PVC Geomembrane / Geosynthetic Interface Strengths

 

APRIL 2000

---

The usual design objective for impoundments and waste containment facilities is to maximize storage capacity. Thus, it is important to construct the side and fill slopes as steeply as possible. To reduce leakage from these facilities, a liner system is usually installed that incorporates a geomembrane. 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 polyvinyl chloride (PVC) geomembrane interface shear strengths that was conducted at the University of Illinois. A forthcoming technical paper in Geosynthetics International will describe the testing and results in greater detail and can be obtained from the PGI.

Torsional ring shear (similar to ASTM D 6467 for cohesive soils) and large-scale direct shear (ASTM D 5321) tests on smooth and faille PVC geomembrane interfaces involving a variety of geosynthetics, e.g., nonwoven geotextiles, a drainage composite, a geonet, and a geomembrane-backed geosynthetic clay liner (GCL), were conducted to quantify the shear behavior of PVC geomembranes. For each interface, no significant difference was found between the ring shear and direct shear test results.
 

Smooth vs. Faille PVC Geomembrane Surfaces:

PVC geomembranes can be manufactured with one side smooth and the other side embossed. The embossing usually produces 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 nonwoven geotextile yielded peak interface friction angles of 30 and 23 degrees, respectively. Additionally, the smooth side 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 higher flexibility and/or softness of the smooth side as compared to 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 focused on the shear resistance of the faille side to set a lower bound for PVC geomembrane/geosynthetic interface shear strengths. The difference in interface strengths between the smooth and faille sides should be considered in the design process.
 

Effect of Geomembrane on Interface Strengths:

PVC, high density polyethylene (HDPE), and very flexible polyethylene (VFPE) geomembranes were sheared against a nonwoven, polyester geotextile with a mass per unit area of 540 g/m2. The interface shear behavior and strength of the HDPE and VFPE geomembrane interfaces were found to be similar, so only a comparison between PVC and VFPE geomembrane interfaces is presented herein. It was anticipated that a VFPE geomembrane would yield similar interface strengths as the PVC geomembrane because of their flexibility. It can be seen from the following figure that the PVC geomembrane exhibited higher peak interface strengths and a lower post-peak strength loss than the textured VFPE geomembrane interface. A possible explanation for this difference in the post-peak strength loss is that the asperities of the VFPE geomembrane tear or pull out a larger amount of the geotextile filaments and orient them parallel to the direction of shearing. The PVC geomembrane extracts a smaller quantity of filaments from the geotextile, allowing the geotextile to stay more intact and maintain a higher interface strength. As a result, there is a smaller post-peak strength loss. This behavior may be advantageous to applications with steep side slopes where shear displacement usually occurs during construction and/or waste placement or where seismically induced permanent deformations could result in a post-peak strength condition. Of course, the PVC and textured VFPE geomembranes exhibited higher peak and residual failure envelopes than the smooth VFPE geomembranes.
 

Summary:

The PVC geomembrane tested in this study yielded a high interface shear resistance with the nonwoven geotextile, drainage composite, geonet, and geomembrane-backed GCL components tested herein investigated. The high interface shear strength is attributed to the flexible and soft nature of the PVC geomembrane. In addition, the interfaces tested exhibited a small (less than 25%) 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. The following table presents the results of the PVC geomembrane interface testing and can be used for estimating the frictional performance of certain geosynthetic interfaces. Since the shear resistance of geosynthetic interfaces is project specific and product dependent, the data should be used for comparison purposes only and not design.

 

Summary of geomembrane/geosynthetic interface friction angles (for comparison purposes only*)

Geomembrane/geosynthetic interface	Peak friction angle(degrees)	Shear displacement at peak (mm)	Residual friction angle (degrees)	Shear displacement at residual (mm)
Faille PVC/non-woven geotextile1	28-25	500-50	28-24	500-650
Faille PVC/non-woven geotextile2**	37-33	700-10	37-26	700-150
Faille PVC/ non-woven geotextile3	25-27	400-13	25-24	400-900
Faille PVC/ non-woven geotextile4	20-22	200-21	20-20	200-300
Smooth PVC/ non-woven geotextile4	29-30	900-400	29-30	900-400
Faille PVC/ non-woven geotextile5	30-27	400-70	30-26	400-550
Smooth HDPE/ non-woven geotextile2**	11-9	4-2	7-5	55-35
Textured HDPE/ non-woven geotextile2**	44-30	11-6	25-15	100-150
Smooth VFPE/ non-woven geotextile2**	11-7	3-1	6-5	50-30
Textured VFPE/ non-woven geotextile2**	38-27	7-5	25-19	150-200
Faille PVC/drainage composite	34-26	500-18	34-23	500-150
Faille PVC/geonet	23-25	1-7	18-21	20-70
Faille PVC/smooth-backed GCL	27-14	1-1	21-10	50-20
Faille PVC/textured-backed GCL	27-24	1-3	21-18	30-20

Note: Each entry corresponds to values at normal stresses of 17 and 400 kPa, respectively. For example, the faille PVC/ non-woven
geotextile1 interface has a secant peak friction angle of 28 degrees at a normal stress of 17 kPa and 25 degrees at a normal stress of 400 kPa.

*Site specific interface testing should be conducted for design purposes.

**Highest normal stress was 285 kPa instead of 400 kPa.

 

Environmental Protection, Inc provides this PGI Technical Bulletin for your information. 

For more information call 800-OK-LINER today!