination of Down-slope Bentonite Erosion in Geosynthetic Clay Liners
downslope erosion , slope , composite liner , exposed liner , GCL
Geosynthetic Clay Liners (GCLs) are commonly paired with a geomembrane and used as part of a composite liner system for landfill barriers. Under some circumstances, leaving a composite geomembrane/geosynthetic clay liner exposed to solar radiation in the field has been shown to cause shrinkage of the underlying GCL. Recent field studies have shown that leaving a composite liner exposed can also lead to down-slope erosion of bentonite from the GCL due to the down-slope movement of moisture. To investigate the factors that can affect the onset of bentonite erosion in a GCL an experimental technique was developed to reproduce similar erosion in the laboratory. The test method simulates the features that occur with the erosion of bentonite caused by down-slope migration of evaporative water in the field. One needle-punched GCL was tested to examine the factors that can affect the onset of erosion of bentonite particles with the flow of water. The factors examined include the effect of the initial wet/dry cycle, water source chemistry, flow rate, slope, prior cation exchange, and the effect of no drying phase in the test cycle. Ten different manufactured GCL products were tested to examine the effect of material properties on the erosion of bentonite from a GCL. The material properties of the products tested differed in terms of the type of carrier and cover geotextiles, bentonite (powdered, fine and coarse grained, and some with a polymer enhancement additive) and the presence of a polypropylene coating over the geotextile. It was found that the most critical factor to trigger the onset of bentonite erosion was the water source chemistry, with the tests that simulated the evaporation and condensation of water (deionized water) below an exposed composite liner leading to the formation of major erosion features. The results of the laboratory testing program also show that erosion features are more visible in products with white coloured geotextiles. The products containing a polypropylene coated geotextile and polymer enhanced bentonite slowed or, in some cases, prevented erosion features from developing.