Introduction

Laurie L. Honnigford
Executive Director, PGI

Industry News

By: David Lauwers

Case studies of geomembranes under actual conditions contain some of the most important information that can be gathered to determine long term performance. Opportunities to evaluate the performance of the geomembrane under these conditions are limited because of the expense of excavating the geomembrane and the fear of disturbing the geomembrane in an attempt to obtain a sample. This leaves only limited opportunities, such as when sites are being expanded or require modifications.

In this case study, the opportunity presented itself in the spring of 1993, when a golf course pond was being enlarged and the existing PVC geomembrane was excavated in the process. The site was at the Lake of the North Golf Course located in the northern part of the lower peninsula of Michigan. According to Jerry Matthews, the golf course architect who originally design the project, the PVC geomembrane was installed in the summer of 1968, almost 25 years ago. The material, a 10 mil PVC geomembrane was originally covered by twelve inches of sand. Approximately six to eight inches of silt had accumulated over the sand during this 25 year period. The pond was designed for irrigation and asthetic purposes and to provide duffers with a means to determine if their golf balls would float.

The conditions in this area of the country provide a good testing ground. The winters are cold, with temperatures falling well below 0°F and rising to 90°F at times during the summer. Also based upon some other previous studies of plasticizer extraction, the rainwater may be more severe than a typical municipal landfill leachate. This phenomenon is due to the fact that there is a larger gradient for plasticizer migration in water than leachate, due to the lack of organic compounds in the water. Lastly, this geomembrane was only 10 mil material.

The changes to the geomembrane would occur more quickly with this gauge than the thicker gauges, 20 to 40 mils, that are typically used on large projects today. For all these reasons, this site provided some very meaningful information relating to the long term performance of PVC geomembranes.

Enough samples were taken to evaluate not only the physical properties of the parent material but also the factory and field seams. All seams were made using a chemical fusion weld. Physical testing according to NSF Standard 54 for PVC geomembrane was conducted, along with chemical analysis of the film. The specific tests that were conducted included thickness, specific gravity, tensile, elongation, 100% modulus and tear resistance. Peel and shear tests were conducted on both the factory and field seams.

Test results are summarized in Tables 1 and 2, and Graphs 1 and 2. There are several things which are immediately apparent from this data.

1.The physical properties still exceed the requirement of NSF Standard 54-83 even after 25 years. In fact they exceeded the NSF 54 Standard by a large margin.

2.There is no deterioration of the seams by the peel and shear values in either the field of factory seams. All the peel tests of the factory and field seams resulted in film tearing bond.

The analytical results confirm that the makeup of this PVC geomembrane has changed very little over the 25 year period. Although an unexposed sample of this geomembrane was not available, if compared against current geomembrane formulas, the plasticizer content has only changed by 12%. Existing PVC geomembrane formulas have about 30% plasticizer, versus the 27.8% found in this geomembrane. The geomembrane had reached a steady state with the surrounding environment and it was not losing any additional plasticizer.

The samples themselves were still very flexible with no sign of deterioration or cracking of the surface. There appeared to be no physical signs that would indicate that the geomembrane had not functioned as designed for the 25 years it was in service. This study only adds to the growing amount of information that indeed PVC geomembranes are a logical choice for a wide range of applications. The fact that they do contain plasticizer is not the Achilles heel as some people would believe, but gives the geomembrane the flexibility that is so important in geosynthetic design, and when properly formulated, provides the permanence to perform in the long haul.

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Environmental Protection, Inc provides this PGI Technical Bulletin for your information. You can obtain an original copy of this bulletin from the PVC Geomembrane Institute, PGI - Technology Program, University of Illinois, 2215 Newmark Civil Eng. Lab, 205 North Matthews Ave., Urbana, IL 61801, or by phoning the PGI at 217-333-3929, or by email at pgi-tp@uiuc.edu

GRAPH 1

TABLE 1 PHYSICAL PROPERTY TESTING

 
GRAPH 2 

TABLE 2  SEAM STRENGTH TESTING