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The engineer’s role in any design pro-ject is to design a project that meetsthe desired purpose, is constructibleand ensures the health, safety andwelfare of the user. However, engi-neers do not always recognize theconsequences of their designs.

Civil engineering is a profession thatholds life and death consequences.When engineers design a project, anerror or mistake could result in proper-ty damage, personal injury or evendeath. Not understanding a product,while allowing its use, may equate tonegligence if injury results. Not readingthe product literature also may consti-tute negligence, should the product’sfailure result in injury or damage.When products fail, the engineer maybe deemed the responsible party.

Many engineers do not understand orappreciate the differences betweenreinforced concrete pipe (RCP) andhigh-density polyethylene (HDPE) pipeand the potential for liability whenspecifying each product. While RCP isa rigid structure that is designed, builtand tested as a structure before itarrives at the construction site, thestructure of HDPE pipe is actually builtand tested in the field. Therefore,HDPE pipe is not an “approved equal”substitute for RCP.

If a design is found to be faulty andthere is injury to either property or to aperson, and the engineer is found notto have met his or her standard ofcare, then the engineer is deemednegligent. This could result in not onlymonetary damages, but ultimately inthe loss of the engineer’s license.Additionally, the engineer may be crim-inally responsible for any death(s)caused by poor engineering. Whileengineers typically are not legallytrained, any engineer who stamps adrawing certifying that a design willmeet the intended purpose of the con-tract should understand the conceptsof standard of care, negligence and lia-bility.

WWhhyy ssppeecciiffyy??Corrugated HDPE pipe, rather than

RCP, is typically specified by designengineers as a result of assertions byHDPE pipe manufacturers regardingits lower cost and superior attributes.In some cases, HDPE may be a rea-sonable alternative to RCP, dependingon the specific project requirementsand design life. However, designersmust be cognizant of all aspects anddesign responsibilities of using anypipe material before specifying thatmaterial because materials and theirservice lives differ.

Many engineers believe that HDPEpipe is an approved equal substitutefor RCP because they believe theassertions stated in the publications ofHDPE pipe manufacturers withoutreading the fine print or the warranty

Concrete pipe offers safety benefitswhen used in storm sewer systems

information. The advertisements byHDPE pipe manufacturers can betempting to the engineer as a per-ceived means of achieving reducedcost and time of construction. In addi-tion, assertions made by the PlasticPipe Institute on service life also mayprovide the engineer assurance thatthe product specified will indeed last aminimum of 100 years. Yet, thesemanufactures’ own warranties andother disclaimers do not support theseassertions.

SSppeecciiffyyiinngg HHDDPPEE An attitude of indifference

seems to permeatethrough the thoughtprocess of someengineers. Anengineer has

installation conditions and externalloads. All of these elements combineto define the behavior of the installedpipe.

RCP is a load-bearing rigid structure,and a single length of RCP isdesigned, built and tested as a struc-ture before it arrives at the constructionsite. In addition, the American Con-crete Pipe Association (ACPA) haspublished the Concrete Pipe DesignManual since 1970. This manual con-tains engineering data on thehydraulics, loads and supportingstrengths and design of concrete pipe.

HDPE pipe is a flexible material andis not an independent structure. In acorrugated HDPE pipe system, thevertical load is transferred to the sidesupport soil and must deflect to func-tion. Specifying corrugated HDPErequires the engineer to provide a dif-ferent design than when specifyingRCP. Therefore, HDPE is not a mate-rial alternative to RCP and cannot beinterchanged in the design.

Factors that determine the successof an underground installation extendwell beyond the attributes of the pipematerial and product itself. The qualityof the environment around the buriedpipe is crucial to the long-term perfor-mance of the piping system. That isbecause buried plastic pipe is a com-posite structure made up of the plasticring and the soil envelope. Both thering and soil play a vital role in thestructural design of plastic pipe. Corru-gated HDPE pipe installed under-ground, being a flexible conduit,deflects under load and interacts struc-turally with the surrounding soilembedment material. Deflection is thechange in the inside diameter thatresults when a load is applied to a flex-ible pipe. The deflection characteristicsare a prime consideration in the struc-tural design.

Engineers must use the soil to con-struct an envelope of supporting mate-rial around the pipe so that the verticalload, applied to the side soil throughpipe deflection, is adequately support-ed. The extent to which the pipedepends on this enveloping soil forsupport is a function of the depth ofcover, surface loading and the ringstiffness of the pipe. This pipe-soilinteraction provides the necessarylong-term support for the pipe. There-fore, the prime structure in an HDPEpipe system is the soil envelope.

Because HDPE pipe is soil-depen-

AASHTO standards in order to ensurethat the design considers all aspects ofboth the standards’ requirements, aswell as the recommendations of thosestandards. Some ASTM/AASHTOspecifications place significant respon-sibility on the engineer regardinginstallation essentials to ensure ser-vice performance. Applicable sectionsin ASTM D 2321-04 include require-ments placed on the engineer withregard to installing thermoplastic pipe.

Engineers also must recognize thatHDPE pipe requires an engineeredinstallation in which the engineer mustbe involved in the construction activi-ties. The engineer must take responsi-bility to ensure that post-installationdeflection testing has been performedand documented. Before making afinal recommendation to the client, theengineer has a responsibility to ana-lyze life-cycle costs, the risks associat-ed with the chosen pipe product and toinform the client about the short-termand long-term costs, as well as any fur-ther risks that are identified during theselection of the pipe material.

HHDDPPEE vvss.. RRCCPP Manufacturer publications are a

small part of what engineers should berelying on when determining whetheror not to specify corrugated HDPEpipe. One of the most importantaspects that engineers must under-stand before specifying corrugatedHDPE pipe is the vast engineering dif-ferences between HDPE pipe andRCP.

Design of any pipe system requiresknowledge of material properties,

the legal responsibility to determinewhether or not the product being spec-ified will perform its intended functionfor the specific project in which thedesign is performed. Therefore, beforespecifying a particular product, theengineer must be aware of the charac-teristics, applications, potential defi-ciencies and limitations of the product.If the product is determined to havebeen unsuitable and damages result,the engineer may be deemed negli-gent and damages will be assessedagainst him.

Engineers have a duty to exercisereasonable professional skill and judg-ment and must adhere to the standardof care ordinarily exercised by mem-bers of their profession. The duty toadhere to the profession’s standard ofcare requires the engineer to use rea-sonable diligence and best judgment,which includes guarding against omis-sions or defects in plans and specifica-tions and keeping abreast ofimprovements.

To apply one’s best judgment, onemust fully understand the differencesbetween HDPE pipe material and theRCP structure when deciding which touse. Before selecting HDPE pipe, theengineer must review the specifiedproducts and their performance for in-situ conditions and evaluate availableliterature to ensure that the proper con-siderations are being made. The engi-neer must be wise to distinguishmanufacturers’ claims from actualfacts regarding their products.

Reasonable diligence also requiresengineers to have read and under-stood the most recent ASTM and

Nesting large RCP on radius.

dent, the behavior of the pipe-soil sys-tem requires a determination of theinteraction that will occur between thepipe, embedment material and thenative soil. The sum of these compo-nents acting together determines thetotal system behavior. Because thesoil in the HDPE pipe-soil interactioncan account for up to 90% of an instal-lation’s success, it is critical that thisdetermination be made by a geotech-nical or soils engineer.

Embedment refers to the materialimmediately surrounding the pipe. Theembedment material must provideadequate strength, stiffness, uniformityof contact and stability to minimizedeformation of the pipe due to earthpressures. The amount of resistancefound in the embedment soil is a directconsequence of the installation proce-dure—the stiffer the embedment mate-rials, the less deflection.

Controlled low-strength material, alsoreferred to as flowable fills or con-trolled-density fill may be used forbackfill and bedding, provided ade-quate flotation resistance can beachieved by restraints, weighting orplacement technique. Designers mustcalculate the width of the trenchrequired to provide sufficient structuralstrength for the system. Minimumtrench width is a function of pipe diam-eter, backfill material and the compaction technique. The engineermust consider not only the embedmentmaterial, but also the undisturbed in-situ soil surrounding the embedmentand the groundwater. The in-situ soil inthe trench wall can migrate through theembedment and affect the pipe’s per-formance.

The HDPE pipe structure is actuallybuilt and tested in the field. The resul-tant deflection of the flexible pipe isdirectly dependent upon the system’sbehavior. Deflection of HDPE pipemust be controlled by the stiffness ofthe pipe, variations in the pipe wallthickness or profile, stiffness of the soilembedment, type of shoring systemused, water table, means and methodsof the contractor, pipe handling, com-paction control, inspection and temper-ature. Therefore, testing for excessivedeflection is not a quality-control checkof the pipe, but the only way to ensurethat the pipe was properly installedwith the soil envelope fully developed.

Research has shown that it takessubstantial time—months to years—forthe full load to reach the pipe in either

state standards and specifications, theanswer has often been “no.” However,when provided to engineers, the detailfound in these types of referencematerials often incurs a reaction ofshock relative to what he or she mayhave specified without understandingthe consequences of that action.

HDPE pipe manufacturers’ war-ranties make it clear that no warranty ismade if the product is not used for theparticular intended use. One manufac-turer’s warranty notes that the HDPEpipe warranty is for the material, andthat the material conforms to ASTMand AASHTO. Significantly, the war-ranty does not discuss the use of thematerial as a structure.

Other manufacturers provide thedisclaimer that, even though the man-ufacturer offers directions, recommen-dations or suggestions for the use oftheir products, it is solely the buyer’sresponsibility to determine whether theproduct is suited for the specific needsof the buyer. Some state that the buyerassumes all risk for unloading, dis-charge, storage, handing and installa-tion, and that they will not beresponsible or liable for any removal orinstallation costs, downtime or otherconsequential damages, even if theyhave been advised of the possibility ofsuch damages.

FFaacciinngg ccoonnsseeqquueenncceess While not all corrugated HDPE pipe

failures end up in law suits and litiga-tion, some do. It is not just the contrac-tor or HDPE pipe manufacturer thatfinds itself potentially liable, but alsothe engineer. The following reasonshave been borne out in law suits andcontroversies over the last decade:

• Failure to understand or performlife-cycle cost analysis;

• Failure to recognize and understandthe differences in pipe design,installation and inspection betweenHDPE and RCP;

• Assuming that HDPE pipe is merelya substitute product for concretepipe;

• Persuaded by HDPE pipe manufac-turer publications touting benefits ofHDPE compared with concretepipe;

• Reliance on HDPE pipe manufac-turer publications without an analy-sis of the specific project conditionsand circumstances; and

• Assuming that he or she is bullet

the trench or embankment conditions.When long-term tests are carried out intrenches and embankments, thechanges in deflection with time are dueto increasing loads and soil consolida-tion. The strength of the HDPE pipematerial reduces under sustainedloads as well.

Deflection, as well as other HDPEpipe performance criteria, is a criticallong-term performance phenomenon.Typically, acceptance deflection ismeasured after the pipe has beeninstalled and backfilled for at least 30days. This gives the soil initial time tosettle and stabilize, although additionalchanges are certain to occur andinspections should be written into theplans. When HDPE pipe exceeds itsdeflection-acceptance limit, it shouldbe uncovered and the embedmentmaterial replaced and compacted.Specifications should also require thatthe pipe be removed and replaced.

Recently, the Florida Department ofTransportation (FDOT) changed itsspecifications to require that all pipesbe videotaped with laser-profile tech-nology. The inspection applies to allpipes with a 48-in. diam. and less,except side drains and cross drainsthat are short enough to inspect fromeach end. The requirements include,among other things, a pipe ovalityreport, deflection measurements withgraphical diameter analysis and a flatanalysis report.

The key factors affecting the successof an HDPE pipe installation include:

• Recognition that corrugated HDPEpipe itself is not a structure likeRCP;

• Recognition of a soil/pipe interactionin the design;

• Chemical composition of thepolyethylene;

• Proper installation techniques; and• Conclusive evidence that the

proven service life of the productmatches the design life of the project.

RReeaadd tthhee ffiinnee pprriinntt Manufacturer information, war-

ranties, standards and specificationsare written to caution, warn andinstruct the user about the specificmaterial, product or design to be per-formed. When the author asked engi-neers whether they have read the fineprint of a manufacturer’s product infor-mation or the applicable national or

proof and that professional liabilityinsurance will cover any problemsor issues arising at the site and thatsite problems are solely the result ofissues of the manufacturer or thecontractor.

One of the more costly examples ofHDPE failure occurred in Shell Lake,Wis., in 2003. Shell Lake is the largestland-locked lake without an outlet inthe state of Wisconsin and is approxi-mately 2,580 acres in size. Becausethe lake has no surface water outlet,the city of Shell Lake commissioned alocal civil engineering firm to design acombination gravity/siphon dischargefrom Shell Lake to the Yellow River, adistance of approximately 4.4 miles.The purpose of the pipeline was toallow the city to lower the elevation ofShell Lake during periods of highwater. The project bid resulted in asystem of corrugated HDPE pipe witha liner.

The pipeline was installed in Novem-ber 2002 and began leaking in January2003. The leaks continued and numer-ous attempts were made to fix theleaks; each time, the gravity/siphondischarge system was shut down.

While the discharge system was shutdown in May 2003, heavy rains causedthe lake level to rise with no way to dis-charge the rising water. Severe flood-ing and water damage wasexperienced by 120 of the lake’s 380residents. Law suits were filed againstthe pipe manufacturer for negligentmanufacturing of pipe products andsupplying pipe products that weredefective and unsuitable for the intend-ed application. The contractor wassued for negligently retaining and train-ing persons on the project performingthe installation. The engineer wassued for negligently designing andengineering the project, approvingunnecessary or harmful changes to theplan and approving the pipe product.The suit was eventually settled, but ata cost of approximately $1.25 million.The engineer and his insurers agreedto pay a significant proportion of thatsettlement.

LLeeaarrnniinngg ffrroomm eerrrroorrss As a result of failures like Shell Lake,

many municipalities and state agen-cies are either discontinuing or limitingthe use of HDPE pipe. In 1997, theDirector of School Planning and Con-struction issued a “departmental mora-

torium on any further use of HDPE forstorm water piping until we can furtherevaluate its validity and longevity.”

In 1998, the city of Knoxville, Tenn.,indicated its exclusive use of concretepipe within city right-of-ways. In 1997,the city of Gallatin, Tenn., advised thateffective that year, corrugatedpolyethylene pipe would no longer beallowed for use in the street crossdrains within the city of Gallatin. TheDepartment of Public Works for the cityof Pueblo has found significant failuresin portions of HDPE storm sewers insubdivisions and will no longer allowHDPE to be placed in any of the city’spublic right-of-ways.

The state of Georgia requires a mini-mum of 25% of a pipe system to bemandrel-tested and any pipe over 5%deflection be removed and replaced.The state of Kentucky has reduced thecondition of payment by up to 50%where inferior installation causes pipeto deflect beyond the 5% deflectionlimit. The state of Wisconsin requires aminimum of 10% of the system bemandrel-tested and any pipe over 5%deflection be removed and either rein-stalled if not damaged or replaced atno cost. The state of Illinois requiresmandrel testing of a piping systemafter 30 days of installation with pipelimited to 5% deflection. The state ofNevada does not allow pipe to exceed5% vertical deflection after 30 days ofinstallation and does not allow re-rounding of the pipe that does not meetthe test.

The Kentucky Transportation Cabinet(KTC) formed a task force in May 2005to evaluate current specification anduse of HDPE pipe on future KTC pro-jects based on performance inspec-tions carried out in 2002 and July2005. The results were published in areport entitled “Evaluation of HDPEPipe Performance on Kentucky DOTConstruction Projects.” The results ofthe report note significant change indeflection since testing at completion.Most deflection was greater than 10%,and it was evident that several pipescontinued to deflect after installation.The report also observed radial crack-ing, invert and crown flattening, rack-ing and sagging. It was evident that radialcracking had occurred after installation.Additional video-laser testing was recom-mended for long-term performance analy-sis, and KTC has since revised sections ofits specifications based on the resultsfound in these reports.

A November 2003 TransportationResearch Board paper, “The Eco-nomics of Culvert Failures,” by JosephPerrin and Chintan Jhaveri of the Uni-versity of Utah, studied culvert failuresin North America. Perrin and Jhavarisurveyed 57 agencies. Of the 25 thatresponded, only four stated that theagency performs a least-cost analysisfor pipe material selection. Differentagencies assumed a different life cyclefor each material. The life cycle forHDPE pipe varied from 30 to 100years. However, Perrin and Jhavariconcluded, “The reality is that pipesare not being replaced as theyapproach their expected service life.”

KKnnooww tthhee ddiiffffeerreennccee Given the differences between RCP

and corrugated HDPE pipe, it is essen-tial that the engineer understands thatthe design of the two piping systems isvastly different. If the engineer doesnot fully recognize these differencesand designs an HDPE system underthe same conditions as a RCP system,there is a high probability that the sys-tem may not perform as intended ormay ultimately fail.

The engineer must take into accountthese differences when designing apipe system using corrugated HDPEpipe. Failing to take these differencesinto account when designing and spec-ifying products places the engineer atrisk. Even if ultimately settled, engi-neers suffer great loss in the course oflitigation—to their professional reputa-tions, as well as to their pocketbooks.

Engineers have a sworn duty to pro-tect the health, safety and welfare ofthe public. Engineers must rememberthis duty when designing, specifying,requiring and enforcing. Failure toheed this duty may result in an engi-neer being the responsible party withall the associated risk, liability and con-sequences.

Author’s Note: Some information in this article wastaken from the PPI and ACPA websites.Dr. Galloway is CEO of the Nielsen-Wurster Group. TheAmerican Concrete Pipe Association contracted withThe Nielsen-Wurster Group to perform independentresearch comparing RCP and HDPE pipe. Gallowaywas one of several team members who were assignedat Nielsen-Wurster to perform that independentresearch. This paper reflects Galloway’s own opin-ions, as a result of her participation in that research.As a matter of accommodation, Galloway would bedelighted to review contradictory assertionsadvanced by others and if appropriate adjust her con-clusions.

© copyrighted Scranton Gillette Communications, Inc.Reprinted with permission, STORM WATER, Fall 2006

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