Properties of exhumed HDPE field waves

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q Revised from a paper "rst presented at the GRI 10 Conference, December 1997.*Corresponding author. Fax: 1-610-522-8441; e-mail: [email protected]

Geotextiles and Geomembranes 17 (1999) 247}261

¹echnical Note

Properties of exhumed HDPE "eld wavesq

George R. Koerner!,*, Anthony W. Eith", Maria Tanese#! Geosynthetic Institute, 475 Kedron Ave., Folsom, PA 19033, USA

" Waste Management Inc., USA# Rust Environmental and Infrastructure, Inc., USA

Received 9 January 1998; received in revised form 11 July 1998; accepted 26 September 1998

Abstract

HDPE geomembranes are the most widely used components of liner and cover systemsof land"lls throughout the world. Millions of square meters of HDPE geomembrane havebeen used as hydraulic barriers over the past twenty years. It is usually assumed thatHDPE geomembranes lie #at against soil subgrade. Unfortunately, this is not always thecase. It is obvious that waves (also called wrinkles) compromise the concept of intimatecontact but equally important, they may in#uence the properties of the geomembrane. Thepurpose of this paper is to examine if waves remain in the back"lled geomembrane and towhat extent they in#uence performance properties. ( 1999 Elsevier Science Ltd. All rightsreserved.

Keywords: Geomembranes; Land"ll; Case history

1. Introduction and description of the site

A municipal solid waste land"ll double liner system constructed in 1988 wasexhumed after it had been in service for eight years. The liner system included primaryand secondary geomembrane layers of 1.5 mm thick high density polyethylene(HDPE). The liner system exhumation was done as part of a reconstruction project on

0266}1144/99/$19.00 ( 1999 Elsevier Science Ltd. All rights reserved.PII: S0266}1144(98)00024}7

Fig. 1. Plan view of Cell 3.

several sumps. The reconstruction was needed to facilitate a lateral expansion of thefacility.

A plan view of Cell 3 of this particular land"ll is shown in Fig. 1. The shaded arearepresents the exhumed sump area. This area is at the convergence of the herring boneleachate collection system and represents the lowest elevation of the cell. A photo-graph of the exhumed sump area is shown in Fig. 2. It represents approximately twopercent of the entire cell volume. A detail of the slope cross section can be seen inFig. 3 in contrast to the cell bottom cross section shown in Fig. 4. The cell bottom isa more conservative design since it experiences higher hydraulic heads than the slopesection. The maximum side slope grades are 3(H)-to-1(V) and the slope on the cellbottom is at a 2% grade. Additional details of the site and exhumation can be found ina paper by Eith and Koerner (1996).

2. Existing conditions

The municipal solid waste (MSW) was excavated by trackhoes which "lled o!-roaddump trucks. The trackhoes were equipped with large buckets and took over a monthto excavate approximately 150,000 m3 of MSW. Care rather than speed was theemphasis for the operation. Within a meter of the liner system the exhumationinvolved hand labor. This was a painstakingly slow process but necessary so as not todamage the liner system.

248 G.R. Koerner et al./Geotextiles and Geomembranes 17 (1999) 247}261

Fig. 2. Overview photograph of Cell 3 sump excavation.

Upon exhuming the waste and revealing the HDPE geomembrane, one couldplainly see a number of large waves in both the primary and secondary geomem-branes. As shown in Fig. 2, the waves appeared to converge towards the sump at thetoe of the slope. The waves varied in size from 50 mm to 800 mm in height. Theexistence of such waves have been analyzed by several researchers (Giroud and Morel,1992) and modeled by others (Soong and Koerner, 1997).

It is surmised that the original back"lling operation had the e!ect of gathering thegeomembrane in the area of the sump. The back"ll was placed from the toe of theslope upwards, starting at the sump and fanning outwards. It appears that the wavesfollowed this back"lling pattern. Note also that this facility makes use of inclinedslope risers for both the leachate detection and leachate collection system. These tworisers complicate grading and back"lling in the area of the sump and perhaps areresponsible for some localized movement in the area.

Figs. 5}7 show various wave examples encountered at the site. When normal stressis applied uniformly and no eccentricities are present, a vertically #attened prayer-wave like those shown in Figs. 5(a) and (b) are formed. If the normal stress is applied

G.R. Koerner et al./Geotextiles and Geomembranes 17 (1999) 247}261 249

Fig. 3. Slope cross-sectional detail of Cell 3.

Fig. 4. Bottom cross-sectional detail of Cell 3.


Fig. 5. (a) Lab prayer wave specimens, (b) "eld prayer wave example.

Fig. 6. (a) Lab &&S'' wave specimens, (b) Field &&S'' wave example.

Fig. 7. (a) Lab mushroom wave specimens, (b) "eld mushroom wave.


Table 1Summary of wide width tension results

Machine direction Cross-machine direction

Yield Yield Stress at Yield Yield Stress atDescription stress! strain" 200% strain stress! strain" 200% strainof wave (MPa) (%) (MPa) (MPa) (%) (MPa)

No wave (#at)geomembrane 16.0 16.3 11.4 15.5 15.0 10.3

Prayer-wave 16.1 20.0 12.3 16.2 16.0 10.4S-wave 15.9 17.2 11.5 15.6 13.0 10.1Mushroom-wave 16.1 21.8 11.6 16.2 17.1 10.2

!Stress calculations are based on an average thickness of 1.54 mm."Yield strain calculations are based on a 100 mm gauge length

nonuniformly, the wave rolls over on itself and produces an S-wave like those shownin Figs. 6(a) and (b). If the prayer wave is con"ned on both sides and then subsequentlyloaded the wave becomes horizontally #attened into a mushroom-wave like thoseshown in Fig. 7(a) and (b).

3. Laboratory results and discussion

Due to the severity and nature of the waves at this site it was decided to conducta laboratory investigation to determine what e!ect (if any) the waves had on theproperties of the geomembrane. It was decided that wide width tensile tests (perASTM D 4885), on the wave compared to #at HDPE geomembrane samples shouldbe conducted. Each di!erent fold geometry was contrasted against non-folded, i.e.,#at, geomembranes. A summary of the results appear in Table 1.

The results presented in Table 1 are the average of three trials for each foldgeometry. Unfortunately, wide width tensile tests were limited to 200% strain becausethe head space on the continuous rate of extension machine could not accommodateenough elongation to break the material at such a slow strain rate. ASTM D4885stipulates that a 200 mm wide specimen gripped across its entire width should bepulled at a strain rate of 1 mm/min. In addition, it should be noted that since each testtook over 3.3 hours to conduct, the continuous rate of extension machine wasoccupied for better than two weeks for this investigation. The test setup as well asa stressed specimen can be seen in Figs. 8 and 9, respectively.

The following observations can be made regarding these wide width tension testresults:

v Within the inherent test variability, there appears to be no statistically signi"cantdi!erence between #at and waved geomembrane tensile strengths.


Table 2Summary of stress crack results

Sample no. Description Replicate Time (hours)

1 Flat geomembrane 1 '2300Flat geomembrane 2 '2300Flat geomembrane 3 '2300Flat geomembrane 4 '2300Flat geomembrane 5 '2300

Average '2300

2 Geomembrane with wave 1 1074Geomembrane with wave 2 885Geomembrane with wave 3 860Geomembrane with wave 4 '1174Geomembrane with wave 5 '1174

Average 1033

3 Flat geomembrane 1 '1823Flat geomembrane 2 '1823Flat geomembrane 3 '1823Flat geomembrane 4 '1823Flat geomembrane 5 '1823

Average '1823

4 Geomembrane with wave 1 696Geomembrane with wave 2 1929Geomembrane with wave 3 913Geomembrane with wave 4 1063Geomembrane with wave 5 1599

Average 1240

v The relatively minor di!erence between #at and waved geomembrane elongationcharacteristics can be explained by the di$culty of placing a folded specimen in thegrips of the CRE machine without slack. Note that all specimens received a preloadof 180}220 N prior to testing.

v In all cases, the folded regions of the test specimens provided a preferentialinitiation point at which the HDPE geomembrane yielded (i.e. #at-wave specimensyielded randomly, prayer-wave specimens yielded primarily in one location, S-wavespecimens yielded in two distinct locations, and mushroom-wave specimens yieldedin three distinct locations).

In addition to wide width tensile testing which challenges the short-term strength,single point notched constant tension load (SP-NCTL) tests as per ASTM 5397(Appendix) were conducted on #at as well as waved geomembrane test specimensto challenge the long-term behavior. Table 2 present a summary of the SP-NCTLresults.


Fig. 8. Wide width tensile test apparatus showing ASTM D 4885 test con"guration.

Fig. 9. Stressed wide tensile specimen.


The following observations can be made about the SP-NCTL test results ofTable 2.

v The geomembrane used at this site is a very high quality stress crack resistantmaterial with failure times in excess of 1823 hours. When the results are comparedwith GRI-GM13 speci"cation for HDPE geomembranes, the results far exceed thespeci"cation requirement of 200 hrs.

v There appears to be a decrease in stress crack resistance of waved versus nonwavedgeomembrane test specimens. Whether this decrease is signi"cant for such aninitially good resin is debatable.

4. Summary and conclusions

Due to the rearrangement of the site plan at a large municipal solid waste disposalfacility, an opportunity presented itself to exhume an eight-year old HDPE geomem-brane liner. Upon exhuming, it was noted that the geomembrane had waves in it. Suchwaves were trapped in the geomembrane when it was back"lled.

In addition to bringing this "eld condition to light, the subsequent test resultscontained in this paper present wide width tensile properties and stress crack resist-ance of #at versus waved material. As a result of the tests conducted, the tensilestrength appears not to be signi"cantly compromised. There is a tendency for thestress crack resistance to decrease, but it is still well above current speci"cation valuesfor this particular property.

Appendix A: Selected aspects of wave management

This appendix is presented as a guide for the installation of geomembranes so as tominimize the incidence of large waves, and (hopefully) to eliminate them altogether. Itis placed here as a counterpoint to the main paper which identi"ed the existence ofrelatively large waves in an exhumed land"ll. It is obviously not possible to present allaspects of wave management, but selected situations based upon our experience areincluded in the appendix. More speci"cally, four areas which have presented pastproblems are described; large #at areas, toe of slope areas and access areas (or ramps).

A.1. Large yat areas

Fig. A.1 shows a wave situation created by seaming adjacent HDPE geomembranepanels which have not reached temperature equilibrium. The work in this project isprogressing from left to right. The left panel (which is signi"cantly waved) was placedthe day before the picture was taken. The seam in question (marked and near thecenter of the picture) was the "rst seam of the day. This seam adjoined two panels, onethat experienced the past environment and one which had just been deployed. Insteadof making this seam in the beginning of the day it should have been made later. In sodoing, the panel to the right of center of the picture could have reached temperatureequilibrium.


Fig. A.1. Photograph showing waves perpendicular to a longitudinal seam. Waves were caused by notallowing the geomembrane to temperature equilibrate.

Fig. A.2 illustrates a large wave caused by not seaming and/or anchoring thelongitudinal panels overnight. By not "xing the ends of these longitudinal panels theytend to shift overnight due to ambient temperature variations. Contraction of thepanels tends to cause signi"cant waves which run at a 453 angle to the longitudinalseams. This particular scenario can be avoided by incrementally conducting tie-inseaming each night in addition to weighing down any free ends with sand bags, rolls ofgeosynthetics or ATVs. This hampers production because tie-in seams require exten-sive preparatory work. However, the suggestion will result in a #atter geomembraneinstallation.

Fig. A.3 illustrates a wave resulting from excessive &&underlap'' of longitudinalpanels in the seaming area. The contractor, while spotting his panels encountereda condition where the longitudinal underlap was approximately 1000 mm at one endof a 300 m roll length and 100 mm at the other. The contractor extruded the 300 mlong seam with disregard to the uneven underlap instead of taking the time and e!ortto trim it uniformly to 100 mm. Unfortunately this excess material under the panelwaved in its own right. This wave translated through the panel and needed to beremedied.

Fig. A.4 shows a modest wave which has manifested itself into a larger wave. Duringthe back"lling process the models wave was not walked-out into two or three smallerwaves. The earth work contractor should have had several laborers 5}10 m in front ofthe construction equipment spreading the drainage gravel walking-out the waves. Inso doing this wave would never have become trapped in the facility. Note that


Fig. A.2. Photograph of large 453 wave transvering cell. Wave was caused by not tieing in or weighingdown panel ends over night.

Fig. A.3. Photograph of longitudinal wave resulting from excessive underlap.


Fig. A.4. Photograph showing waves being back"lled by drainage gravel. Situation created by improperlywalking-out the wave in advance of gravel placement.

cutting-out waves at this stage of the construction sequence is time consuming andless than ideal.

A.2. Toe of slope areas

Fig. A.5 shows a common situation of smooth HDPE collecting at the toe of theslope. This phenomenon is known as &&caterpillaring'' and occurs when the geomem-brane has not been back"lled in its anchor trench. Fig. A.6 shows laborers cutting thewave out of region. At this particular site which is of a double-liner type, this reworkinvolved cutting and repairing seven geosynthetic layers. In each case, the excessmaterial needs to be removed and a new seam created.

On the subject of back"lling geomembranes, it is interesting to note that there areseveral pro-active situations that can be taken to create less waves in geomembranes.Geotextiles, often used as puncture protection layers, can in#uence the temperature ofunderlying geomembranes. Cadwallader et al. (1993) and Koerner and Koerner (1995)have shown that during the hottest time of the year (August) during the hottest time ofthe day (afternoon) a light gray geotextile kept a deployed geomembrane on average123C cooler than a black geotextile. Furthermore, Daniel and Koerner (1993) suggestcontrolling the amount of slack in the geomembrane by back"lling at night or earlymorning when ambient temperatures are cool and geomembranes are taught to thesubgrade.


Fig. A.5. Photograph showing geomembrane that has caterpillared down the slope and gathered at thebreak in grade at the toe of slope.

Fig. A.6. Photograph showing wave repair at the toe of slope. Excess material was removed from "vegeosynthetic layers in this instance prior to re-seaming.


Fig. A.7. Photograph showing the use of textured geomembrane on side slopes and sump area to reduce theamount of waves.

Fig. A.8. Photograph showing the use of textured geomembrane in the cell access area to reduce theamount of waves.


A.3. Access areas (or ramps)

Fig. A.7 shows the use of textured geomembrane to mitigate caterpillaring ofgeomembranes on side slope or breaks in grade. Fig. A.7 even shows texturedgeomembrane being used in the sump area. Textured geomembranes interact withunderlying layers by virtue of high interface friction values. Therefore, texturedgeomembranes tend to wave less than smooth geomembranes.

Fig. A.8 shows textured geomembrane being used in the access area of a cell. Liveloads in this area are generally high. Dynamic forces mobilized by ample truck tra$calso tends to accumulate the geomembrane to the toe of the slope. If textured HDPEis used in lieu of smooth HDPE in these areas, caterpillaring will be greatly mini-mized.

References

American Society for Testing and Materials (ASTM) D 4885, 1995. Standard Test Method for DeterminingPerformance Strength of Geomembranes by the Wide Strip Tensile Method. West Conshohocken, PA,USA.

Cadwallader, M., Cranton, M., Peggs, I.D., 1993. White-surfaced HDPE geomembranes: Assessing theirsigni"cance to liner design and installation. Proceedings of Geosynthetics 93 Conference, Vancouver, BC,pp. 1065}1079.

Daniel, D.E., Koerner, R.M., 1993. EPA Technical Guidance document: &&Quality assurance and qualitycontrol for waste containment facilities''. EPA/600/R-93/182. Cincinnati, OH, pp. 167}170.

Eith, A.E., Koerner, G.R., 1996. Assessment of HDPE geomembrane performance in a municipal wasteland"ll double liner system after eight years of service. Proceedings of the GRI-10 Conference on FieldPerformance of Geosynthetics and Geosynthetic Related Systems, Koerner, R.M., Koerner, G.R., Hsuan,Y.G. (Eds.), GII Publications, Philadelphia, PA, pp. 101}115.

Geosynthetic Research Institute (GRI Standard GM13), Standard Speci"cation for Test Properties, TestingFrequency and Recommended Warrant for High Density Polyethylene (HDPE) Smooth and TexturedGeomembranes. Folsom, PA, USA.

Giroud, J.P., Morel, N. 1992. Analysis of geomembrane wrinkles. Geotextiles and Geomembranes 11 (3),255}276.

Koerner, G.R., Koerner, R.M., 1995. Temperature behavior of "eld deployed HDPE geomembrane.Proceedings of Geosynthetics '95 Conference, Nashville, TN, pp. 921}938.

Soong, T.-Y., Koerner, R.M., 1997. HDPE waves in the laboratory. Proceedings of the GRI-11 Conferenceon Field Installation of Geosynthetics, Koerner, R.M., Hsuan, Y.G. (Eds.), GII Publications, Philadel-phia, PA, pp. 128}155.


Properties of exhumed HDPE field waves

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