ALTERNATIVE CONCEPTS FOR WATER AND SEWER MAIN ACCESS IN THtrNORTHWEST TERRITORIES

K.R. Johnson, IJMA Engineering Ltd. (kjohson@uma$oup.com)B.C. Grieco, tlMA Engineering Ltd. (bgrieco@umagoup.com)

ABSTRACT

A study to investigate altemate less costly access systems for below ground water and sewermain servicing was undenaken by UMA Engineering Ltd. on behalfofthe Govemment oftheNorthwest Territories. The system most commonly used in permafiost areas ofthe NofihwestTerritories is a buried insulated steel access vault.

The study was based on a decision analysis ofa number ofaltemative concepts. lnfbrmationfor the concepts was based on a survey of inftastucture access systems in Aiaska, the yukonTerritory, the Northwest Teritories and the Nunavut Tgrritory. These concepts were thenrefined, and combined to provide a large number ofpotential access systems.

A total of l7 altematives were evaluated using a Kepner-Tregoe Decislon Alalysis. Theevaluation indicated that the three highest ruking altematives for access to sewer and watermains (in decreasing rar*) are:

. Common below ground mains with insulated steel access vaults.

. Common below ground mains with insulated High Density polyethylene (HDPE)access vautts.

Separate below ground mains with shallow insulated HDPE water access vaults,(requiring a portable shelter), and insulated HDPE sewer access vaults.

1.0 INTRODUCTION

A study was undertaken to investigate altemate,less costlyconcepts to the insulated steelwaterand sewer access vault currently adopted by the Govemnent ofthe Northwest Territoies forbuied water and sewer main servicing in the NWT.

Altemative concepts were developed and assessed using the Kepner-Tregoe method ofDecision Analysis. This method requires that a hierarchy ofobjectives be established, againstwhich the altemate concepts are evaluated.

The current design which utilizes a corDmon insulated steel access vault for both water andsewersystems is the result ofnearly 20 years ofdesign evolution. Prcblems which comrnonlyoccuned with the earlier systems ofwater and sewer access vaults included water infiltrationinto the vaults, physical and moistue danage to the intemal urethane insulation, difficulty ofaccess to appurtenances, and freeze breakage ofpiping and appurtenances.

The insulated steel access vault which is presently used, has rectified all ofthese problemsassociated with the previous designs. However, these vaults may cost in excess of$39,000.This costincludes supply and installation ofall fittings and appurtenances, but does not includemobilization and demobilization. The cost ofthe access vaults may represent 30o% ofthecontract p ce for the piped utilities system.

2.0 METHODOLOGY

The data collection was based on a survey of access systems in Alaska and the Yukon.Govemment agencies other than water and sanitation departments such as telephone utilitieswere also co[tacted. None of the groups contacted had developed a special design forpermafiost areas; rather, they used standard prefabricated concrete or metal sections.

The altemative concepts for accessing water and sewer mains w€re developed through groupdiscussions ofUMA staffexperienced in cold region, municipal, and mechanical engineering.Additional input was obtained from the Municipal and Community Affairs Department oftheNWT eovemment.

3.0 ALTERNATIVE CONCEPTS

3.1 Access From Wilhin A Structure

The altemative concepts within this group, as sumnrarized in the following Table 1, maybefurther classified as having fittings and appurtcnanccs accessed in a below ground structure,and fittings and appu enances accessed in an above ground structufe.

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3.2 Direct Buried Access

The direct buried conceptinvolves fittings and appurtenances which arc individually insulatedand individually accessed. Methods for insulating and accessing the various fittings andappurtenances arc described in the followins sections.

3.2.1 Water Main Appurtenarces

The thaw access for water main would employ athaw lance to water mains in either direction.The structure would be built from two back to back 45o laterals. Freezing ofthe watet rn thelaterals may occur, however, expansion ofthe ice would take place lowe;own in the lateral,so that damage fiom freeze expansion would be rmlikely.

The drain for water main would employ one oftwo systems to drain a direct buried water mainwithout access vaults. The thaw access shown in Figures 2 and 3 could be used to gain accessfor pumping out the system. Altematively, a drain and valve can be placed at low points in thesystem with the dtain pipe extended to a ditch or natural water course.

The fire hydrant would be covered with field sprayed insulation and sheet metal. Altematively,the hydrant and tee could be covered with preformed urethane halves and preformed galvanizedsheet steel split covers. The hydrant banel would be filled with propylene glycol antifteeze.The method ofinsulation and cover for the valve would be the same as for the hvdrant.

3.2.2 Sewer Main Appurtenances

The surface clean out and thaw access for sewer main shown in Figue 3 may be utilized as asewer maln thaw access and clean out. A second arrangement, shown on Figure 2 may alsobe used for sewer thaw and clean out. With this altemative, more piping is required, however,only one clean out cover is available/requited for access.

The manhole for the sewer main would employ an uninsulated body (standard precast concrete)and a fabricated, insulated, removable cover made to suit each manlole pipe configuration, andall piping would be insulat€d. A second t)?e of manhole would utilize a small, shallowmanhole with access to the sewer lines from the top ofthe manhole only. The manhole wouldbe insulated on the interior and the manhole would be watertisht.

4.0 DECISION ANALYSIS

The Kepner-Trego€ method of Decision Alalysis involves the systematic evaluation of anumberofdeveloped feasible altematives against a hierarchy offrxed objectives. Any feasiblealtemative is subsequently adjudicated intems ofthepossible adverse consequences ofusingthe altemative.

4.1 Must Objectives

Must objectives arc defined as those objectives which must be met for an altemative to becoNidered further. The list ofmust objectives were developed in consultation with all thoseinvolved in the project as follows:

. Access for sewer cleal out;

. Access for thawing sewer main,

. Access for draining water main,

. Access for thawing water main;

. Access to operate, maintain and repair appurtenances;

. Hy&ant drain and hydrant contents do not freeze;

. Resistarce to all uplift forces;

. Resistance to thaw settlement;

. Prevents all ingress ofwater;

. Minimum 20 year life;

. Prctection against freeze damage by utilizing the heat within the water ard sewer main;and

. Accommodates thrust forces due to expansion and contraction ofpipe.

4.2 Want Obiectiv€s

The want obj ectives have weights assigned to them which indicates their relative importance.All altematives are subsequently rated as to how well they satisfy each wart objective. Theuse ofwant objectives provides for a comparison ofalternatives.

Capital Cost Objectives' Low fabrication cost;. Low installation cost;. Easy shipping and handling for ground or sea tansportation; and. Minimized field installation work.

Maintenance Objectives. Maintenance wo.k to bedone in a sheltered portable environment with powerfacilities

for heat and light;. Spills or leakage to be easily removed;

Durable construction for known operating conditions;Vandalism resistance;Maintenance and repairs done using local resources;Minimurn but adequate space to perfom r€pair; andEasy locating and accessing under poor w€ather conditions.

Other Objectives. Competitive production in the NWT;. Field modifications or alterations possible for unplanned futue extensions;. Minimum tumaround for manufacture ofcomponents in system;. Easy rcmoval and/or rcplacement;. Minimum potential ofcontamination ofwater main from sewer main or other sources;. Original ground thermal regime maintained; and. Occupational Health and Safety Standalds met.

4.3 Analvsis

The water and sewer main access conc€pts and systems werc rcfined and combined to produceall possible access systems. Before sfaxting with the formal Kepner-Tregoe analysis, some ofthe more impractical altematives were removed Aom the list-

A total of 13 access systems met all l2 must objectives and were analyzed using the 18weighted want objectives listed. The access systems with their individual weighted scores foreach objective, subtotal weighted scores for each ofthe three categories ofobjectives, and theirtotal weighted scores were calculated.

The thr€e most pr€ferred access systems, with their subtotal and total weighted scores are:

1. Common below ground mains with insulated steel access vaults (existing system -Table 1 - AlL 1, Figure 1)

Capital Cost Score SubtotalMaintenance Objectives Score SubtotalOther Objectives SubtotalTotal Weighted Score

Capital Cost SubtotalMaintenance Objectives SubtotalOther Objectives SubtotalTotal Weighted Score

5371,350

9942,881

Common below ground mains with insulated HDPE access vaults (Table I - Alt. I,Figure 1)

800I ,032

9 1 82,7 50

3. Separate below ground mains with shallow insulated HDPE water access vault, whichrequires a portable shelter, and insulated HDPE sewet access vault (Table t - Alt. 7,Similar to Figure 3 except with separate sewer access vault in place of surface sewercleanouts)

Capital Cost SubtotalMaintenance Objectives SubtotalOther Objectives SubtotalTotal Weighted Score

692|,046

8502,588

5.0 CONCLUSION

As a result ofthis study and Decision Analysis ofaltemative methods ofaccessing water andsewer mains, thtee impotant conclusions can be drawn. The conclusions are:

2.

3 .

1 . At this time the existing buried insulated steel access vault is the highest ranking designfor accessing below ground water and sewermains and appurtenances in the NorthwestTerritories, for the purpose ofopention, maintenance and repair.

The buried insulated High Density Polyethylene (HDPE) common access vault is apotential lower cost altemative to the insulated steelaccess vault which maybe suitedfor cetain applications. The suitability of the HDPE access vault requires moreinvestigation before it can be considered for installation in the Northwest Tenitories.

A system including an insulated HDPE sewer (only) access vault and a separateshallow insulated HDPE water (only) access vault is also a potential lower costaltemative, although less promising than others, to the insulated steel accessvault. Thesuitability of the HDPE access vault requires more investigation before it can beconsidered for installation in the Northwest Territories. This system utilizes a portableshelter which also requires investigation and design with input from experiencedspecialists.

PLANNTS

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SIEEL ACCESSAULI (NSULATED)

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COMMON BELOWGROUND STRUCTURE(TNSULATED)

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