a 13-year study of an exposed green geomembrane cover€¦ · gseworld.com technical note a 13-year...

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TECHNICAL NOTE

A 13-YEAR STUDY OF AN EXPOSED GREEN GEOMEMBRANE COVER In the summer of 2001, a 60 mil High Density Polyethylene 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. 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.

OVERVIEW

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 high density

polyethylene (HDPE) 60 mil geomembrane as the capping system based on several of

the considerations listed above.

Author Contact:Boyd Ramsey

Chief Engineer

GSE Environmental

281-230-2598

[email protected]

[GSE Geomembranes]

A 13-Year Study of an Exposed Green Geomembrane

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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.

EVALUATION PROGRAM:

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 below (figure 3) as it existed in

early 2015. 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 (figure 4) and repairs completed at the time of sampling.

Fig. 1: Polk County Site Before Fig. 2: Aerial View Circa 2005

Fig. 4: 2014 SampleFig. 3: Sampling Area


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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 (figure 5).

Test Method Units Description Original Specification 2013 Properties

ASTM D 638 Mpa MD Yield Strength 14.9 22.3

ASTM D 638 Mpa TD Yield Strength 14.9 24.4

ASTM D 638 Mpa MD Break Strength 10.3 22.1

ASTM D 638 Mpa TD Break Strength 10.3 18.3

ASTM D 638 % MD Yield Elongation 13 18

ASTM D 638 % TD Yield Elongation 13 17

ASTM D 638 % MD Break Elongation 150 411

ASTM D 638 % TD Break Elongation 150 156

ASTM D 4885 Mpa MD Yield Strength none 18.7

ASTM D 4885 Mpa TD Yield Strength none 19.2

ASTM D 4885 % MD Break Elongation none 21

ASTM D 4885 % TD Break Elongation none 19

ASTM D 3895 minutes OIT 125 10

ASTM D 3895 minutes HP-OIT none 121

ASTM D 5397 hours NCTL 500 >500

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

Fig.5: Physical Property Comparison - 2013 Properties vs. Original Specification


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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”. The following table (figure 6) illustrates this for 2006, the

first year that testing was structured in this manner.

Test Method Units Description Original Specification

Field Testing 2006

No UV 2006

Warehouse 2006

ASTM D 638 Mpa MD Yield Strength 14.9 20.5 20.1 20.6

ASTM D 638 Mpa TD Yield Strength 14.9 21.0 20.1 20.9

ASTM D 638 Mpa MD Break Strength 10.3 21.5 20.5 23.2

ASTM D 638 Mpa TD Break Strength 10.3 17.1 18.2 18.8

ASTM D 638 % MD Yield Elongation 13 18 20 18

ASTM D 638 % TD Yield Elongation 13 18 19 17

ASTM D 638 % MD Break Elongation 150 477 466 515

ASTM D 638 % TD Break Elongation 150 271 387 456

ASTM D 4885 Mpa MD Yield Strength none 16.4 18.0 17.7

ASTM D 4885 Mpa TD Yield Strength none 17.1 18.5 17.6

ASTM D 4885 MD Break Elongation none 20 19 21

ASTM D 4885 TD Break Elongation none 18 18 20

ASTM D 3895 OIT 125 38 49 88

ASTM D 3895 HP-OIT none 276 278 468

ASTM D 5397 NCTL 500 >500 >500 >500

Fig.6: Physical Property Comparison - 2006 Properties(No UV and Waehouse) vs. Original Specification

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. The table below (figure 7) 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.

Fig.7: OIT Over Time for Three Samples


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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.

INSPECTION, SETTLEMENT AND REPAIR

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.”

Fig.8: Repair at Gas Vent Stack


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“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 (figure 9) most repaired throughout the entire installation. This drain

was installed to reduce surface runoff, which, ironically has not been a problem at the site.

CONCLUSIONS AND COMMENTARY:

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.”

Fig.9: Fixed Drain with Excess Slack and Repairs


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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.

ACKNOWLEDGEMENTS

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.

REFERENCES

Adams M.W. and Wagner N. Forensic Study of an HDPE Liner after 10 years of Exposure. Web

reference: http://geosynthetica.net/tech_docs/10yearExAda msWagner.pdf

ASTM D7238 - 06 Standard Test Method for Effect of Exposure of Unreinforced Polyolefin

Geomembrane Using Fluorescent UV Condensation Apparatus.

Canal Lining Demonstration Project - Year 10 Final Report November 2002, R-02-03 Authors:

Jay Swihart and Jack Haynes U.S. Department of the Interior, Bureau of Reclamation

Case, M. E., Koerner, G. R., Hsuan, Y. G. and Koerner, R. M. (2010), “Case History of a 16-Year

Old Exposed HDPE Geomembrane Cover,” 9th ICG, Brazil (on CD).

Case, Mark E., Koerner, George R., Koerner, Robert M., Hsuan, Yick (2010), Case History

of a 16-Year Old Exposed HDPE Geomembrane Cover, Proceedings of 9th International

Conference on Geosynthetics, Guarujá, Brazil

“Designing with Geosynthetics” Koerner, Robert M., Xlibris Press, Corp.; 6th edition (January

16, 2012) Available through Amazon linked here

Giroud, J.P., Pelte, T. and Bathurst, R.J., 1995, “Uplift of Geomembranes by Wind”,

Geosynthetics International, Vol. 2, No. 6, pp. 897-952.

Gleason, M.H., Houlihan, M.F., and Palutis, J.R., “Exposed Geomembrane Cover Systems:

Technology Summary,” Proceedings of the Sixth International Conference on Geosynthetics,

Portland, Oregon, February, 2001, pp. 905-918.

Gleason, M. H., Houlihan, M. F. and Giroud, J. P. (1998), “An Exposed Geomembrane Cover

System for a Landfill,” Proc. 6th ICG, Atlanta, IFAI, pp. 211-218.

GSI White Paper (2003), “The Questionable Strategy of Soil-Only Landfill Covers,” GFR, Vol.

21, No. 2, March, pp. 18-23

Ivy, Nathan (2002), HDPE geomembrane after 20 years of service, GFR Magazine June/July

2002


This Information is provided for reference purposes only and is not intended as a warranty or guarantee. GSE assumes no liability in connection with the use of this Information. Specifications subject to change without notice. GSE and other trademarks in this document are registered trademarks of GSE Environmental, LLC in the United States and certain foreign countries.1APR2015

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Koerner, R. M., Koerner, G. R. and Hsuan, Y. G., “Lifetime Prediction of Exposed

Geomembranes,” Proceedings of the GRI-18 Conference on “Geosynthetics Research &

Development In-Progress” at GeoFrontiers, January 26, 2005, GII Publication, Folsom, PA.

Koerner, R. M. (2011), “Traditional Versus Exposed Geomembrane Landfill Covers: Cost and

Sustainability Perspectives,” Proc. GRI-24, GII Publ., Folsom, PA, pp. 182-197.

Tarnowski C. and Baldauf S.. “ Ageing Resistance of HDPE-Geomembranes – Evaluation

of Longterm Behavior under Consideration of Project Experiences” Web reference:

http://www.gseworld.com/content/documents/articles/Ageing%20resistance%20of%20

HDPEgeomembranes.pdf

The EGC Takes On Three Hurricanes in Polk County. January 2005 Municipal Solid Waste

Management

MSW Management (April 2005), “Exposed Geomembrane Liner Cap Takes on Three

Hurricanes in Polk County, South Central Florida” March/April, MSW Management magazine,

www.mswmanagement.com , pp. 72-77. Web reference: http://www.erosioncontrol.biz/MSW/

Articles/The_EGC_Takes_On_Three_Hurricanes_in_Polk_County_1545.aspx

Thiel, R., Purdy, S. and Yazdani, R. (2003), “Case History of Exposed Geomembrane Cover for

Bioreactor Landfill,” Session D06, Proc. Sardinia 2003

Yako, M. A., Koerner, G. R., Hsuan, Y. G. and Koerner, R. M. (2010), “Case History of a 20-Year

Old Exposed HDPE Surface Impoundment Liner,” 9th ICG, Brazil (on CD).

a 13-year study of an exposed green geomembrane cover€¦ · gseworld.com technical note a 13-year...

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