Interface Shear Damage to a HDPE Geomembrane. II: Gravel Drainage Layer

An experimental program of large-scale direct shear tests has indicated that shear displacement of a gravel drainage layer and nonwoven geotextile protection layer over a high-density polyethylene (HDPE) geomembrane under moderate to high normal stress conditions can cause much greater damage to the geomembrane than static pressure alone. Essentially, no damage was observed at low normal stress. The greatest damage occurred at high normal stress (1,389 kPa) using a lightweight geotextile ($335\hspace{0.17em}\text{g}/{\text{m}}^2$) and yielded an average of $31\hspace{0.17em}\text{holes}/{\text{m}}^2$, with a maximum hole size of 29 mm. Surprisingly, geomembrane damage measured using a lightweight geotextile was greater than that measured using no geotextile due to a change in failure surface location. For the same conditions, shear-induced damage was slightly less for a geomembrane placed on a compacted sand subgrade than on a compacted clay subgrade. Interface shear strength increased significantly with decreasing geotextile mass/area due to greater out-of-plane deformation of the geomembrane. The findings suggest that the placement of a gravel drainage layer on top of a HDPE geomembrane, even with a protection nonwoven geotextile, should be viewed with caution for landfill bottom liner systems and other moderate- to high-stress applications. If there is a reasonable expectation for interface shear displacement, project-specific direct shear tests should be conducted to determine the potential for shear-induced geomembrane damage. Recommendations are provided for the performance of such tests and for design options when damage mitigation is necessary.