Examination of PVC in a "Top Cap" Application

 

EXAMINATION OF PVC IN A 'TOP CAP' APPLICATION

Samuel B. Levin and Mark D. Hammond


 

REFERENCE: Levin, S. B. and Hammond, M. D., "Examination of PVC in a 'Top Cap' Application," Geosynthetic Testing for Waste Containment Applications, ASTM STP 1081, Robert M. Koerner, Ed., American Society for Testing and Materials, Philadelphia, 1990.


ABSTRACT: The PVC liner installed over Phase I of the Dyer Boulevard Landfill provides us with an opportunity to examine the material after 5+ years of exposure to landfill gas and other environmental stresses in a top cap application. Test results for samples extracted from the cap are compared to test results obtained at the time of installation, to material properties included within the original material specification, and to material properties from a 'control' sample of excess PVC material from this closure project kept in a warehouse since 1983/84.


KEYWORDS: landfill closure, landfill lining, geosynthetics performance
 

INTRODUCTION

Thee   Dyer Boulevard Landfill services all of Palm Beach County, Florida. It is located over the Turnpike Aquifer, an important local source of drinking water. When initially opened in 1968, it was one of several unlined landfills operated within the County. With time, it became a major disposal area for both municipal solid waste and sewage sludge.

In late 1970's, evidence of contamination of the shallow aquifer surrounding this high rise landfill was observed within the site's monitoring network. Under the terms of a consent agreement between Palm Beach County and the Florida Department of Environmental Regulation, executed in 1982, it was agreed that:

  • The Phase I (unlined) landfill was to be closed in an environmentally sound manner.
  • A new lined landfill was to be developed to provide for future solid waste disposal capacity.
  • An alternative means of wastewater treatment plant sludge and septic tank pumpings disposal was to be utilized.

Post, Buckley, Schuh & Jernigan, Inc. (PBS&J) was selected by Palm Beach County, and subsequently by the Solid Waste Authority of Palm Beach County (SWA) when it assumed responsibility for the landfill in 1983, to develop the closure design for the existing 190 acres ± of landfill cells in Phase I.

The closure design developed for the site included a low permeability 'top cap', well vegetated side slopes, and an integrated drainage system to capture and remove surface runoff, reducing percolation and subsequent leachate generation. The top cap was designed for installation over slopes of less than 10 percent, based on water balance calculations which indicated that only minimal percolation was anticipated through well vegetated landfill side slopes.

The selection of a liner material for use in the top cap at the Dyer Boulevard Landfill proved to be an arduous task, with properties of various materials reported in differing units, or obtained using differing test methods. Suppliers assisted in the selection process by noting the superior properties of some liner materials relative to competing materials. Plasticizer loss, ultraviolet degradation, questionable chemical resistance with respect to landfill gas exposure, and more limited elongation properties were cited as reasons to consider materials other than PVC. Environmental stress cracking, seaming difficulties, and poor strength characteristics upon exposure to bidirectional forces were cited as reasons to consider materials other than High Density Polyethylene (HDPE).
 

PURPOSE AND SCOPE

During the past several years, geosynthetics testing has matured to the point at which properties of virgin materials ' are widely available and in many cases readily comparable. Data concerning the properties of liner materials which have been in service remain scarce, although there is a growing body of information concerning exposure in the laboratory to simulated in-service environments [1], [2].

The top cap material in-service at Dyer Boulevard provides an opportunity to examine the properties of material which has been in service for over five years. Properties of this material will be compared to the properties determined by quality assurance testing during its manufacture and installation, in an attempt to assess the change in properties resulting from material exposure. Excess material stored in a warehouse since its purchase in 1983/84 will serve as a control.
 

DESIGN AND SPECIFICATIONS

The site closure design included the placement of PVC sheet, soil bedding and cover material, a passive landfill gas venting system, drainage improvements, and seeding, mulching and sodding of the completed landfill. A typical cross section through the final cover is provided in Figure 1. Physical properties specified for the 20 mil PVC liner are presented in Table 1. Tensile strength at the seam was required to be at least 80 percent of that of the parent material, or 1760 psi. Also required in the specification was the sampling and testing of the production run for tensile strength and elongation at break.

General Contractor Crabtree Construction Company, Inc., purchased 3.32 million square feet of PVC material, Product Number 1951, manufactured by Dynamit Nobel of America, Inc. and fabricated by the Watersaver Company, Inc. The surface of the subgrade prepared as liner bedding was treated with Hyvar X-L Herbicide prior to placement and seaming of the liner panels. The coarse grained sand used for liner bedding and backfill was obtained by dredge from a near site borrow area.

One hundred and sixty-six panels of PVC, most of which measured 400 feet by 70 feet, were installed by Wright/Kohli Construction Company, a specialty liner subcontractor. Watersaver WS-70 splicing solvent was used for seaming the panels together.

 

TABLE 1 -- Specified Physical Properties for 20 Mil PVC

Physical Property

Value

ASTM Test

     

Thickness

+/- 10%

D 1593

Specific Gravity

1.24 - 1.30

D 792

Tensile Strength

2200 psi

D 882 or D 412

Elongation

300%

D 882 or D 412

100% Modulus

1000 psi

D 882

Graves Tear

270 lbs/in

D 1004

Water Extraction

0.35% (max)

D 1239

Volatility

0.70% (max)

D 1203

Impact Cold Crack

-20ºF

D 1790

Dimensional Stability

5% max @ 212ºF

D 1204-54

 

 

 

 

 

 

 

 

 

 

 

 

 






 

PVC SHEET (LINER) TESTING

Testing of the properties of the PVC sheet installed over the Dyer Boulevard landfill included:

  • Quality control (QC) testing of the parent material at the time of manufacture by Dynamit Nobel of America, Inc.
  • Non destructive (air lance) testing of all field seams during the time of installation.
  • Destructive testing of the parent material, factory and field seam samples obtained throughout the ten month installation period.

Table 2 provides a summary of the manufacturer's QC data. Please note that the data presented is the average for a number of rolls tested. One sample was obtained for testing purposes for each 86,000 square feet of manufactured product.

 

TABLE 2 -- Manufacturer's QC Data

         


ASTM

Manufacturer's

Test Result

Test Result

Test Result

Test Result

Properties

Test Method

Specification

(Ave.11 Rolls)

(Ave.6 Rolls)

(Ave.13 Rollsb)

(Ave.7 Rolls)

Thickness (mils)

D-1593

20 +/- 5%

19.9

19.5

19.8

19.9

Specific Gravity (min.)

D-792

1.23

1.25

1.25

1.2

1.25

Tensile Strength

D-882

2400

MD 2999

MD 2836

MD 2487

MD 2977

(psi)

   

TD 2768

TD 2692

TD 2464

TD 2781

Modulus @ 100%

D-882

1000

MD 1446

MD 1270

MD 1384

MD 1291

Elong. (psi min.)

   

TD 1333

TD 1202

TD 1332

TD 1203

Elongation, % min.

D-882

300

MD 414

MD 423

MD 336

MD 453

     

TD 426

TD 435

TD 354

TD 449

Tear Strength (lbs.)

D-1004

5.5

MD 7.46

Not

8.51

Not

     

TD 7.92

Reported

9.18

Reported

Low Temperature

D-1790

-20

Pass

Pass

Pass

Pass

Impact °F

           

Volatile Loss max.

D-1203

1

0.79

0.70%

0.87%

1.13%

(@70°C for 24 hrs.)

           

Water extraction

D-1239

0.3

0.08

0.18

0.15

0.3

% Loss max

           

(1040F, for 24 hrs.)

           

Dimensional Stability

D-1204

±5.0

MD -1.8

MD -1.8

MD -1.8

MD -1.27

% change, max.

   

TD 0.6

TD 1.7

TD 0.86

TD 0.51

212?F for 15 min.

           

a Provided by the Watersaver Company, Inc.

   

MD - Machine Dire

 

b Number of rolls tested estimated based upon reported material weight.

 

TD - Transverse D

 


Destructive testing of field samples taken throughout the liner installation process was performed by QC Metallurgical Inc., Hollywood, Florida. A total of 236 tensile strength tests were performed on field samples. These test results are summarized in Table 3. All tensile tests were performed in shear.
 

TABLE 3 -- Summary of Tensile Test Data for Field Samples of PVC when Installed

   
   

Specified Values

Range of Results

Average Value

Sample

Sample Type

Sample Quantity

(psi)

(psi)

(psi)

Standard Deviation

Field Seam in Shear

195

1760

1490 - 2550

1937

165

           

Factory Seam in Shear

30

1760

1825 - 2568

2132

186

           

Parent Material

11

2200

2040 - 4750

2857

712


LINER RESAMPLING/TESTING PROGRAM

In December, 1989, the soil overburden was carefully removed from the top cap in several locations of the Phase I area to expose the liner material. According to the Solid Waste Authority's manager of landfills, the exposed material appeared to exhibit few signs of degradation upon visual inspection.

The surface of the material exhibited minor undulations, a few millimeters in size, where the material apparently elongated to conform to the surfaces of soil particles. The samples were highly pliable, exhibiting no apparent brittleness.

Eight coupons of the parent material and one field seam coupon were sliced from the cap by the Solid Waste Authority's manager of landfills. Added to the package of nine samples was a coupon sliced from a factory panel of PVC stored by the Solid Waste Authority as surplus in a warehouse since its purchase in 1983/84 for capping the Phase I Dyer Boulevard site. The ten samples were transmitted to Geosyntec, Inc., Boynton Beach, Florida for laboratory testing.

A summary of test results for these samples is provided in Table 4. The tests were selected to correspond with tests performed by the manufacturer in 1984. Low temperature impact testing was not performed due to its limited applicability to the sub-tropical West Palm Beach environment. Dimensional stability was not determined due to an insufficient quantity of sample material.

 

TABLE 4 -- Liner Resampling/Testing Program

       
 

Specific

Tensile

Modulus @ 100%

Elongation

Tear

Volatile Loss

Water Extraction

 

Gravity

Strength

Elongation

(%)

Strength

(%)

(%)

   

(psi)

(psi)

 

(lbs.)

   

ASTM

             

Test Method

0-792

0-882

D-882

0-682

D-1004

D-1203

D-1239

Field Seam

N/A

2204 shear

N/A

407

N/A

N/A

N/A

   

1140 peel

         

Field Panel 1

1.265

MD TD

MD TD

MD TD

MD TD

   
   

2744 2642

1865 1721

341 381

10.1 9.3

1.33

2.81

Field Panel 2

1.282

MD TD

MD TD

MD TD

MD TD

   
   

3072 2642

1865 1721

341 381

10.1 9.3

0.4

1.08

Field Panel 3

1.267

MD TD

MD TD

MD TD

MD TD

   
   

2740 2544

1968 1828

341 334

9.9 9.6

1.41

2.06

Field Panel 4

1.264

MD TD

MD TD

MD TD

MD TD

   
   

2614 2633

2043 1904

340 324

9.5 9.8

2.02

1.85

Field Panel 5

1.267

MD TD

MD TD

MD TD

MD TD

   
   

2634 2327

1884 1879

311 309

9.5 8.6

3.26

1.99

Field Panel 6

1.252

MD TD

MD TD

MD TD

MD TD

   
   

2496 2360

1667 1794

356 270

8.8 8.9

4.23

2.63

Field Panel 7

1.27

MD TD

MD TD

MD TD

MD TD

   
   

2446 2149

1746 1672

309 331

9.2 8.5

0.85

1.69

Field Panel 8

1.278

MD TD

MD TD

MD TD

MD TD

   
   

2627 2628

1763 1781

362 345

8.9 8.6

0.96

2.58

Warehouse

1.279

MD TD

MD TD

MD TD

MD TD

   

Panel

 

2662 2439

1557 1476

429.4 417.4

8.3 7.8

0.78

1.29

           

MD - Machine Direction

           

TD - Transverse Direction

 

COMPARISON/EVALUATION OF TEST RESULTS

An insufficient number of field seam samples (one) and warehouse parent material samples (one) were retested to provide for a reliable comparison of test results. Data from the testing of these samples is provided for information only. Emphasis will be placed on comparing the data from the factory QC parent material test results with testing of the eight parent material samples obtained in December, 1989. This data is presented for comparison in Table 5.

 

TABLE 5 -- Summary of Test Results

       
   

Parent Material (Manufacturer's)

Parent Material

Parent Material
(field samples

   

QC Data

(from Warehouse)

December 1989)

Specific Gravity

Range

1.26 - 1.25

 

1.25 - 1.28

 
 

Ave.

1.25

1.28

1.27

 

Tensile Strength

MD Range

2487 -2999

2662

2498 - 3072

 

(psi)

Ave.

2789

 

2672

 
 

TD Range

2464 - 2781

2439

2149 - 2725,

 
 

Ave.

2651

 

2502

 

Break Elongation

MD Range

336 - 453

429

309 - 362

 

(%)

Ave.

395

 

335

 
 

TD Range

354 - 449

417

270 - 381

 
 

Ave.

407

 

327

 

Tear Resistance

MD Range

7.5 - 8.5

8.3

8.9

-10.1

(lbs.)

Ave.

8

   

9.5

 

TD Range

7.9 - 9.2

7.8

8.5

-9.8

 

Ave.

8.6

   

9.1

Secant Modulus

MD Range

1270 - 1446

1557

1667 - 2243

 

(psi)

Ave.

1366

 

1897

 
 

TD Range

1202 - 1333

1476

1672 - 2027

 
 

Ave.

1287

 

1826

 

Volatile Loss (%)

Range

0.70 - 1.13

0.78

0.4 - 4.23

 
 

Ave.

0.87

 

1.81

 

Water Extraction (%)

Range

0.08 - 0.30

1.29

1.69 - 2.81

 
 

Ave.

0.16

 

2.09

 
           

a Average values presented here are average of the average QC data reported by the manufacturer, weighed by the number of rolls tested.

 

         
       

The specific gravity of the 'aged' PVC appears to be somewhat greater than that at the time of manufacture. This increase is consistent with a loss of plasticizer in the aged samples, since the specific gravity of the PVC resin (approximately 1.4) exceeds that of the plasticizer (approximately 0.98). Based on the apparent increase in specific gravity, the aged samples have lost approximately 13 percent of their initial (factory) plasticizer content.

Plasticizer loss would expectedly be evidenced by an increase in tensile strength. The aged samples did not reflect this, with a loss in the average tensile strength of about 4 and 6 percent, machine direction and transverse direction respectively.

Conversely, break elongation results followed a highly predictable path. The average aged sample elongation was 15 percent (machine direction) and 20 percent (transverse direction) less than that reported on the factory QC sheets.

The tear resistance test, as its name implies, is a measure of the force necessary to initiate a tear in the plastic sheet. The averaged test sample tear resistance values for the aged samples exceeded the average values observed at the time of manufacture by 6 and 19 percent.

Secant modulus at 100% strain provides a measure of the stiffness of the sheet, and should increase as plasticizer is lost from the material. The modulus at 100% strain of the aged samples averaged 42 and 39 percent above the average modulus data reported at the time of manufacture.

The volatile loss test is subject to some variability, depending on the type of plasticizer used [4],[5]. The large difference between the average volatile loss tested at the time of manufacture and that of the aged samples likely exceeds variations inherent in the test. The average volatile loss of the aged samples exceeded the average loss reported by DNA by over 100 percent.

An even larger increase was observed between average values for the water extraction test. The average loss values for the aged samples exceeded those reported at the time of manufacture by over 1000 percent.
 

DISCUSSION

The above described apparent changes in properties of the PVC sheet, with the possible exception of tensile strength, are consistent with changes associated with plasticizer loss. Since designers, owners, and operators of solid waste landfills are most concerned with the long term effectiveness of the top cap, what we really wish to know is of what point will placticizer loss be sufficient to result in cap failure, and how long will it take for this failure to occur?

It would appear from both visual observation and laboratory testing that for a South Florida landfill, this period exceeds five years. With few exceptions, the properties which relate to the ability of the PVC to continue to function as a low permeability moisture barrier, (tensile strength, elongation, tear resistance, secant modulus) exceeded the originally specified values in each of the eight aged samples.

How long beyond this five year period will the PVC sheet continue to provide its intended top cap function? Extrapolation of the data obtained by testing the eight aged samples is tenuous, at best.

Some insight may be provided through review of the comprehensive study performed by Morrison and Starbuck [5]. Testing was performed on 10 mil PVC linings within eight canals which had been in service for periods ranging from 0.6 to 19 years. The changes in properties observed by Morrison and Starbuck were mostly consistent with those observed in the eight landfill top cap samples, and were attributed to plasticizer loss. The landfill top cap environment does not appear to have been more hostile to PVC than the canal lining environment, although the difference in thickness (10 mil for canal linings, 20 mil for the top cap) may have impacted the relative magnitude of the observed/reported changes in properties.

The Morrison and Starbuck study provides data for 10 mil canal lining samples after as many as 18 and 19 years in service. Loss in elongation for the 19 year old sample is as high as 63.3%, with a corresponding loss of plasticizer of 45.7%. These property changes were not sufficient to cause failure of the canal lining.

An additional potential difference between the landfill top cap environment and the canal lining environment is temperature. The top cap may be exposed to elevated landfill gas temperatures resulting from waste decomposition. Elevated temperatures are a driving force for plasticizer loss. In-situ temperatures were not noted in the canal lining study, making comparisons with the top cap environment more difficult.

How long a service life is required for a landfill top cap? In Florida, the post closure period for a solid waste landfill extends a minimum of 20 years [6]. The latest proposed federal rules [7] require a post closure period which extends for a minimum of 30 years.

The stresses imposed on a top cap in a solid waste landfill subsequent to its installation, are in our opinion primarily a result of differential waste settlement. Sufficient elongation to accommodate this settlement is of paramount importance.

The rate and uniformity at which landfills settle is highly variable depending to a large extent on the type of waste accepted, compaction procedures, moisture content, and overall landfill height. Landfill settlement tends to be most pronounced in the initial several years after closure. It is during this period that the PVC exhibits its greatest elongation properties.
 

SUMMARY AND CONCLUSIONS

The PVC top cap at the Dyer Boulevard Landfill appears to have maintained its integrity after its initial 5+ years in service. Test values from sample coupons extracted from the top cap differ from test values for sample coupons taken from the parent material at the time of manufacture. These apparent property changes are consistent with those attributed to plasticizer loss, published for PVC which has been in service as canal lining, although the canal lining and top cap environments may differ.

The aging of PVC in both the top cap and canal environments appears to be consistent with changes in properties associated with plasticizer loss although other factors in the landfill environment or canal environment may be responsible for these observed changes. In the canal environment, the PVC exhibited significant loss of plasticizer over 18 and 19 years of exposure. Corresponding elongation properties were reduced by as much as 63%. The material was still serving its intended function despite these changes in properties. Additional long term data (preferably from landfill top cap environments) will assist in projecting the anticipated service life of PVC for landfill closures.
 

ACKNOWLEDGMENTS

We wish to express our sincere appreciation to the Solid Waste Authority of Palm Beach County for funding the liner testing program. Additional thanks are due to Art Arena, Technical Director, Huls America, Inc., and Richard Dickenson, a technical consultant to Huls America, Inc.
 

REFERENCES

W.R. Morrison and L.D. Parkhill, Evaluation of Flexible Membrane Liner Seams after Chemical Exposure and Simulated Weathering, U.S. Bureau of Reclamation, Engineering and Research Center, Denver, Colorado, Feburary 1987.

Haxo, H.E., et.al., Liner Materials Exposed to Municipal Solid Waste Leachate, third Interim Report, EPA-600/2-79-038, July 1979.

Post, Buckley, Schuh & Jernigan, Inc. , Contract Specifications for Dyer Boulevard Landfill Closure for Palm Beach County, Florida, June 1983.

ASTM, 1989 Annual Book of ASTM Standards, Section 8, Volume 08.01.

W.R. Morrison and J.G. Starbuck, Performance of Plastic Canal Linings, U.S. Bureau of Reclamation, Engineering and Research Center, Denver, Colorado, January 1984.

Florida Administration Code, Chapter 17-701.07.

[7] Federal Register, Volume 53, No. 168, Tuesday, August 30, 1988/Proposed Rules, Page 33408.

 

Mr. Levin is Assistant Manager of the Solid Waste/Resource Recovery Division of Post, Buckley, Schuh & Jernigan, Inc., 800 North Magnolia Avenue, Suite 600, Orlando, Florida 32803. Mr. Hammond is Director of Operations, Solid Waste Authority of Palm Beach County, 5114 Okeechobee Boulevard, Suite 2-C, West Palm Beach, Florida 33417.


 

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