Alternative Concepts for Water and Sewer Main Access in the Northwest Territories

ALTERNATIVE CONCEPTSFOR WATER AND SEWER MAIN ACCESSIN THtr NORTHWEST TERRITORIES K.R. Johnson,IJMA EngineeringLtd. (kjohson@uma$oup.com) B.C. Grieco,tlMA Engineering Ltd. (bgrieco@umagoup.com) ABSTRACT A studyto investigatealtematelesscostly accesssystemsfor below ground water andsewer main servicingwasundenakenby UMA EngineeringLtd. on behalfofthe Govemmentofthe NorthwestTerritories. The systemmost commonlyusedin permafiostareasofthe Nofihwest Territoriesis a buried insulatedsteelaccessvault.

The studywasbasedon a decisionanalysisofa numberofaltemative concepts.lnfbrmation for the conceptswasbasedon a surveyof inftastucture accesssystemsin Aiaska, the yukon Territory, the Northwest Teritories and the Nunavut Tgrritory. Theseconceptswere then refined,andcombinedto provide a largenumberofpotential accesssystems.

A total of l7 altemativeswere evaluatedusing a Kepner-TregoeDecislon Alalysis. The evaluationindicatedthat the threehighestruking altemativesfor accessto sewerandwater mains(in decreasingrar*) are: .

Commonbelowgroundmainswith insulatedsteelaccessvaults.

.

Commonbelow groundmainswith insulatedHigh Densitypolyethylene(HDPE) access vautts.

Separatebelow groundmainswith shallowinsulatedHDPE water accessvaults, (requiringa portableshelter),andinsulatedHDPEseweraccessvaults.

1.0 INTRODUCTION altemate,less costlyconcepts to theinsulated steelwater A studywasundertaken to investigate andseweraccess vaultcurrentlyadoptedby theGovemnentofthe NorthwestTerritoiesfor water and sewermain servicingin theNWT. buied Altemativeconceptswere developedand assessed using the Kepner-Tregoe methodof DecisionAnalysis.Thismethodrequiresthata hierarchyofobjectivesbeestablished, against whichthe altemateconceptsareevaluated.

The currentdesignwhich utilizesa corDmoninsulatedsteelaccessvault for bothwaterand sewersystems is theresultofnearly20 yearsofdesignevolution.Prcblemswhichcomrnonly vaultsincludedwaterinfiltration occunedwith the earliersystems ofwater andseweraccess insulation,difficultyof intothevaults,physicalandmoistuedanageto theintemalurethane access to appurtenances, andfreezebreakage ofpiping andappurtenances.

The insulatedsteelaccessvaultwhich is presentlyused,hasrectifiedall oftheseproblems with the previousdesigns.However,thesevaultsmay costin excessof$39,000. associated ofall fittingsandappurtenances, butdoesnotinclude Thiscostincludes supplyandinstallation ofthe mobilizationand demobilization.The costofthe accessvaultsmay represent30o% contractp ce for the pipedutilitiessystem.

2.0 METHODOLOGY The data collectionwas basedon a surveyof accesssystemsin Alaska and the Yukon. otherthanwaterandsanitationdepartments suchastelephone utilities Govemmentagencies were also co[tacted. None of the groupscontactedhad developeda specialdesignfor permafiostareas;rather,they usedstandardprefabricatedconcreteor metal sections. waterandsewermainswâ‚Źredevelopedthroughgroup Thealtemativeconceptsfor accessing in coldregion,municipal,andmechanical engineering. discussions ofUMA staffexperienced ofthe AdditionalinputwasobtainedfromtheMunicipalandCommunityAffairs Department NWT eovemment.

3.0 ALTERNATIVE CONCEPTS 3.1

AccessFrom Wilhin A Structure

withinthisgroup,assumnrarized in thefollowingTable1, maybe Thealtemative concepts accessed furtherclassifiedashavingfittingsandappurtcnanccs in a belowgroundstructure, andfittingsandappu enances accessed in an abovegroundstructufe.

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3.2

DirectBuriedAccess

Thedirectburiedconceptinvolves fittingsandappurtenances whicharcindividuallyinsulated and individuallyaccessed.Methodsfor insulatingand accessingthe variousfittings and appurtenances arcdescribedin thefollowinssections.

3.2.1 WaterMain Appurtenarces Thethawaccess for watermainwouldemployathawlanceto watermainsin eitherdirection.

Thestructurewouldbe built from two backto back45olaterals.Freezingofthe watetrn the lateralsmay occur,however,expansion ofthe ice wouldtakeplacelowe;own in thelateral, sothatdamagefiom freezeexpansion wouldbe rmlikely.

The drain for water main would employoneoftwo systemsto drain a direct buriedwatermain withoutaccess vaults.Thethawaccess shownin Figures2 and3 could beusedto gainaccess for pumpingoutthesystem.Altematively,a drainandvalvecanbeplacedat low pointsin the systemwith the dtainpipeextended to a ditchor naturalwatercourse.

Thefire hydrantwould becoveredwith field sprayedinsulationandsheetmetal. Altematively, thehydrantandteecouldbecoveredwith preformedurethanehalvesandpreformedgalvanized sheetsteelsplit covers.The hydrantbanelwouldbe filled with propyleneglycol antifteeze.

Themethodofinsulationandcoverfor thevalvewouldbe the sameasfor thehvdrant.

3.2.2 SewerMain Appurtenances The surfacecleanout andthaw accessfor sewermain shownin Figue 3 may be utilized asa sewermaln thaw accessandcleanout. A secondarrangement, shownon Figure2 mayalso beusedfor sewerthawandcleanout. With thisaltemative, morepipingis required,however, only onecleanout coveris available/requited for access.

Themanholefor thesewermainwould employanuninsulatedbody (standardprecastconcrete) anda fabricated, insulated, removable covermadeto suiteachmanlolepipeconfiguration, and all piping would be insulat€d.A secondt)?e of manholewould utilize a small,shallow manholewith access to thesewerlinesfromthetop ofthe manholeonly. Themanholewould be insulatedon theinteriorandthemanholewouldbe watertisht.

4.0 DECISIONANALYSIS The Kepner-Tregoâ&#x201A;Ź methodof DecisionAlalysis involvesthe systematicevaluationof a numberofdeveloped feasiblealtematives againstahierarchyoffrxedobjectives.Any feasible altemativeis subsequently adjudicated intems ofthepossibleadverse consequences ofusing thealtemative.

4.1

Must Objectives

Must objectivesarc defined as thoseobjectiveswhich must be met for an altemativeto be coNideredfurther.The list ofmust objectivesweredeveloped in consultation with all those involvedin theprojectasfollows: . . . . . . . . . . . .

Accessfor sewercleal out; Accessfor thawingsewermain, Accessfor draining water main, Accessfor thawing water main; Accessto operate,maintain andrepair appurtenances; Hy&ant drain and hydrantcontentsdo not freeze; Resistarce to all uplift forces; Resistanceto thaw settlement; Preventsall ingressofwater; Minimum20 yearlife; Prctectionagainstfreezedamageby utilizing theheatwithin the waterard sewermain; and Accommodates thrustforcesdueto expansion andcontractionofpipe.

4.2

WantObiectivâ&#x201A;Źs

The want objectiveshaveweights assignedto themwhich indicatestheir relativeimportance.

All altematives aresubsequently ratedasto how well theysatisfyeachwart objective.The useofwant objectivesprovidesfor a comparison ofalternatives.

Capital CostObjectives ' Low fabricationcost; .

Low installationcost; .

Easyshippingandhandlingfor groundor seatansportation;and .

Minimizedfield installationwork.

Maintenance Objectives .

Maintenance wo.k to bedonein a sheltered portableenvironment with powerfacilities for heatandlight; .

Spillsor leakageto be easilyremoved;

for knownoperatingconditions; Durableconstruction Vandalismresistance; andrepairsdoneusinglocalresources; Maintenance Minimurn but adequatespaceto perfom r€pair; and Easylocating and accessingunderpoor w€atherconditions.

Other Objectives .

Competitiveproductionin theNWT; . possiblefor unplanned futue extensions; Field modificationsor alterations .

Minimum tumaroundfor manufactureofcomponentsin system; .

Easyrcmovaland/orrcplacement; .

Minimumpotentialofcontamination ofwatermainfrom sewermainor othersources; .

Original groundthermalregimemaintained;and .

OccupationalHealth and SafetyStandaldsmet.

4.3

Analvsis

Thewater andsewermain accessconc€ptsandsystemswerc rcfined andcombinedto produce analysis,someof all possibleaccess systems.Beforesfaxting with the formalKepner-Tregoe the more impractical altemativeswere removedAom the listA total of 13 accesssystemsmet all l2 must objectivesand were analyzedusing the 18 weightedwantobjectiveslisted.Theaccess systems with theirindividualweightedscoresfor eachobjective,subtotalweightedscoresfor eachofthethreecategories ofobjectives,andtheir totalweightedscoreswerecalculated. with theirsubtotalandtotal weightedscoresare: Thethr€emostpr€ferredaccess systems, 1.

Commonbelow groundmainswith insulatedsteelaccessvaults(existingsystemTable1 - AlL 1, Figure1) CapitalCostScoreSubtotal Maintenance ObjectivesScoreSubtotal OtherObjectivesSubtotal Total WeightedScore

537 1,350 994 2,881

Commonbelowgroundmainswith insulatedHDPE accessvaults(TableI - Alt. I, Figure1) Capital Cost Subtotal MaintenanceObjectivesSubtotal Other ObjectivesSubtotal Total Weighted Score

800 I,032 918 2,750

3.

Separate belowgroundmainswith shallowinsulated HDPEwateraccess vault,which requiresa portableshelter,andinsulatedHDPEsewetaccessvault (Tablet - Alt. 7, Similarto Figure3 exceptwith separate seweraccessvault in placeof surfacesewer cleanouts) CapitalCostSubtotal MaintenanceObjectivesSubtotal OtherObjectivesSubtotal Total WeightedScore

692 |,046 850 2,588

5.0 CONCLUSION As a resultofthis studyandDecisionAnalysisofaltemativemethodsofaccessingwaterand sewermains,thteeimpotant conclusions canbe drawn. The conclusions are: 1.

At thistimetheexistingburiedinsulated steelaccess vaultis thehighestrankingdesign for accessing belowgroundwaterandsewermains andappurtenances in theNorthwest Territories,for thepurposeofopention, maintenance andrepair.

2.

The buriedinsulatedHigh DensityPolyethylene (HDPE)commonaccessvault is a potentiallowercostaltemativeto theinsulatedsteelaccess vaultwhichmaybesuited for cetain applications.The suitabilityof the HDPE accessvault requiresmore investigation beforeit canbe considered for installationin the NorthwestTenitories.

3.

A systemincludingan insulatedHDPE sewer(only) accessvault and a separate shallowinsulatedHDPE water (only) accessvault is also a potentiallower cost altemative, althoughlesspromisingthanothers,to theinsulatedsteelaccessvault.

The suitabilityof the HDPE accessvault requiresmore investigationbeforeit canbe considered for installationin theNorthwestTerritories.This systemutilizesa portable shelterwhich also requiresinvestigationand designwith input from experienced specialists.

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