13-Year Study of Exposed, Green Geomembrane Cover

By Boyd Ramsey In the summer of 2001, a 60- mil high-density polyethylene (HDPE) green geomembrane was installed as a cap at the Polk County Florida solid waste disposal facility (The North Central Sanitary Landfill Class 1 Disposal Area). This cap covers approximately 4 hectares (16 acres) of surface area on a mass of waste with a relatively flat top surface and benched side slopes with an angle of 29 degrees or less.

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 1 – Side slope overview with inset image of the site prior to capping.

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 2 – Boyd Ramsey (GSE Environmental) and Allen Chote (Polk County Waste &
Recycling Division)

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 3 – Sampling zone

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 4 – Physical Property Comparison – 2013 Properties vs. Original Specification

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 5 – Property Comparison – 2006 Properties(No UV and Waehouse) vs. Original Specification

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 6 – OIT Over Time for Three Samples

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 7 – Repair at Gas Vent Stack

13-Year Study of Exposed, Green Geomembrane Cover

Fig. 8 – Fixed Drain with Excess Slack and Repairs

The cap area utilizes an active vacuum gas collection and control system. This cap application has been extensively monitored with weekly site inspections, regular rainfall, gas and leachate generation measurements and recordkeeping, annual testing of materials and retains from the site, subsidence measurements, documented repairs (when needed) and an overall very complete and careful recording of the site conditions and material performance.

The cap has performed extremely well and has well exceeded the design and lifespan expectations that existed when the site was installed.

Also, the site has been subjected to severe and unusual weather events and has performed exceptionally well. This paper presents a summary of the testing and performance of the materials, describes the current status of the cap and materials both relative to the initial condition of the material and the specification requirements that existed when the material was installed and offers a projection of the future performance of the material.


Increasingly, over the past decade, geosynthetic caps have been utilized at sites where a “traditional” soil covered cap may be problematic. Site conditions such as steep slopes that may exist on pre-Subtitle “D” facilities, sites with limited or no available “borrow” soils, bioreactors and other sites where large levels of subsidence are anticipated and/or locations with unusual erosion issues are all good candidates for the use of an exposed geomembrane cover system. This site chose an exposed green geomembrane–a 60-mil high-density polyethylene (HDPE) liner–as the capping system based on several of the considerations listed above.

The green color was selected to present a more aesthetically pleasing appearance. This material variety had been previously installed at a site in Sabine Parish, Louisiana, and at the time, the initial results were encouraging. The Sabine Parish installation (the first of this material type and formulation) followed a series of white geomembrane installations; a similar stabilization package and resin selection was utilized in the manufacture of the green materials with obvious differences in pigmentation.

In addition to the concern regarding life expectancy for the Polk County installation, another significant concern was wind uplift. While the site normally utilized a vacuum gas extraction system the behavior of the exposed geomembrane during a significant storm event when both high winds and loss of electrical power were reasonably anticipated. To address this issue, vertical anchor trenches were installed in the cover. The engineering of this is addressed in the Giroud reference. In subsequent years, the system was found to have excellent performance even in very difficult conditions.


As a part of the permitting approval process a fairly rigorous annual testing program was established for the cover material. Properties required to be tested are defined in the sites Long-Term Care Plan and include Wide Width Strength of Geomembranes (ASTM D 4885), Oxidative Induction Time (ASTM D 3895), and Environmental Stress Cracking (ASTM D 5397). Additionally, tensile and elongation testing per ASTM D 638and ASTM D 4885 was performed on an annual basis. This testing was a part of the installation and operational plan for the site and a section of the cover was prepared with existing sampling sections pre-prepared for annual evaluation; this facilitated the sampling of the materials without requiring a penetration of the cover. The test area is shown in Fig. 3. Ten sample sections were prepared at the initial installation. As the material has well exceeded the initial 10-year lifespan estimate, subsequent to 2011, samples have been cut from the cover and repairs completed at the time of sampling.

The methodology and evaluation criteria for degradation of polyolefin geosynthetics are fairly well established. A detailed discussion of the lifespan and durability of polyethylene geomembranes is not within the scope of this paper, only a related topic. The author would refer the reader to multiple references, particularly the Geosynthetic Institute/Koerner papers and publications: “GRI White Paper #6 on Geomembrane Lifetime Prediction: Unexposed and Exposed Conditions” is a very good starting place for the new reader.

In very short summary, there are three phases of degradation: Stage A – Antioxidant Depletion Time (as measured by Oxidative Induction Time (OIT) and OIT retention). Stage B – Induction Time to the Onset of Degradation (measured by a statistically significant change in physical properties). Stage C – Time to Reach 50% Degradation (i.e., the half-life as a measured property reaches 50% of the original value). To that end, the most recent sampling of the exposed cover materials physical properties as compared with the original values and the specification requirements at the time of installation is below (Fig. 4).

The materials still have retained OIT, physical properties are in excellent, near original conditions, the stress crack performance of the material is still extremely strong and the physical appearance of the site is very good. To quote form the 2013 inspection report “The 2013 results of the analyses discussed above in conjunction with the observed performance of the material in the field support the conclusion of previous reports that the geomembrane continues to perform as expected.”

The green geomembrane cover has now been exposed for more than 13 years, significantly longer than the 10 year estimate that was made at installation. With some remaining OIT, the materials are still within the “Stage A” portion of their lifespan.

Due to the excellent testing and record keeping program, additional data and information is available regarding the performance of these materials. In addition to the testing of exposed portions of the geomembrane cover testing was also conducted on samples of the geomembrane which had been used in constructing a portion of the cover, yet were not continuously exposed to ultraviolet (UV) light. UV light is recognized as being a very significant contributor to the degradation of geomembranes and an environmental force that the geomembrane should be stabilized to protect against to the greatest extent possible.

These samples are identified in the following tables as “no UV”. Also, retains of the materials had been stored in a warehouse against the potential for large scale repairs being needed. Over the years, this material, which was stored inside and not subjected to the environment or continuous sunlight (UV) was also sampled and tested. These samples are identified in the following tables as “warehouse”. Fig. 5 illustrates this for 2006, the first year that testing was structured in this manner.

As the physical properties are maintaining the original levels it is apparent that the materials are currently in the “stage A” of the lifespan and thus our attention turns to OIT and stabilizer consumption. Fig. 6 illustrates the results of OIT testing for the field samples and for the “no UV” and “Warehouse” samples where testing of OIT began with the 2006 sampling period.

Arguably, there is stabilizer remaining in the product. There are those who would claim that an OIT less than 20 minutes is essentially a ‘zero’ OIT and that indicates that the materials have moved to “stage B”, however this author would disagree and would offer as evidence the fact that the last four OIT data points covering a four year period were all less than 20 minutes, yet the presence of a measureable OIT persists and the physical properties has indicated no statistically significant changes.

In summary, a detailed and regularly scheduled sampling and testing program has indicated that the 60 mil High Density Polyethylene green geomembrane has performed extremely well, with no significant loss of physical properties over a 13 year period and retention of OIT with a reasonable expectation that the cover materials are still in the early stages of their lifespan and should be expected to perform their function for many more years, if not decades.


It should be noted that this is not an academic exercise. The materials supplied and installed are necessary to perform a critical function of containment. The materials have also performed very well in this regard. Regular inspections have been conducted throughout the lifespan of the site and where necessary, small repairs have been made. Inspection and repair programs such as these have demonstrated their value over and over again allowing for timely and very inexpensive repairs particularly when compared to the costs associated with repairs on sites that are not inspected on a regular schedule. Further, the site has undergone the anticipated consolidation and the geomembrane has handled those changes in configuration very well.

Again, to quote from the most recent report:

“Settlement along the east/west axis of the site ranged from 7.6 feet to 11.5 feet with an average settlement of 9.0 feet. Settlement along the north/south axis of the site ranged from 7.0 feet to 10.2 feet with an average of 8.0 feet. The combined settlement for the site at these 18 data points averaged 8.7 feet.”

“Polk County EGC inspections observed a number of areas requiring minor repairs; however, no significant EGC damage was identified. The majority of the damage identified represented minor mechanical damage and minor extrusion-weld defects similar to those observed during the previous reports. Although an increase in the number of peeling and splitting welds and minor damage have been observed over the years, this trend declined during the 2013 reporting period; from 74 to 32. Creasing and surface cracking observed in previous reports on terraces and berms did not continue. Eliminating vehicles traveling on the EGC, for operation and maintenance of the gas collection system, contributed to the improvement.”

The comments regarding vehicle traffic on exposed geosynthetics are consistent with issues at other sites and manufacturer’s recommendations to minimize the traffic on geosynthetics and when it is essential to work directly on the geosynthetic, to utilize low ground pressure apparatus operated in a very careful and conservative manner.

One specific design suggestion for improvement should be mentioned. A fixed surface drain was included in the design and installation. This structure is not affected by subsidence and as a result has been the area (Fig. 8) most repaired throughout the entire installation. This drain was installed to reduce surface runoff, which, ironically has not been a problem at the site.


This installation has been a very successful demonstration of the benefits and performance of exposed geomembrane cover systems. After a 13 year exposure period, not only is the geomembrane performing well significantly beyond the initially anticipated 10 year lifespan, but the material has physical properties nearly identical to those at installation and is very likely still in the first phase of its lifespan. This would allow for lifespan predictions of 50 or more years based on existing models and methodologies which are very conservative.

While it is not specifically related to lifespan, the reader should also be aware that this site was subjected to three direct strikes by hurricanes during 2004. The site and cover performed extremely well. Quoting from the referenced article: “Throughout all three storms, Polk County’s EGC withstood approximately 20 inches of heavy rainfall and hurricane-force winds originating from all points of the compass.”

Ana Wood, Polk County Waste Resource Management Director, states, “I am very pleased with the EGC’s performance not only through these recent violent storms, but also in its overall performance during the last four years. The EGC can provide landfill owners and operators value in many different areas. Polk County has seen savings related to the EGC with lower construction and maintenance costs, and fewer compliance-related issues, and has contributed to the overall efficiency of our operations.”

The materials and design are performing very well, much better than anticipated and there is a very reasonable expectation that the materials will continue to perform well for many years into the future, if not decades.


The design and the vast majority of the subsequent annual testing were completed by Jones, Edmunds and Associates of Gainesville, FL. Allan Chote of the Polk County Waste & Recycling Division has conducted the inspections and has personally monitored the site over the years, his commitment to serving the citizens of Polk County and the state of Florida is extremely admirable.

Find more technical notes, project stories, and geosynthetics and engineering information at GSE Environmental’s website: www.gseworld.com.


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