The usual design objective for impoundments and waste containment facilities is to maximize storage capacity. Thus, it is important to construct the side slopes as steep as possible. To reduce leakage from these facilities a liner system is usually installed that incorporates a geomembrane. For example, municipal and hazardous waste containment facilities in the United States are required to have a liner and cover system that usually consist of a compacted clay liner and geosynthetic materials. The geosynthetic components of these systems routinely include layers of geonet or drainage composite, geotextile cushions and/or filters, and a geomembrane. An important characteristic of these liner systems with respect to slope stability is the shear resistance along the various component interfaces. A number of case histories suggest that the geomembrane can create a problematic interface due to low frictional resistance between it and another geosynthetic component or soil. This technical bulletin describes some of the research on PVC geomembrane interface strengths that is being currently conducted at the University of Illinois. A forthcoming technical paper will describe the testing and results in greater detail and can be obtained from the PGI.
Torsional ring shear and large scale direct shear (ASTM D 5321) tests on smooth and faille PVC geomembrane interfaces involving a variety of non-woven geotextiles, a drainage composite, a geonet, and a geomembrane-backed geosynthetic clay liner (GCL) are being conducted to quantify the shear behavior of PVC geomembranes.
Smooth vs. Faille PVC Geomembrane Surfaces:
PVC geomembranes are usually manufactured with one side being smooth and the other side being embossed. Many PVC geomembranes are produced using a faille emboss on one side. The embossing usually results in a surface that looks like a file and thus is referred to as the faille side. It was found that the smooth side yields a higher interface shear resistance than the faille side. For example, the smooth and faille sides sheared against the same non-woven geotextile yielded peak interface friction angles of 30 and 23 degrees, respectively. The smooth side also exhibited little, if any, post-peak strength loss whereas the faille side exhibited a residual interface friction angle of 20 degrees versus a peak friction angle of 23 degrees. The higher frictional strength of smooth PVC geomembranes is attributed to the greater interface contact area during shear and the more "sticky" and flexible nature of the smooth side versus the faille side. Since it was demonstrated that the faille side of a PVC geomembrane renders a lower interface shear resistance than the smooth side, the interface research is focusing on the shear resistance of the faille side to set a lower bound for PVC geomembrane interface shear strengths.
Geomembrane/Non-Woven Geotextile Interface Strengths:
PVC, high density polyethylene (HDPE), and very flexible polyethylene (VFPE) geomembranes are being tested with a non-woven, polyester geotextile with a mass per unit area of 540 g/m2. The testing of the smooth and textured VFPE geomembranes is ongoing and will be included in the forthcoming paper. In the interim, peak and residual failure envelopes for faille PVC and textured HDPE geomembrane/non-woven geotextile interfaces are shown in Figure 1. The two interfaces have similar peak failure envelopes. However, there is a difference in the post-peak strength loss, as reflected in the residual failure envelopes. Specifically, the textured HDPE geomembrane interface undergoes a larger post-peak strength loss compared to the faille PVC geomembrane interface. A possible explanation for this post-peak decrease is that the asperities of the HDPE geomembrane tear or pull out the filaments of the geotextile and orient them parallel to the direction of shearing. The PVC geomembrane extracts a smaller quantity of filaments from the geotextile than the HDPE geomembrane, allowing the geotextile to stay more intact and maintain a greater interface strength. As a result, there is a smaller post-peak strength loss. This smaller strength loss may be beneficial to applications with steep side slopes or seismically induced permanent deformations that could result in a post-peak strength condition.
Other findings of the interface testing include a polyester based geotextile yields higher peak and residual PVC geomembrane interface strengths than a similar mass per unit area polypropylene based non-woven geotextile. Non-woven geotextile fiber type has an impact on PVC geomembrane/geotextile interface shear strength. Staple fiber geotextiles appear to yield higher interface strengths than continuous single filament geotextiles for the PVC geomembrae tested herein. A non-woven geotextile mass per unit area of 205 g/m2 resulted in higher peak interface strengths than the same geotextile with a mass per unit area of 540 g/m2 for the PVC geomembrane tested.
PVC Geomembrane/Unreinforced GCL Interface:
Hydrated bentonite is one of the weakest natural materials in terms of shear strength. One technique for maintaining the shear strength of bentonite in the field is to encapsulate it between two geomembranes. It is anticipated that encapsulating the bentonite between two geomembranes may reduce the extent of bentonite hydration and thus maintain the shear strength and bearing capacity of dry bentonite. This is often accomplished by using the geomembrane-backed GCL and covering the bentonite with a PVC geomembrane. Therefore, the interface between unreinforced bentonite and a PVC geomembrane is being investigated. The use of a PVC geomembrane to encapsulate an HDPE geomembrane-backed GCL results in a high peak interface friction angle (26 degrees) due to the flexible PVC interlocking with the granular bentonite. This high interface friction angle will also result in smaller geomembrane wrinkles, which may reduce the possibility of areal hydration of the bentonite.
The PVC geomembrane being tested yielded a high interface shear resistance for the non-woven geotextile, drainage composite, geonet, and geomembrane-backed GCL interfaces investigated during this study. The high interface shear strength is attributed to the soft, "sticky," and flexible nature of the PVC geomembrane. In addition, the interfaces tested exhibit a small (less than 20%) post-peak strength loss, which may be beneficial in applications where a post-peak strength is applicable to design such as steep side slopes or seismically active regions.
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