geocell design principles

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August 2004Volume 22, Number 6

1Copyright 2004 GFR Magazine.

Reprinted with permission of Industrial Fabrics Association International. 1

Late 2004 opportunitiesThe latter months of the year offer many con-struction and materials conferences. 2630 September, Dam Safety 2004,Phoenix . Organized by the Association ofState Dam Safety Officials (ASDSO), the an-nual Dam Safety conference gives consultingengineers, water management professionals,regulators, construction industry representa-tives and many others a way into the deeper

level discussions, trends and concerns incontemporary dam design and operation. For more information, contact Associationof State Dam Safety Officials (ASDSO), 450Old Vine St., Fl. 2, Lexington, KY 40507-1544; +1 859 257 5140, fax +1 859 3231958, e-mail [email protected], Web sitewww.damsafety.org. 2630 September, MINExpo Interna-tional 2004, Las Vegas . Global raw materialdemand has risen sharply during the pastfew years. The trend has revitalized the min-ing industry, and presented engineers withmany new design opportunities and environ-mental challenges. For more information, contact Hall-Erickson Inc., 98E. Naperville Rd., Westmont, IL 60559-1559; +1 630 4347779, fax +1 630 434 1216, e-mail [email protected], Web site www.minexpo.com. 2327 October, GeoQuebec 2004, Quebec City .The 57th Canadian Geotechnical Conference offers

more than 400 papers, numerous discussions andchances to sample materials and evaluate servicesup close. The event brings together a broad swatch ofengineering disciplines. For more information on the geosynthetic sessions,contact Eric Blond (SAGEOS/CTT Group), +1 450771-4608, fax +1 450 778 3901, e-mail [email protected]. For information regarding the general conference,please write to [email protected], or visit the

Opportunities, ownership and education

Geocells, nonwoven geotextiles and drainagecomposites being installed at the new Sofiaairport, Bulgaria.

Reminder: 2005 Specifiers GuideThe deadline is rapidly approaching for submis-sions of product data to the 2005 Specifiers Guide.Manufacturers of geosynthetic materials are encour-aged to contact GFRs editors to confirm that theirsubmissions have been received and processed, orto inform the magazines staff that submissions areforthcoming. The deadline is 15 September 2004. The SpecGuide, GFRs annual volume of polymeric product

data and professional resource directory, will bepublished in December and distributed to magazinesubscribers, at shows throughout the forthcomingyear, and through the IFAI Bookstore. (See insideback cover.) Product data submission tables areprovided for geotextiles, geogrids, geomembranes,rolled erosion control products, drainage products,geocells and geosynthetic clay liners. Contact The Editors, GFR Magazine, 1801County Rd. B W., Roseville, MN 55113-4061; +1 651225 6988, fax +1 651 225 6966, e-mail [email protected], Web site www.gfrmagazine.info.


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Copyright 2004 GFR Magazine.Reprinted with permission of Industrial Fabrics Association International. 2

Web site at www.geoquebec2004.org. 2729 October, IFAI Expo 2004, Pitts-burgh . Organized by the Industrial Fabrics

Association International (IFAI), IFAI Expo isNorth Americas largest annual specialty fabricsindustry exposition. The events mix of educa-tional seminars, short courses, presentationsand exhibits reveal niche markets, trends anddesigns in a number of fields: architecture,engineering, agriculture, industrial and manymore. This years event includes an educationtrack to help impart more engineering knowl-edge to architects.

For more information, contact IFAI Confer-ence Management, 1801 County Rd. B W., Ros-eville, MN 55113-4061; +1 651 222 2508, fax +1651 631 9334, e-mail [email protected], Website www.ifaiexpo.info. 34 November, 2004 Design-Build Expo,Chicago . Without a doubt, design-build strategies are reshaping how engineers do business. The Design-Build Expo brings architects, engineers and contractors together for sharing experience and concerns. For more information, contact Patrick Wilson, Design-Build Institute of America, 1010 Massachusetts

Ave. N.W., Fl. 3, Washington, DC 20001-5402; +1 202 454 7535, fax +1 202 682 5877, e-mail [email protected], Web site www.eshow2000.com/dbia.

Giroud to deliver 2005 Terzaghi Lecture

By Lara Peggs

In March 2001, Dr. J.-P. Giroud was invited to give the 2005 Terzaghi Lecture organized every other yearby the Technical University of Vienna, Austria. It is an exceptional honor, as it is one of the most prestigiouslectures in the field of geotechnical engineering. Dr. Giroud will present the Terzaghi Lecture on the firstday of the Austrian Geotechnical Conference, which will take place in Vienna on 2122 February 2005.

The conference typically attracts 300400 participants from about 20 countries. The title of the lecture will be Geosynthetic engineering: successes, failures and lessons learned. Thetentative synopsis is: Karl Terzaghi at Mission Dam (now called Terzaghi Dam), and his first experience with a geosynthetic:a failure and a success. Failures and lessons learned. The selected examples will be of interest to a large audience, and willshow the degree of sophistication in geosynthetic engineering, such as: geomembrane cracking patterns,and the triumph of rational analysis; the effect of differential settlement on geosynthetics and the conceptof co-energy, an original application of mechanics; influence of water on stability of geosynthetic systems,

Q & A: DABFETIts one of the more peculiar professional acronymsto follow an engineers name: DABFET. It standsfor Diplomate of the American Board of ForensicEngineering and Technology. A good number ofchemical specialists and expert witnesses acquireand maintain this credential, since projects ofteninvolve a great mix of disparate materials: concrete,steel, geosynthetics, polymer-modified soils, etc.Certifications and degrees can be a differencemaker in a clients selection of an engineer, or aregulators input on code requirements.

For more information, contact American Collegeof Forensic Examiners International, 2750 E. Sun-shine, Springfield, MO 65804; +1 417 881 3818, fax+1 417 881 4702, Web site www.acfei.com.


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and the defeat of common sense. Successes and lessons learned. Tentative subjects include: some applications of geosynthetics in dams,where the durability of geosynthetics may be better than the durability of traditional materials; geotextilefilters as a design success for geosynthetic engineering that could transfer technology toward geotechnicalengineering. This will end the lecture on a Terzaghian note.

Lara Peggs is the content manager for geosynthetica.net.

Are you qualified?In February, the American Society of Civil Engineers (ASCE) raised the question of whether a bachelorsdegree in civil engineering was enough, given the scope of infrastructure that civil engineers are responsiblefor (e.g., urban transportation systems). Since then a number of field publications and society newslettershave given space to the discussion. Of course, it is unlikely that a 23- or 36- or 52-year-old engineer recently graduated from an under-graduate program or having just qualified for recognition as a professional engineer would easily win a

job designing a subway system or remediating a brownfield. Years of dues (both financial and figurative)must be paid before the major jobs may realistically be competed for. Still, the question is a valid one. Though we rarely see the sort of development that transforms entire societiesthe standard exampleis the invention (and mass availability) of the televisionwe have reached a point of technological researchand development where a tremendous number of products, services and techniques are available. It issaid that our cars lose half their value the moment we drive them off the lot, but many of our computersreach a near zero value within 18 months, especially when we need to upgrade our computers to accom-modate increasingly vital design and management programs. Its easy to see this conflict of choice in electronic technologies, and weve grown used to it. But thisconflict is played out in many other areas, including engineering. And this is where extended education

begins to play a much deeper role. Consider the myriad concerns of standard construction practices: tightbid competition; rising raw material costs that in turn alter the as-built cost; labor, materials and insurance;etc. (Insurance alone is worth deep consideration.) The rapid proliferation of choices threatens confidencein any design.Is research losing its value? One of the real struggles presented by new technologies is that the faster they appear, the less likelylong-term studies will be taken up. Road construction is a segment especially vulnerable to this. A trans-portation department might embark on a five- or seven-year study on the performance of various asphaltmixes within that departments primary area of responsibility, but a few advances in mix and applicationtechnologies in the studys first year or two could render the entire project moot. Globally, roadway studiesare being abandoned because new choices of asphalt mixes are available. This comes at the disadvantageof what we might have learned from the performance of the old mixes, or about the other factors involvedin roadway performance: drainage control, reinforcement, frequency of maintenance. Certainly, new technologies are being built upon previous advances, but were seeing change come atan alarming ratealarming because it encourages complacency or outright resistance. That is, we becometoo trusting of what is new, or we become too hesitant to investigate a newer option. Education can provide a valuable buffer here. A deeper understanding of the primary design consid-erations can deliver the proper perspective, an essential mix of questioning and analysis.


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Polymers in the field Polymeric construction products have advanced quickly in the last few decades, as have polymericmaterials in most fields. Composites, plastics, fabrics in engineering: The world is no longer just concrete,wood and steel. We are more efficient and economical, but we certainly have not simplified things. Trueenough, the incorporation of fibers (e.g., carbon) into materials means that even concrete, wood and steel

are different than they were a decade ago. Specialization is a must to gain real confidence in where ourprojects are going. For polymeric products, new uses of resin formulations and manufacturing advances are leading toabrupt leaps in the mechanical performance values of materials. And though one strength test value mayout-muscle our instinct to fully consider other material choices, we must. Education is an insurance policy. A deeper level of understanding of the forces acting upon our de-signse.g., hydrostatic, seismic, creepis vital, as is the ability to understand how our select materialsrespond. Not every engineer needs a doctorate, but we must continue to gain knowledge, refine our practice,and collect resources to assist us in selecting the right materials. And where we are not expert, we mustseek the proper project agents to verify our decisions. Technology is a difficult thing to keep pace with, but having the right background or the correct peopleon call goes a long way towards minimizing the confusion caused by progress.

Five for oneGFRs new online edition gives subscribers the print edition, unlimited access to the online archives, andthe ability to share the electronic-half of the subscription with four colleagues. This will help keep smalloffices and company divisions up-to-date on the use of polymeric materials in contemporary design andconstruction. Visit www.gfrmagazine.info for subscription information. Writers interested in submitting case studies, research or other articles for simultaneous print and elec-

tronic publication should contact The Editors, GFR Magazine, 1801 County Rd. B W., Roseville, MN 55113-4061; +1 651 225 6988, fax +1 651 225 6966, e-mail [email protected], Web site www.gfrmagazine.info.

D35 meets D18 ASTM Internationals Committee D35 on Geosynthetics held its mid-year meeting in June in Kansas City.It was suggested that members of subcommittee D35.05 (geosynthetic erosion control) be contacted inregards to related activities of Committee D18 on Soil and Rock. Specifically, the request was made inregards to D18.25 (erosion and sediment control). ASTM Internationals committees invite participation from the field. They use consensus to revise oldor write new standards. Committees with cross-over interests and memberships are encouraged to shareinformation and ideas, but it must be noted that in order to receive applicable committee ballot items, eachparticipant must apply for the right subcommittee memberships. GFR readers are encouraged to look into ASTM committee memberships and opportunities. For information regarding Committee D35 on Geosynthetics, contact Christi Sierk, staff manager, at+1 610 832 9728, e-mail [email protected]. For information regarding Committee D18 on Soil and Rock, contact Bob Morgan, staff manager, at+1 610 832 9732, e-mail [email protected]. Both committees may be reached by mail at ASTM International, P.O. Box C700, West Conshohocken,PA 19428-2959. Visit ASTM and its individual committee Web sites online at www.astm.org.


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Big Dig author Dan McNichol selected as guest lecturerOrganizers of Geo-Frontiers 2005 have chosen Dan McNichol as the events guest lecturer. The congresstakes place 2426 January at the Hilton Austin Convention Center Hotel in Austin, Texas. More than 550technical paper abstracts have been submitted for potential presentations. McNichol, author of the best-selling book, The Big Dig, is considered an expert on Bostons Central/

Artery Tunnel Project. In Washington, D.C., he received a White House appointment to serve the thenSecretary of Transportation, Andrew H. Card (who currently serves as President Bushs Chief of Staff).McNichol will speak about his new book, The Roads that Built America: the Incredible Story of the U.S.Interstate System. Geo-Frontiers 2005 is being organized by the Geosynthetic Materials Association (GMA), a divisionof the Industrial Fabrics Association International (IFAI); the Geo-Institute (G-I) of the American Society ofCivil Engineers (ASCE); and the Geosynthetic Research Institute (GRI). Technical papers presented during this broad-based congress, which combines Geosynthetics 2005TM,the G-I Congress and GRI-18, will be presented in the following educational tracks: Earthquake engineering and soil dynamics Erosion control Foundations Geotechnical professional issues Pavements Site characterization Slopes and retaining structures Soil improvement and grouting Waste containment and remediation In addition to technical paper presentations, Geo-Frontiers 2005 will include hands-on workshops,short courses, field demonstrations and tours, and an expansive exhibition floor featuring a full range ofproducts, technologies and services to support your designs.

For more information, contact IFAI Conference Management, 1801 County Rd. B W., Roseville, MN55113-4061; +1 651 222 2508, fax +1 651 631 9334, e-mail [email protected], Web site www.gmanow.com or www.geofrontiers.org.


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The practice: ownershipBy Jean E. Bierwirth, P.E.

Editors introduction: Many (if not most) engineers

give serious thought to establishing an independentconsulting practice. Work within this profession canbe considerably adversarial, usually demands longhours, and ultimately encourages specialization, evenif it is not the sort of specialization one intended whenfirst entering the field. As such, working independentlybecomes quite attractive, even if we are still workinglong, often pressured hours. Engineers and doctorshave similar career arcs in this regard. (True, the simi-larities end quickly. Imagine a doctor showing up towork in a dusty pickup truck, or an engineer visiting alandfill site in a Mercedes and a pair of loafers.) Red Mesa Consulting has operated in GrandJunction, Colo., since 1996. Its owner, Jean Bierwirth,who last wrote for GFR in April 2003 (Surface impoundment rehabilitation), recently made thedecision to take her independent practice one step further, choosing to buy a facility rather thancontinue to rent. For readers considering establishing their own practice, or faced with a similar cost and com-mitment decision, her story is one to learn fromespecially since she was able to use her firmsexpertise to secure an advantageous locations approval.CK

To rent? or own?I hate paying rent! My accountant can debate at length the tax advantages of leasing, but instinctivelyit feels like flushing money down the toilet. So when financial circumstances allowed for real estateinvestment, my firm started scouring the town for office buildings. There were a wide variety of choices: pre-fab metal warehouses, high-rent brick and glass build-ings, and old residential properties converted to office space. The conditions along with the pricesvaried widely. And, inevitably, some sellers were more impressed with their property than seemedappropriate. Unable to locate a suitable and affordable building, buying land and building our ownfacility was explored. Options were discussed, but when it came down to it, trying to find a largeenough parcel with ample parking and negotiating our way through the permitting process provedtoo time consuming and expensive. So back to existing structures and, finally, we found a 12,600

ft.2 facility with a generous fenced-in yard and parking area in lower downtown Grand Junction. Further investigation revealed that the property was located in the Mesa County Economic En-terprise Zone. This meant that a percentage of monies invested in new staff and equipment wouldbe reimbursed through tax credits, an unexpected bonus. Our financing was provided in part by the U.S. Small Business Administration (SBA). Their re-quirements included a Phase I Environmental Site Assessment, which Red Mesa conducts routinely.However, because of potential for conflict, we had to hire an outside firm to perform the assessment.Low and behold, there are 11 leaking underground storage tanks (LUSTs) listed in the regulatory

Red Mesas new building in downtown GrandJunction, Colo.


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database for the area surrounding our building. Perhaps someone less familiar with environmentalassessments would have been concerned, but after reviewing the specific entries, estimating ground-water flow direction, and speaking with the tank owners, we were able to show the lenders that theLUSTs were of minimal concern. After all, this is a well-developed area of town and zoned industrial;

it would have been surprising not to find regulatory entries. We closed on the property last June and moved in September after spending approximately$120,000 on remodeling and bringing the building up to fire codes. Last fall, the voters of GrandJunction approved a $75 million road project that includes a parkway south and east of our prop-ertyfurther enhancing visibility and access. Not only did we pick an area of growth, but to sweetenour pot, we have located a major corporation as a long-term tenant to offset some of the costs. The purchase has given Red Mesa a new sense of independence and prosperity. This changecan be made by anyone looking to invest and build equity. Here are some things that we took into consideration when purchasing our office building: Location and zoning. That is, where are the areas of greatest growth and appreciation, and whatzoning offers the greatest flexibility. Always keep your eye on return on investment should you decideto sell. Tax breaks. Are there areas in your town that provide economic incentive to development or remod-eling? Establishing your own equity. Why would you pay someone elses mortgage if you dont have to? Will you be able to rent portions to reduce overhead? Be warned; it often takes months to find ten-ants, but under the right circumstances, you can offset the mortgage and reduce remodeling costsby requiring that your new neighbor pay for tenant finish. There are risks if real estate is not appreciating where you work, if the cost of unexpected repairsis onerous, or if your income stream suddenly decreases, but the upside is the pride of ownershipand the feeling of building for the future.

Red Mesa Consulting Inc., www.redmesa.net, provides civil and environmental engineering ser-vices to clients throughout the Southwest. See Jean Bierwirths April 2003 Designers Forum article on a surface impoundment rebuild projectat www.gfrmagazine.info.


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By Robert M. Koerner, P.E.,director of GSI

By way of the Geosynthetic Institutes (GSI) Geosynthetic Research Institute (GRI), we have been writingstandards (which include test methods, guides and specifications) since 1986. From the very beginningour concept was to provide starter-standards for ASTM International and, more recently, the InternationalStandards Organization (ISO). Whenever these groups formalize a standard on the same topic, we depre-ciate ours so different procedures that could potentially cause confusion are not available. An accountingof GSIs activities to date in this regard is noted in Table 1. Once a need is expressed for a particular standard, we work within the GSI focus group that is mostinvolved in the potential standard. When a draft is ready and has general or majority agreement (not

necessarily a consensus), we send it out to the entire membership for their review and comment. Some-times it goes back to the focus group, but generally it does not, and it is adopted with minor changes.For example, the Hanging bag test for geotextile tubes and containers has just been adopted. It had itsbeginning in December 2003 and took six months to finalize. Conversely, some standards never seem toget finalized. In general, specifications are the most difficult, and guides are the quickest. Test methodsfall in between. For the at-large field of geosynthetics, GRI specifications and guides are openly available on our Website. These are free for anyone to download, and the version on the Web site is always the most recentmodification. Also, test methods (along with the specifications and guides) are bound in a 250+ pagepublished book form. GSI charges $100 plus shipping and handling. We plan to continue with this activity in the hope that it serves not only GSI members, but the entire

geosynthetics industry. For more information, contact Geosynthetic Institute, 475 Kedron Ave., Folsom, PA 19033; +1 610 5228440, fax +1 610 522 8441, e-mail [email protected], Web site www.geosynthetic-institute.org.

The role of GRI standards

Geosyntheticcategory

GRI Standards Depreciatedto ASTM or ISOTest methods Guides Specifications

Geotextiles 9 3 2 5Geogrids 8 3 0 3Geonets 1 0 0 1Geomembranes 12 5 6 6GCLs 2 0 0 2Geocomposites 8 0 0 1Multipurpose 8 2 0 4

Table 1 . A breakdown of standards (test methods, guides and specifications) created by GRI since1986. As other organizations create their own acceptable standards, GRI retires its standards.


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Part 1: roadway applications

During the summer of 1977, I visited theCorps of Engineers Waterways Experi-ment Station at Vicksburg to discussresearch that I was performing on theuse of geotextiles to improve the per-formance of track support structures.Both the heat and humidity of a Mis-sissippi August day and the Corps in-novative research on the use of whatwould become geocells left a lastingimpression in my memory. Contained inthe largest Quonset hut that I had everseen was a roadway test sectionused for full-scale tests of alterna-tive rapid deployment military roadsfor challenging weak subgrades.The search for a modern alterna-tive to the steel mats made famousin both fronts during World War IIwas in full stride. The specific topicwas focused on improving tacticalbridge approach roads across softground (Webster 1977, 1979), butthe fundamental nature of the research was self-evident even then. A young (we all were) Steve Webster had the luxury in this test site to create a variety of challengingsubgrades, construct a full-scale alternative military road, and then run cycles of actual military equipmentover the road. As a young academic, I was flush with the thought of what optional research use I couldfind for an Army tank in North Carolina. Dove hunting took on a new meaning. The geocell test section Iwatched under construction was formed of thousands of short corrugated plastic pipe sections standing

vertical on the native ground or a geotextile, see Photo 1 . The pipe sections were mechanically attachedtogether and then filled with sand. Given only a thin sand surfacing to bury the plastic pipes, the perfor-mance of the roadway under heavy traffic loading was amazing. The ability of the geocells to limit roadwaydisplacements far exceeded the simple separator geotextiles that I had been investigating.

In the years since that steamy day in Vicksburg, I have been disappointed with the near absenceof research papers on geocells in all of the geosynthetic national and international conferences. EvenKoerner allocates only a handfull of pages in his textbook to this most interesting geo topic (Koerner1996). With applications of geocells now encompassing roadway reinforcement, erosion control, retainingwalls, and even emergency flood walls, a more applied review of geocells is overdue. This two-part series

Geocells: a 25-year perspective

Commercial geocell Flexural stiffness, EI (lb-in2)GeoProducts 3 Smooth 65,255GeoProducts 4 Smooth 58,003GeoProducts 4 Smooth 43,117

Presto 3 Textured 23,976Presto 4 Textured & Perforated 34,096

Table 1 . Geocell flexibility.

By Gregory N. Richardson, Ph.D., P.E.

Photo 1 . Corps of Engineers geocell test section, Vicksburg, 1977.


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will focus on the basic theory behindgeocell performance and its role in theinitial application to roadways over poorsubgrades. Part Two of this series willextend the application of geocells to

erosion control and retaining walls. Inall applications, an attempt is made toclearly identify the important physicalproperties of both the geocell manu-factured component and the granularmaterial used to fill the cells.

Todays geocell productWhile Webster explored a wide rangeof potential geocell mats, todays com-mercial products are almost exclusivelyformed of 50-mil thick high-density poly-ethylene (HDPE) strips factory welded to form panels having a honeycomb structure. The panels areshipped collapsed but are quickly expanded and staked in place ( Photo 2 ). The individual cells of thegeocell panels are then filled with gravel or sand. The use of cohesive fills is physically impractical due tothe inability to compact such soils in the small cells and lack of physical benefits for such soils. The indi-vidual cells have a height to diameter ratio in roadway applications of approximately one. The HDPE geocells are available with the plastic sheets smooth or textured and with or without per-forations. The role of these options is discussed in the technical discussions in this series. Additionally,a geotextile separator is typically placed beneath the geocell honeycomb on clayey subgrade to preventpumping of subgrade fines into the geocell granular fill.

The role of confinement for granular subgradesWebsters early research showed thatthe geocells provided an effective con-finement of their contents when theheight of the cell was equal to or greaterthan the diameter of the cell. This con-finement may be thought of as similar tothat provided by the bag in conventionalsandbags. As load is applied to the con-fined granular material, its expansionperpendicular to the load is limited bythe tensile strength of the bag. This cre-ates a confining stress that increasesthe strength of the granular fill. Thiseffect is shown in Figure 1 using theMohrs circle model (Hausemann 1976)that most civil engineers are familiarwith. Here the stress r is the lateral

Photo 2 . Todays geocell mats expand to form honeycombs.

Figure 1 . Composite Mohr envelope; reinforced earth.


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stress taken by the reinforcement. This reduces the horizontal stress in the granular fill and producesthe apparent cohesion. The lateral confinement of the geocell produces pseudo-cohesion strength in thegranular fill that is critical to its performance. The amount of pseudo-cohesion developed is influenced bythe stiffness of the geocell walls and the ability of the geocell to contain the granular fill. This pseudo-cohe-sion model has been successfully applied to all forms of soil reinforcement.

The role of the pseudo-cohesion in roadway applications can be clearly demonstrated using conven-tional bearing capacity analysis. The bearing capacity, Q ult', of a soil having both cohesive and frictionalstrength subjected to a uniform circular loading (e.g., tire load) is given as follows:

Where c is the cohesion of the soil, is the unit weight of the soil, R is the radius of the load, and N c andN are bearing capacity factors that are a function of the frictional strength of the subgrade. The first halfof the equation represents the bearing capacity due to cohesion; the second half represents the bearingcapacity due to the frictional strength of the subgrade.

The apparent cohesion from one commercial brand of geocells formed of 50 mil polyethylene is re-ported to be 3,000 psf (Presto 2003). Assuming conservative physical properties for the granular fill of =100 psf and a friction angle of 30 , the bearing capacity for these 50 mil HDPE walled geocells subjectedto a vehicle tire load would be approximately

This simple example shows thatcommercial geocells would provide a300-fold increase in the bearing capac-ity as compared to the layer of sand.This assumes that the thickness of thereinforced granular layer is greater thanthe radius of the applied wheel load.

Lacking a rigorous method for evalu-ation of the bearing capacity of a specificgeocell and granular fill, the pseudo-co-hesion must be based on laboratorytesting. A design engineer must havea feel for the relative impact of geocellproperties on actual performance.

Geocell rigidity for weak claysubgrade (CBR


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honeycomb structure of the geocell tospread out concentrated wheel loads.

An excellent laboratory study that quali-tatively examined this mechanism wasperformed by Richard Bathurst and

Peter Jarrett at the Royal Military Col-lege of Canada in the 1980s (Bathurst1981). Figure 2 shows the deformationof the center of a various geocell/gravelsystems constructed over one meterof compressible peat. The Presto Ge-oWeb in Figure 2 is similar to thatshown in Photo 2 . The geocellular sys-tem in Figure 2 is actually more of aflexible honeycomb system formed fromstrips of geogrid joined with steel rodbodkin connections. Laboratory datashows that the geocell formed of the welded HDPE strips had significantly less deformation that the geocellformed of geogrids or the unreinforced gravel. This study qualitatively shows that the stiffness of the plastichoneycomb system is very important but does not provide guidance on quantifying this role.

The role of the stiffness of the plastic honeycomb system should be remembered when selecting geocellmats. Many of the available commercial geocell mats have a significant percentage of the side walls of thecells removed by perforation of the strips. These perforations greatly reduce the stiffness of the honeycombstructure. To illustrate this increased flexibility, beams of the flat as-shipped geocells were subjected toa central point load. Table 1 shows the resulting decrease in the flexural stiffness of the geocell beams.For roadway applications, I see no advantage in using perforated or textured geocells, since the flexibilityof the honeycomb is highly compromised. The drainage offered by the perforations is simply not neededin roadway applications. An interesting analytical approach to bearing capacity based on cell rigidity estimates the amount ofvertical stress that is transferred to the honeycomb cell structure. The load transferred to the cell structureis assumed to transfer laterally such that it does not influence the local bearing capacity. This approachrequires testing such as performed by Barthurst and Jarrett to confirm sufficient rigidity in the cell struc-ture to actually support the loading being transferred to it. The vertical stress at the top and bottom of anindividual cell can be estimated for uniformly loaded line and circular loads centered over a single geocellusing the following equations (Presto 2003):

Line load

Circular load

Figure 3 . Variables for the calculation of vertical stress in subgrade.


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The variables used in these equa-tions are shown on Figure 4 . The lineload would be appropriate for tank/dozertracks, while the circular load is good forrubber tires. Note also that these elastic

equations do not account for the pres-ence of the geocell honeycomb and aretherefore quite approximate. Having calculated the verticalstresses, z, acting on a geocell, thehorizontal stresses, h, are simply takenas z times the active earth pressurecoefficient, K a . The active earth pres-sure coefficient is related to the internalfriction angle, ,of the granular fill asfollows:

Figure 4 shows the assumed stressconditions acting on the single cellbeing evaluated. The average horizon-tal stress acting on the inside of the cell,havg is assumed to be simply the aver-age of the top and bottom horizontalstresses. The load in the geocell being trans-ferred to the geocell honeycomb sys-tem, F transfer , is simply the one half theaverage horizontal stress times the interior surface area of a cell times the tangent of the interface friction,, between the granular fill and the interior walls of the geocell. This can be expressed as follows:

Note that the 1 2 factor conservatively accounts for the nonlinear distribution of the vertical stress withdepth. This force is assumed to be transferred laterally by the geocell honeycomb and is subtracted fromthe vertical stress previously calculated at the base of the geocell, (sz)base, as follows:

The allowable stress on the geotextile underlying the geocell is assumed to equal 2.8 times the co-hesive strength, c, of the subgrade. This is based on early recommendations by the U.S. Forest Service(Steward 1977) for geotextile stabilized haul roads designed for high traffic counts and light rutting. If thecorrected vertical stress acting on the geotextile, ( z)corrected, is greater than 2.8c, then the thickness ofthe granular cover over the geogrid must be increased and the evaluation repeated.

Figure 4 . Assumed stress conditions acting on a single cell.


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The force being transferred to the geocell system is dependent on the interface friction of the granularfill to the HDPE walls of the geocell. This is commonly referenced in terms of the ratio, r, of the interfacefriction angle to the internal friction angle of the granular fill. Typical published values for this interface fric-tion ratio are compared in Table 2 to those obtained specifically for this article. The performance of thegeocells formed of textured HDPE sheet and those having perforations are essentially identical for mostgranular materials. Ironically, the perforations were introduced to allow drainage of geocell mats used onslopes for erosion control and not to improve bearing capacity applications. For typical bearing capacityapplications, the perforations are not required for drainage and detrimentally impact the stiffness of thegeocell. This will be discussed later. As an example calculation, lets look at the problem shown on Figure 5 . The vertical stress on the topof the geocell can be calculated as

In a similar manner, the vertical stress on the bottom of the geocell is calculated to be 16.7 psi. Theaverage horizontal stress acting on the geocell, havg , is then calculated to be 9.7 psi. The total shear force

transferred to the geocell, F transfer , is equal to

The resulting vertical stress acting on the subgrade, (sz)corrected, is then calculated to be

1 + (4.6/4 )2

Granular Material Geocell Wall Published* r = d/f Measured** r = d/f

Coarse Sand/Gravel Smooth 0.71 0.71Textured 0.88 0.83Smooth-Perforated - 0.85Textured -Perforated 0.90 0.89

#40 Silica Sand Smooth 0.78 0.68Textured 0.90 0.87Smooth-Perforated - 0.87Textured -Perforated 0.90 0.93

Crushed Stone Smooth 0.72Textured 0.72Smooth-Perforated -Textured -Perforated 0.83

* = Hausemann 1976. ** = Steward 1977.

Table 2 . Peak interface friction angle ratio, r.


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Given the allowable contact stressof 2.8 x 2.1 psi = 5.9 psi, the geocelldesign meets bearing capacity require-ments.

Note that this design method is ap-

proximate and assumes that the struc-ture of the geocell honeycomb is ad-equate to transfer the load laterally.Obviously this may not be true if thethickness of the plastic forming the cellsis significantly reduced, if the weldsholding the cells together are inade-quate, or if the modulus of the plastic issignificantly reduced. Since no calcula-tions are performed for this honeycombstiffness, the designer must cautiouslyaccept manufacturers recommenda-tions. For bearing capacity applications,I would recommend avoiding geocellsformed with perforations or texturing.

Geocell specificationsPersonally, I find evaluating specifications for commercial geocell products more difficult than reviewingthe design concepts. Fortunately, all the commercial geocell products that the author reviewed are underlicense to the Army Corps of Engineers and shared many fundamental properties. Based on the designconcepts previously reviewed, it is apparent that a geocell system for roadway applications must provide

the following: A system stiffness that ensures applied loads are distributed laterally by the honeycomb structure Adequate frictional bond to the enclosed granular fill to ensure that either confinement or load transferoccurs Sufficient robustness that the HDPE used to form the geocell will survive both installation and service As with all commercial products, the designer should understand the applicability of each specifica-tion requirement to the function being required of the product. The following discussion relates only to theroadway application of geocells.

System stiffness requirementsThe work by Bathurst and Jarrett showed that geocells of the proper size and formed of welded smooth

50 mil thick HDPE sheet could provide adequate stiffness to distribute applied loads laterally. The geocellsformed of geogrids did not perform as well. Four factors can have significant influence on the stiffnessof the geocell honeycomb: weld strength, height-to-diameter ratio of the individual cells, panel thickness,and perforations to the sheet. Weld strength requirements for geocells are based on Corps of Engineersresearch (U.S. Army Corps) as summarized on Table 3 . All commercial geocell products reviewed ap-peared to use and meet these requirements.

For roadway applications, the depth-to-diameter ratio of the individual cell should be approximatelyone. Commercial geocells are available in depths of 75 mm (3 in.), 100 mm (4 in.), 150 mm (6 in.), and 200

Figure 5 . Peak interface friction angle ration, r.


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m (8 in.) with nominal areas of 289 cm 2 (44.8 in. 2), 460 mm 2 (71.3 in. 2) and 1.2 m 2 (187 in. 2). The depth-to-diameter ratio criteria would suggest that cost effective reinforcement would be obtained with 150 mmhigh cells having a nominal area of 289 cm 2 (H/D = 0.80) or 200 mm high cells having a nominal area of460 mm 2 (H/D = 0.82).

The thickness of the HDPE forming the geocells is nominally 50 3 mil. A factor that I have not seen

addressed by direct research is the impact of texturing on this thickness. Typically we think of texturingas roughness added to the surface of a geomembrane so that the nominal thickness actually increases.However, the texturing in geocells is embossed into the sheet and results in a significant percentage of thesheet having a nominal thickness of only 31 mils by my measurement. Bending tests presented in Table 3indicated that the embossed texturing significantly reduces the stiffness of the geocell honeycomb. Perforations are made in the strips forming geocells to allow lateral drainage when the system is in-stalled on a slope or to improve the interface friction between the HDPE and granular fill. For roadwayapplications, lateral drainage is typically not a concern. Since excessive perforations could significantlyreduce the stiffness of the geocell system (recall the poor performance of the geogrid formed geocells inthe tests by Bathurst and Jarrett), the perforations should be only enough to improve the interface frictionand not so many as to reduce the rigidity of the honeycomb system.

Frictional bond Table 3 shows that the interface friction ratio (interface friction/internal friction angle of fill) between thegeocell wall and the granular fill ranges from 0.71 for smooth to 0.9 for textured/perforated HDPE geocells.With the exception of crushed stone fill, the combination of texturing and perforations does not have merit.For smooth sheet, a minimal number of perforations dramatically increases the interface friction ratio. Thenumber of perforations however must be limited to retain the strength of the geocell side wall.

Strenth Property Geocell Depth Seam Peel StrengthShort-term Seam Strength 75 mm (3 in)

100 mm (4in)150 mm (6in)200 mm (8in)

1,060 N (240 lbf)1,420 N (320 lbf)2,130 N (480 lbf)2,840 N (640 lbf)

Long-term Seam Strength 'Seam Hang Strength": a 100 mm welded joint must support a load of 72.5kg (160 lbs) for 30 days minimum or a load of 72.5 kg (160 lbs) for 7 daysminimum undergoing a temperature change from 23 C (74.5 F) ro 54 C(130 F) on 1 hour cycles.

Table 3 . Minimum geocell seam strength requirements.

Material Property Test Method Geogrid GRI-GM13Minimum Polymer Density ASTM D1505 0.940 g/cm 3 0.940 g/cm 3

Minimum Carbon Black Content ASTM D 1603 1.5% 2.0-3.0%Environmental Stress Crack Resistance ASTM D 1693 4,000 hr 200 hr

Table 4 . Survivability physical properties of HDPE Geocells.


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Survivability The key physical properties of the HDPE specified for commercial geocells are shown on Table 4 withsimilar properties for HDPE liner material. Note that the geocell HDPE has greater stress crack resistance,which is needed since stone may be compacted direct on the HDPE in the geocell. Also, the liner HDPE

has greater UV protection since it maybe exposed for extended periods of time in some applications. Con-versely, the geocell HDPE is typically exposed to UV for only a short period of time during construction.

SummaryGeocells provide the most dramatic geo improvement in bearing capacity possible. For either sandy orweak clay subgrade, the performance is outstanding. Some 6.4 million ft. 2 (595,000 m 2) of geocells wereused in the first Desert Storm to ensure mobility of the U.S. Army. Fortunately, they work as well in peace-time. This article has focused on the use of geocells in unpaved haul roads. Design procedures have alsobeen developed by the Corps of Engineers to incorporate geocell systems in paved roadways. In such

applications, the geocell honeycomb structure leads to a significant increase in the structural number, SN,of that layer. The readers are directed to the web sites of the manufacturers for guidelines on installationof geocells in roadway applications. Rob Swan of SGI Testing Services performed the interface friction and bending tests of geocell productsfor the new data presented in this article. His ability to work out of the box is greatly appreciated.

Part Two of this series will examine the use of geocell systems in erosion control applications and inthe construction of retaining walls. These out-of-the-box applications of geocells are now a principal useof these systems.

ReferencesBathurst, R.J. and Jarrett, P.M. 1981. Large-scale model tests of geocomposite mattresses over peat

subgrades. Transportation Research Record 1188. Transportation Research Board, Washington,D.C.

Hausemann, M.R. 1976. Strength of Reinforced Soil. Proceedings of 8th Australian Road ResearchConference, vol.8.

Koerner, R.M. 1996. Designing with Geosynthetics. Prentice Hall, Engelwood Cliffs, N.J.Presto Products Co. 2003. The Geoweb. Load Support System Technical Overview. Presto Products

Company, Appleton, Wis.Steward, J.E., Williamson, R. and Mohney, J. 1977. Guidelines for use of Fabrics in Construction and

Maintenance of Low-Volume Roads. USDA Forest Service, Portland, Ore.US Army Corps of Engineers. Tech Report GL-86-19.Webster, S.L. and Watkins, J.E. 1977. Investigation of Construction Techniques for Tactical Bridge Ap proach Roads Across Soft Ground, Report S-77-1. Soils and Pavements Laboratory, U.S. Army Wa terways Experiment Station, Vicksburg, Miss.Webster, S.L. 1979. Investigation of Construction Concepts Across Soft Ground, Report S-79-20. Geo technical Laboratory, U.S. Army Waterways Experiment Station, Vicksburg, Miss.

Greg Richardson is president of G.N. Richardson & Associates, Raleigh, N.C.; www.gnra.com.


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The design of landfill liner systems authorized under NewYork States solid waste management regulations, 6 NYCRRPart 360 (Part 360), has evolved systematically over timeas design engineers have sought improved liner systemperformance and optimized disposal capacity. Actual linerperformance data from previous design improvements isgrowing and substantiating the merit of these subtle varia-tions above and beyond the minimum prescribed regulatoryliner system requirements. The design of the Section IV, Cell

1 and 2 expansion of the Broome County, N.Y. landfill is aprime example of this ongoing design evolution. The designwas a first in Upstate New York for both its liner cross-sectionand the approach to documenting its performance.

Liner design evolutionThe standard liner cross-section required by the Part 360regulations, including a pore pressure relief system (PPRS)and frost protection (a necessity in central New York State ifwaste cannot cover the liner system before winter sets in), isgenerically depicted in Figure 1a . This section utilizes onlytwo geosynthetic components: a geomembrane in both theprimary and secondary composite liner systems. The totalthickness of the standard prescribed liner system is 9.5 ft.(290 cm) with the underlying pore pressure relief and frostprotection layers placed above the liner system. With design engineers seeking means to optimize land-fill capacity and construction efficiency, this section quicklyevolved into the typical section shown in Figure 1b . Thissection utilizes a geosynthetic clay liner (GCL) for the upper 6in. (150 mm) of low permeability soil in the primary composite

liner system, a drainage composite (DC) for 12 in. (300 mm) of drainage material in the secondary leach-ate collection and removal system (SLCRS), and a similar drainage composite for the drainage materialin the pore pressure relief system. This section reduces the liner section by 2.5 ft. (750 mm), resulting inincreased airspace, which equals increased revenue for the project owner and decreased constructiontimean important consideration in central New York where it can snow from October to May. Since the12 in. (300-mm) structural fill layer required by the regulations has no specific permeability requirementsin the regulations, designers typically specify a relatively high permeability material for this layer to supple-ment the flow capacity of the SLCRS. This section evolved based on the proven performance of properly

A case history of the Broome County Landfills pioneering expansion

The evolution of a better landfill liner system

Photo 1 . The Broome County landfill has beena seminal project in New York State's evolvingunderstanding of liner systems.

Photo 2 . Installation of the secondarygeomemrane (foreground) and drainagematerial (background).


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designed GCLs and drainage composites used in specific components of landfill liners. For the Broome County Landfill Expansion project, the owner, engineer and regulatory agency workedclosely together to conceive, permit and construct the next evolution of landfill liner sections. As shown inFigure 1c , this section eliminated the 12 in. (300 mm) of structural fill, eliminated 6 in. (150 mm) of mate-rial in the primary drainage layer, and added 12 in. (300 mm) of waste tire derived aggregate on the topof the section to meet both hydraulic flow capacity and frost protection requirements. This section further

Figure 1 . Evolution of a lining system.


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reduces overall thickness by 2.5 ft. (750 mm), addsmore airspace, and streamlines construction. Thecounty was very interested in streamlining con-struction, since the project required the excava-tion of over 900,000 yd.3 (700,000 m3) of material

prior to beginning construction of the liner system.More importantly in the eyes of the design engi-neer, it eliminated the need to place soil materialin the middle of the liner system, except for pipingtrenches and bedding material required for acces-sibility. After installation of the lower two feet of lowpermeability material, the liner section is comprisedsolely of geosynthetics until the 18-in. (450-mm)primary drainage layer. This evolution in the liner section was proposedbased on several premises. Reduced need for structural fill layer. The primarypurpose of the structural fill layer in the standardsection was to provide a foundation for compactionof the low permeability soil above. Once the lowpermeability soil was replaced by a GCL, there wasless need for the structural fill component, exceptto provide additional flow capacity for the SLCRSand as a physical separation between the primaryand secondary liner systems. Performance of typical liner section. The doublecomposite liner sections currently in service atlandfills throughout New York State are performing well, with the mean SLCRS flow rate below 8 gallonsper acre per day (gpad) [80 liters per hectare per day (lphd)] and rates as low as 0.6 gpad (6 lphd). It wasanticipated that by limiting the liner components that could contribute water to SLCRS, the Broome Countydesign will demonstrate even better performance as we move toward the theoretical deminimus goal of 1gpad based only on vapor transmission through the geomembrane. Rapid attainment of primary liner leakage rate (ALR) data. During design of the expansion, time wasof the essence, as the current capacity was being consumed. By eliminating the soil components in themiddle of the liner section, the true performance of the primary liner system should be determined morequickly. Further, if the primary liner performance data did not meet the regulatory limit of 20 gpad (200lphd), there would be no debate as to whether it was due to construction waterthe initial reasoning

that engineers typically use when they dont want to admit that everything may not have gone perfectlyduring construction. Occurrence of liner defects. Literature shows that a vast majority (97%) of the defects identified in geo-membranes occur during construction of the landfill. The majority (73%) of those construction relateddefects are caused by placing of the soil drainage layer above the geomembrane. Literature summarizingthe findings of electrical leak location surveys indicates that landfill liner construction will typically resultin as many as 69 liner defects per acre. It was assumed that by properly designing the liner system andeliminating the need for heavy construction equipment during installation of the middle soil portion of theliner section, a significant percentage of those defects could be eliminated.

Figure 2 . A point of interest: The few defects found in theliner seemed to have been clustered.


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Performance of waste tire derived aggregate. There is agrowing body of literature that documents the effective per-formance of waste tire derived aggregate both as a drainagematerial and as an insulating layer. New York States solidwaste management regulations allow for substitution of

waste tire derived aggregate in a landfills primary leachatecollection and removal system, contingent upon the properengineering design and specification of tire derived aggregateequivalent design applications. The merits of such designshave demonstrated that tire derived aggregate applications: - Meet and exceed the minimum regulatory required hy-draulic permeability needs of the landfills primary leachatecollection and removal system - Provide 78 times the thermal protection as a conven-tional soil aggregate, greatly improving upon the frost pro-tectiveness of the primary leachate collection and removalsystem - Provide enhanced armoring protection and visual iden-tification properties of the upper layer of the landfills primaryleachate collection and removal layer - Help minimize transportation related impacts and costs,since waste tire derived aggregate is much lighter then con-ventional soil aggregate These equivalent design proposals are consistent withNew York States recently enacted Waste Tire Managementand Recycling Act of 2003. This legislation was enacted tohelp ensure the proper management of waste tires in NewYork State. The use of waste tire derived aggregate in landfillleachate collection and removal systems is considered anenvironmentally acceptable beneficial use as a civil engi-neering application and is not considered disposal under theprovisions of the act. However, that could be another case history, as this article focuses on the liner system prior to placingthe tire shreds.

Design and construction challengesEven though the engineer could cite literature for performance data and make logical extrapolations in

support of the proposed liner section, the regulatory agency expressed concerns with three primary issuesrelated to eliminating the structural fill layer that could not be entirely alleviated with equations and logic: Double damage. By eliminating the structural fill layer, the potential to compromise both composite lin-ers with one error during construction or operation of the cell increased. Factors that were considered inapproving the proposed liner section included:

- The fact that the section included a PPRS allowed another layer of monitoring that could be imple-mented in the event that both liner systems may have been damaged - The fact that the low permeability of the in situ glacial till will also help minimize potential impacts to

Photo 3 . Cover material was applied ahead ofheavy equipment, thus minimizing the potentialfor damage caused by equipment on the geo-synthetics.

Photo 4 . Eletrical leak location measurespinpointed the first defect: a single piece ofgravel. The flag used to mark the spot did notpenetrate the GCL.


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the environment - The fact that the landfills containment system could also become an intra gradient design, ifnecessary - Emphasis was placed on initial operation of the cell, and the existing landfill operations could be usedto better segregate suitable waste material for use in the initial lift of waste in the new cell Maintaining intimate contact. Without any soil components in the middle of the liner system, the potentialfor wrinkles increases. The exceptional performance of the composite liner systems identified earlier ispredicated on intimate contact between the geomembrane and the underlying low permeability soil barrier

(or equivalent) component. This contact minimizes the potential for leachate that breaches the geomem-brane to spread laterally to find a preferential flow path through the soil barrier. A wrinkle in a lower layermay also increase the potential for and/or magnitude of wrinkles in subsequent components. Intermedi-ate soil layers provide a nominal normal stress (weight) that both anchors the geosynthetics in place andinsulates them from temperature changes. The creation of wrinkles is exacerbated during the day whenthe sun is out and when wide daily temperature swings occur. The fact that its rarely sunny for extendedperiods in central New York was not a sufficient argument for regulatory team players. Ultimately, whilethis issue was not solved during design, it was a focus during construction, and the general contractorimplemented means and methods during construction that greatly minimized wrinkles.

Figure 3 . Secondary LCRS flow data.


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SLCRS flow capacity. Even though the required flow capacity in the SLCRS can be evaluated and veri-fied via material testing, the need for additional flow capacity at critical locations (low points) remaineda concern. The grading of the base of the 12 acre (4.85 ha) cell was partially controlled by bedrock anddirected all flow to an outer corner of the cell. Over the lowest acre of the base of the cell, an additionallayer of drainage composite was added to help address this regulatory concern.

The final step in gaining approval of the proposed liner section was to evaluate the integrity of theprimary liner system as a part of construction by specifying electrical leak location testing of the primarygeomembrane as part of the construction quality assurance (CQA) process. As one of the co-authors ofthis paper has expressed in his regulatory presentations: You will probably need to perform a leak locationtest on your liner installation sooner or later, so we elected to do it sooner. This was the first applicationof electrical leak location testing as part of the upfront standard CQA process in New York State, and anapproach that would allow the team to rapidly identify potential problems and quickly address them. Be-cause the technology was relatively new at the time, the electrical leak location testing was kept part of theengineers contract, and proposals were sought from qualified, recognized providers of the technology. Acorresponding request to increase the spacing (reduce the frequency) of destructive sampling to greaterthan 500 ft. (150 m) was not accepted. However, if the CQA data demonstrated acceptable performance,

a variance to the destructive testing criteria would be considered for construction of future cells.

Team approachIn order to achieve the results that we anticipated on this project, everyone on the team needed to under-stand the overall goals of the project, the pros and cons of the design, and those aspects that were criticalto construction, such as wrinkle management. This would involve the owner, regulatory agency, engineer,general contractor, geosynthetic installer and CQA subconsultant.The entire team accepted the challenge and was willing to demonstrate the quality that each member,individually, knew they were capable of. The teamwork process began during the pre-bid meeting wherepotential bidders were informed of the electrical leak location testing criteria. During the pre-construction

conference, various factors affecting performance of the system were discussed, including CQA/CQC(construction quality control), wrinkle management, and the benefits of having the electrical leak locationsurvey. Because there was a large volume of soil that required excavation prior to beginning constructionof the liner system, a separate geomembrane pre-installation conference was held later to review our goalsand the components of the electrical leak location testing system that needed to be installed during con-struction. To their credit, rather than viewing the electrical leak location survey as questioning their ability,the geosynthetic installer [certified by International Association of Geosynthetic Installers (IAGI)] took theaddition of an electrical leak location survey as an opportunity to prove their skills and support their beliefthat fewer destructive tests should be required for qualified contractors.

Construction After a few months of soil excavation, installation of the PPRS and 24 in. (600 mm) of low permeabilitysoil barrier layer, construction of the critical middle geosynthetic portion of the liner system began. Oneof the most obvious aspects of the construction sequencing was that as soon as a portion of the cell wasdetermined acceptable via the required CQA testing, the general contractor and geosynthetics installerimmediately began installing the next component of the liner system. This is shown in an aerial photographof the site ( Photo 1 ), where: Compaction of the low permeability soil in being completed (lower right portion of the photo) Installation of the secondary HDPE geomembrane is seen (upper right)


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Drainage composite is being placed (lower left) GCL is being placed (central left) Primary geomembrane, cushion geotextile and primarydrainage stone are being placed (upper left) This approach limited the amount of time any single

component of the liner system was exposed to the elements,reducing the potential for development of wrinkles, and ex-pedited the installation of the soil drainage material (a con-fining normal stress). Photo 2 , taken about the same time,reiterates this point and shows installation of the secondarygeomembrane in the foreground and drainage material in thebackground. The general contractor also focused drainage materialinstallation activities during the morning hours before theoccasional glimpse of the sun had an opportunity to createwrinkles. Finally this experienced general contractor em-ployed construction means and methods that are somewhatunusual. Trucks hauling drainage material were confined torelatively small access roads with a sufficient thickness ofdrainage material to protect the underlying geomembrane.This kept the truck drivers who were relatively inexperiencedwith landfill construction away from troublestandard practicein good construction. A low ground pressure (LGP) bulldozer(in the hands of an operator experienced in landfill construc-tion) was then used to push the material to an excavator thatsat on thick area of drainage material. Rather than using thebulldozer to push drainage material (which can encourageand propagate wrinkles), the excavator was used to care-fully place material ahead of itself ( Photo 3 ). Once a givenarea of geotextile was covered and anchored in place, theLGP bulldozer was used to final grade the drainage material.The writers believe that this approach helped minimize thedevelopment of wrinkles in the geosynthetics components inthe middle of the liner section. Another critical consideration is installation of the electrical leak location testing system componentsduring the construction process and providing the proper perimeter conditions (electrical isolation) toimprove the quality of the electrical leak location results. While this step was met with the typical general

contractor grumbling, the benefits were explained, and the contractor provided an excellent set up.

ResultsOne of the goals identified previously was to rapidly establish the primary liner systems performance andidentify potential problems. During the design process this was envisioned as completing the electricalleak location survey as soon as construction of the liner system was completed. In keeping with the moveahead as soon as its ready approach, the general contractor requested that the survey begin immedi-ately after the drainage material was installed in the base of the cell, while material was still being placed

Photo 5 . A few tine holes excaped vacuum boxtesting but were found through visual checks.

Photo 6 . A small scratch, potentially the resultof construction equipment contact, was foundthrough visual examination of the site afterbeing missed by vacuum box testing.


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on the side slopes. Of the 12 acre (4.85ha) landfill footprint, approximately 10 ac.(4 ha) comprising the landfill base weretested. A total of 8 defects were identifiedand located using electrical leak location

testing. This provided an average defectrate of less than 1 defect per acre (2 de-fects/ha), that compares very favorablyto the typical reported values for linerinstallations of 69 defects/acre (1522defects/ha) detected using electrical leaklocation testing. Photo 4 depicts the first defect found(the small orange flag is gently placed inthe defectit does not penetrate the GCL!). Upon opening the cushion geotextile, a single piece of gravelwas found, probably dropped from a workers boot during installation of the cushion geotextile. Photo5 shows the decreasing size of the defects found (in the white circle near the bottom of the photo). It issurmised that a needle in the underlying GCL caused this defect, despite normal manufacturer and fieldscans for needles. Note that vacuum box testing of this area did not identify the defect. A visual inspectionwas used to locate the defect. No needle was found in the cushion geotextile. Defect 7 was at first thoughtto be located in a seam; however, vacuum box testing did not reveal a defect. Upon further examina-tion, a small scratch, most likely caused by installation equipment, was observed ( Photo 6 between thefinger and the white line). Table 1 summarizes the defects found during testing and how they were cre-ated. Defect 4 correctly identifies a rock as the culprit. Although all the drainage stone was screened, thegeneral contractor monitored for oversized material, and the field representative monitored for oversizedmaterial, an 8 x 12 in. (200 x 300 mm) cobble managed to find its way onto the site. Although this stonewas not in direct contact with the liner system, its position must have transferred enough stress from theconstruction equipment to the stone and liner below to cause a small penetration. Finally, the cause ofDefect 8 is unknown. The leak location survey initially identified a signal in this area. After cleaning andvacuum box testing the area, nothing was found. The area was resurveyed, the signal reappeared, andanother investigation ensued. Again, nothing was found, and a large patch was placed over the entire areain question. Assuming the three equipment damage defects are attributed to installation damage, approximately30% of the defects occurred during installation. This is comparable to the 24% value reported in the litera-ture. The literature also reports that approximately 60% of the installation defects are found in extrusionwelded joints and pipe penetrations. One interesting result of the electrical leak location survey was thatnone of the defects identified were located in seams. This dramatically contrasts with the reported data

(and greatly supports the geosynthetic installers position that fewer destructive tests could be taken). Atthe same time, the results of the survey make intuitive sense. If the geomembrane CQA process is imple-mented correctly, defects in the seams should be identified and repaired during geomembrane installation.The only defects the electrical leak location survey finds should be located in those areas that werentspecifically tested previously. It is also interesting to note that when plotted, a majority of the defects are located either in a relativelysmall cluster in the southern portion of the cell or along the high ridge in the cell, which runs diagonallyacross the base ( Figure 2 ). Therefore, these defects would not likely have been detected via the primary

Number / Cause1; Stone below cushion geotextile2: Stone below cushion geotextile3: Equipment scratchespossible staples at end of roll

4: Rock in middle of drainage material5: Pinhole, probably needle from below6:Equipment damage possible dropped pliers or simular 7: equipment scratch, unknown cause8: May be part5ial penetration of needle from below

Table 1 . Number of defects discovered and possible causes.


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liner performance monitoring portion of the construction certification process. A plot of the SCLRS flow (primary liner performance data) data is presented in Figure 3 . Primary liner

performance data is presented in the bottom plot, with daily precipitation shown in the middle plot and asummary of pertinent construction activities in the upper plot. As can be seen in the figure, the liner perfor-mance data prior to the electrical leak location survey was slightly less than 1 gpad (10 lphd). Even though

the cell had eight defects, all were small, and a majority of the defects were located near the ridge in thecell. Both points will tend to reduce the amount of water available to flow through a defect. The primaryliner performance data continues to decrease as the geomembrane repairs were made. The installationcontractor provided a special tape that was placed over the penetrating defects upon location over theweekend prior to making repairs so performance data could begin being collected without lost time. A fewother notes on this data: Drainage material installation continued on side slopes during this period. Two small blips in the primary liner performance data were noted, at two and five weeks after the linerrepairs. Based on the construction schedule, it was determined that these increases were likely in responseto the increased normal and live loads associated with placing tire shreds near the lowest point in the cellexpressing construction water from the drainage composite. The ongoing post-construction primary liner performance data over the first two months was barely 0.1gpad (1 lphd).

ConclusionsSeveral conclusions can be drawn from this project. The Section IV, Cell 1 expansion of the Broome County Landfill was a successful first step in construct-ing a better landfill liner system in New York State. From the modifications to the liner system, to the useof tire shreds for drainage and frost protection, to the use of electrical leak location testing as a CQA tool,each step has been proven to hasten construction and improve overall quality. The increased airspaceand resulting revenues are an added bonus.

The modified design resulted in a rapid determination of an accurate ALR for the cell. By eliminating thesoil components from the middle of the double liner sandwich, the issue of construction water was mostlyremoved from the equation and left primarily leakage due to defects. The geotextile components of thedrainage composite may hold minor amounts of water, but this should be released fairly quickly after theapplication of the drainage stone (if it will be released at all). The GCL may also hold minor amounts ofwater, but the tenacity of the sodium montmorillonite used in these products should continue to hold thatmoisture. Any areas of GCL that become saturated and swell during construction should be removed andreplaced prior to covering with the primary geomembrane. A team approach to construction creates understanding, ownership and pride on behalf of everyoneinvolved in the construction process. Although both the general contractor and the geosynthetics installerwere widely experienced and respected in New York State, they had never worked together on a project.

The fact that their team produced the results seen is a testament to both their abilities and the approach.The question has been raised whether the level of care on the project was heightened because theelectrical leak location testing was included in the construction process. Its a good question and may betrue for most instances. However, based on past projects with the construction team, the primary authorbelieves that the construction team on this project performed as they normally doit just happens to beat a higher level. Electrical leak location testing, as a part of the CQA process, is both valuable and accurate. Despite ateam approach and careful construction practices, things still happen. Electrical leak location testing can


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identify and locate the defects that occur and should be considered for any landfill liner construction proj-ect. Not only will the testing identify defects, but the location of those defects can also give an indicationof the quality of the geomembrane CQA seam testing process. A number of defects in the seams mayraise flags as to the quality of the work and the seam testingboth destructive and non-destructive. When it comes time to construct that next cell, well have to remember to get that variance on the de-

structive testing frequency.

AcknowledgementsThe authors of this article would like to acknowledge Mario Mike Nirchi (project manager for the NewYork State Department of Environmental Conservation [NYSDEC], now retired), Phil Hale and Mike Masur(president and project manager for Marcy Excavation), and Carl Burdick and Chuck Rhoades (ChenangoContracting) for their efforts in making this project a success. Additional thanks from the primary authorto Hale and Masur for not complaining too much when they found out how much they had to assist withthe electrical leak location survey. Their assistance produced an excellent testing environment and qualityresults. Wed also like to thank Joe Torre (manager, New York Yankees)just to see if anyone is reallyreading this part.

ReferencesNew York State Department of Environmental Conservation . 6NYCRR Part 360 Solid Waste Manage-ment

Facilities. Title 6 of the Compilation of Codes, Rules and Regulations, Subpart 360-2 Landfills.Phaneuf, R. 2000. Landfill Construction QualityWhat Weve Learned from Electrical Resistivity Test-ing.

Interface Friction/Direct Stability Testing & Slope Stability. TRI Short Course, Albany, New York.Phaneuf, R. and Glander, C. 2003. Using Tire Chips in Landfill Leachate Collection and Removal

Systems Under Part 360. Proceedings of the Federation of New York Solid Waste Associations

Solid Waste/Recycling Conference & Trade Show, Bolton Landing, New York.Phaneuf, R. and Peggs, I. 2001. Landfill Construction QualityLessons Learned From ElectricalResistivity Testing of Geomembrane Liners. Geotechnical Fabrics Report, v. 19, no. 3, pp. 2835.

Thiel, R., Darilek, G. and Laine, D. 2003. Cutting Holes For Testing vs. Testing For Holes. GFR, v. 21,no. 5, pp. 2023.

Project InformationOwner : Broome County, N.Y.Design Engineer : Stearns & Wheler, LLC, Cazenovia, N.Y.General Contractor : Marcy Excavation Co. Inc., Frankfort, N.Y.Geomembrane Supplier : Poly-Flex, Grand Prairie, TexasGeosynthetics Installer : Chenango Contracting, Johnson City, N.Y.Leak Survey Consultant : Leak Location Services, Inc., San Antonio, Texas

Bradford L. Smith, P.E., DEE is the director of Solid Waste Engineering for Stearns & Wheler, Cazenovia, N.Y.Robert J. Phaneuf, P.E., is now the chief of Hazardous Waste Engineering Western Section with

the New York State Department of Environmental Conservation (NYSDEC), Albany, N.Y.Kevin Roche is the deputy commissioner of Solid Waste for Broome County, N.Y.


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A recent GFR article described the potential interdis-ciplinary approaches of using phytoremediation andgeosynthetics in environmental applications (Kelsey2004). Increasingly, the disciplines of engineering andnatural sciences are cooperating more closely to ad-dress complex environmental challenges. While wesometimes still make fun of each other, the implemen-tation of sound remediation systems clearly requiresboth skill sets. Geologists, chemists, biologists and soil

scientists are trained to understand natural processes(e.g., groundwater flow regimes and geochemical pro-cesses, fate and transport of contaminants, toxicologyand risk assessment, natural resources management,etc.), but civil engineers are needed to properly designand construct the identified remediation systems. Evenmore passive, natural systems like phytoremediationor constructed wetlands need engineering inputs. This article aimsto further narrow the cultural gap between engineers and scien-tists by introducing the engineering community to major conceptsof phytotechnologies (formerly referred to as phytoremediation) andsome of the exciting opportunities that are available in this field.

PhytotechnologiesPhytoremediation is the use of plants to remediate or containcontaminants in soil, groundwater, surface water and sediments.More recently, the term phytotechnologies has been introducedinstead of phytoremediation, since this remedial approach cov-ers a number of technologies and applications. Over the last twodecades, phytoremediation has emerged as a feasible alterna-tive to more active and costly technologies, especially for large

areas with relatively low levels of contamination in shallow soils orgroundwater. The technology is rapidly gaining acceptance withinregulatory agencies as well as the public. In general, six mainmechanisms are involved in the application of phytotechnologies(ITRC 2001): Phytostabilization is the use of plants to immobilize (inorganicand organic) contaminants in soil, sediments and groundwaterthrough absorption and accumulation into the roots, the adsorption

Using plants to mitigate environmental problems

Phytotechnologies in current designs

Photo 1 . Phytotechnologies are moving from the labinto the field, in part through the convergence of engi-neering disciplines with the natural sciences.

Photo 2 . Common reeds (phragmites) arecommonly used in sonstructed wetlands.


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onto the roots, or the precipitation or immobilization within the root zone. Rhizodegradation refers to the breakdown of (organic) contaminants in soil through the bioactivity thatexists in the rhizosphere (an area a few millimeters away from a root surface). Phytoaccumulation is the process of metal- or salt-accumulating plants translocating and concentrating(inorganic) contaminants into the roots and aboveground biomass. In general, plants are harvested, andrecovered inorganics are either recycled (e.g., mining of metals from the harvested plant materials), orthe dried plant biomass is disposed of at an appropriate facility. Phytodegradation refers to the uptake of (organic) contaminants from soil, sediments and water withsubsequent transformation within plant tissues. Plant enzymatic transformation products are often lesstoxic compounds or result in bound residues that are less bioavailable. Phytovolatilization is the mechanism of uptake and translocation of the (inorganic and organic) contami-nants into the leaves with subsequent release to the atmosphere through transpiration. Evapotranspiration (ET) of plants can be used to significantly affect the local hydrology through intercep-tion of rain on leaf surfaces and transpirational uptake by the plant root system. This process has alsobeen referred to as phyto-pumping. The first five mechanisms have been successfully used to remediate or contain contaminated soils andshallow groundwater, while the use of ET applies more to the management of water (e.g., infiltration control

Figure 1 . Phytoaccumulation of arsenic using the fern pteris vittata .


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on landfill caps), and control of plume migration in groundwater (i.e., hydraulic containment). Furthermore,more than one of these processes can sometimes be operational at the same time. For example, con-structed wetlands or alternative landfill covers (i.e., phytocaps) might simultaneously involve the processesof evapotranspiration, rhizodegradation and phytoaccumulation to meet the project objectives. It becomes apparent that the concept of phytotechnology is fairly broad and includes many more

applications than just the classical contaminant uptake and harvest model that used to be the defaultimage of phytoremediation. Conceptually, phytotechnologies include a variety of applications ranging fromconstructed wetlands to alternative landfill covers, from tree plantations for hydraulic control to the use ofplants for slope stabilization, from planted (riparian) buffers for nutrient management and sediment controlto the classical applications of contaminant uptake and degradation.

If applied to remediation, phytotechnologies are limited by the effective rooting depth of plants, as wellas the phytotoxicity and/or plant-availability of contaminants. Quite often, only a fraction of the total con-centration of a specific contaminant is in a potentially bioavailable form that is accessible for plant uptake.It is therefore of utmost importance to carefully characterize and delineate the contaminants present ata site prior to selecting phytotechnologies as a potential remedial alternative. In some cases, this mayinclude additional characterization work (and therefore cost) not routinely used when employing more con-ventional technologies, such as excavation and disposal or pump-and-treat. For example, if one wants toconsider phytoaccumulation of metals as a potential remedial technology, a subset of soil samples shouldbe submitted for specialty analysis called sequential extraction procedures (SEPs). SEPs have beendeveloped to estimate which fraction of the total concentration of a given contaminant is bioavailable ornon-bioavailable. It may well be the case that a site contains an inorganic contaminant (e.g., arsenic) at300 mg/kg (ppm), and only 50% of that total concentra-tion is plant-available. If the remedial goal is to clean thesite to 25 ppm within a reasonable time frame, one mayexclude phytoremediation from the start of the project.

Again, phytoremediation as a stand-alone technologyis best suited for sites exhibiting relatively low levels ofcontamination in shallow soils or groundwater. However,systems have been implemented in which deeper soilshave been excavated and phytoremediated on-site. Forthese systems, geosynthetics could be used to constructtemporary staging/treatment areas in order to preventcross-contamination of un-impacted soils with contami-nated materials. A good example of this approach is theuse of geosynthetics in a constructed treatment area toprevent leaching of lead to deeper soils and groundwa-ter after lead has been mobilized (e.g., using chelating

agents) for plant uptake. Since most of the soil-boundlead is not available for plant uptake, lead has to bemade plant-available in such systems. The exact dos-age of a mobilizing agent is a tricky undertaking, andachieving a balance between rendering a contaminantplant-available and avoiding deep leaching is not easy.

In summary, phytotechnologies have broad applica-tions and provide great opportunities for engineers andscientists to pool their respective skill sets in order to meet

Photo 3 . Newly constructed wetland cells for treatment of landfill leachate.


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project objectives. The following case stu

geocell design principles

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