Effect of geomembrane colour and cover soil on solar-driven down-slope bentonite erosion from a GCL

Abstract

The surface temperature of an exposed composite liner consisting of a geosynthetic clay liner (GCL) covered by a black geomembrane (GMB) can routinely exceed 60°C during summer solar exposure before falling to ambient air temperature at night. Observations at the Queen's University Environmental Liner Test Site (QUELTS) have recently shown that the evaporation and cooling of the air space at the GMB–GCL interface causes condensed water to form underneath the GMB. These water drops flow down-slope until they are released onto the top surface of the GCL at points of contact, seams, or changes in geomembrane curvature. An unforeseen consequence of this solar-driven generation and release of distilled water onto the GCL was discovered to be down-slope erosion of bentonite from the GCL. The objective of the current study was to repeat the QUELTS field exposure experiment to confirm whether the immediate covering of the GMB will supress the mechanism of down-slope bentonite erosion through the elimination of the air gap at the GMB–GCL interface and a reduction in thermal cycling. Further, this study examined whether the use of a white-coloured GMB can reduce the time to onset of erosion if immediate cover is not possible. Results confirm that no erosion occurred on the composite liner test section covered by 0.3 m of gravel after the 15 month duration of the study. In contrast, erosion was observed to occur on the exposed white GMB section, but at a slower rate than that observed on the exposed black GMB section. The timely placement of cover soil on the composite liner as per manufacturers' recommendations is therefore the most effective strategy to avoid the issue of down-slope bentonite erosion.