Damage to HDPE Geomembrane from Interface Shear with Gravel Drainage Layer and Protection Layer

Bottom liner systems for landfills and other waste containment facilities often include a gravel drainage layer and protection layer overlying a geomembrane. The protection layer can be made of different materials but is typically a nonwoven geotextile, as first proposed by Giroud in 1971 for several geomembrane-lined reservoirs (Giroud 1973). Extensive research has been conducted over the last 20 years to assess the performance of geomembrane protection layers using static pressure tests. However, this work has not accounted for possible additional geomembrane damage that can occur when static pressure is combined with interface shear. Shear displacements can be expected within liner systems, especially on or near side slopes, due to various mechanisms including waste settlement, seismic loading, and strength mobilization for stability.

An experimental program of large-scale direct shear tests was conducted to investigate damage that occurs when a gravel layer and nonwoven geotextile are sheared over a high-density polyethylene (HDPE) geomembrane. The specimens measured $152\times \mathrm{1,067}\hspace{0.17em}\text{mm}$ in plan and, from top to bottom, consisted of angular gravel (particle size range = 25–38 mm), nonwoven geotextile ($\text{mass/area}=335–810\hspace{0.17em}{\text{g/m}}^2$), textured geomembrane (thickness = 1.5 mm), and compacted sand. Normal stress levels ranged from 345 to 1,389 kPa. Tests were also conducted using no geotextile and a compacted clay subgrade. Each test included a 24-h static pressure stage followed by a shearing stage, during which the specimen was sheared to a final displacement of 200 mm at a rate of $1\hspace{0.17em}\text{mm/min}$. The geomembranes were inspected for damage after each stage of testing.

An example of the experimental results is shown in Fig. 1. Geomembrane specimen P8 was covered with a lightweight geotextile ($335\hspace{0.17em}{\text{g/m}}^2$) and tested at a normal stress of 1,389 kPa. After the static pressure stage, the geomembrane displayed minor damage in the form of indentations under the gravel contacts as shown in Fig. 1(a). Static pressure damage decreased with increasing geotextile mass/area and decreasing normal stress. No holes were created in the geomembranes for any of the static pressure tests.

Fig. 1(b) shows the same geomembrane specimen after the shearing stage. Failure occurred at the geotextile/geomembrane interface. Shear displacement of the gravel layer and nonwoven geotextile over the geomembrane caused much greater damage than static pressure alone. Geomembrane specimen P8 displayed the most damage in the experimental program and yielded an average of $31\hspace{0.17em}{\text{holes/m}}^2$, with a maximum hole size of 29 mm. At the same normal stress level, significant wrinkle and indentation damage was also observed with a heavyweight geotextile ($810\hspace{0.17em}{\text{g/m}}^2$). Shear-induced geomembrane damage also decreased with increasing geotextile mass/area and decreasing normal stress. However, geomembrane damage measured using a lightweight geotextile was greater than damage measured using no geotextile because of a change in failure surface location. At constant normal stress, geotextile/geomembrane interface shear strength increased significantly with decreasing geotextile mass/area because of the greater out-of-plane deformation of the geomembrane along the interface.