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napcopipe.com | 1.855.624.7473 ©2019 NAPCO, a Westlake company All rights reserved PI-TB-015-US-EN-0119.3

NAPCO, a Westlake company 2801 Post Oak Blvd., Suite 600n Houston, Texas 77056

Tel 713.840.7473n Fax 713.552.0087

RigidVs.FlexiblePipeLoadingConditionsOverviewDifferencesinpipematerialswillleadtodifferentpipe-soilinteractionsoncethepipeisburied.Akeydifferenceinthepipe-soilinteractionistheflexibilityofthepipematerialinresponsetoearthandsurfaceloadinguponburial.FlexiblepipessuchasPVC,andHDPEaredefinedbytheirabilitytoyieldunderloadingwithoutfracturing.Rigidpipessuchasconcrete,clay,andductileironpipesarelimitedintheirabilitytoyieldunderloadwithoutsustainingdamage.Thiskeydifferencebetweenflexibleandrigidpipesleadstotwodistinctloadingconditionsimposedonthepipeonceitisburiedinthetrench.Thetrenchloadconditionappliesprimarilytorigidpipes,whiletheprismloadconditionappliesprimarilytoflexiblepipes.Understandingthedifferencesbetweenthetwocanaidinthedesign,selection,andinstallationofpipematerials,embedmentmaterials,andsafedepthofbury.

RigidPipe(TrenchLoadCondition)Whenrigidpipe,suchasconcrete,clay,andironpipeswhicharestifferthanthesurroundingsoilisburied,thesoilinthesideprismswillcompressfurtherthanthepipewilldeflect.Frictioninthesoilinthetrenchabovethepipe,betweenthethreeprismsresultsinanadditionalloadonthepipegreaterthanjustthecentralprism.Thisisdueinpartbysettlementofthesideprismswhichbegintopulldownwardonthecentralprismastheysettle.Theresultisthatthepipebeingstosupportthesoilandliveloadingofboththecentralprism,andthetwoadjacentsideprisms.

Thisphenomenonisoftenmodeledbythreesprings,withtherigidpipebeingthestiffestspring.Whenaload(thesoilandsurfaceloads)isappliedtothetrench,thethreespringscompress.Becauseonespringisstifferthantheothertwo,itabsorbsmoreoftheload.Inthiscase,therigidpipebeingstifferthanthesurroundingembedmentmaterial,andmodeledbythestifferspring,resultsinthepipeabsorbingthemostloading.

Thelongerthearrow,thegreatertheloadactinginthedirectionofthe

arrowatthatlocation.

FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipeDesignandConstruction5thEd.

FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipeDesignandConstruction5thEd.

OverviewDifferences in pipe materials will lead to different pipe-soil inter-actions once the pipe is buried. A key difference in the pipe-soil interaction is the flexibility of the pipe material in response to earth and surface loading upon burial. Flexible pipes such as PVC, and HDPE are defined by their ability to yield under loading without fracturing. Rigid pipes such as concrete, clay, and ductile iron pipes are limited in their ability to yield under load without sustaining damage. This key difference between flexible and rigid pipes leads to two distinct loading conditions imposed on the pipe once it is buried in the trench. The trench load condition applies primarily to rigid pipes, while the prism load condition applies primarily to flexible pipes. Understanding the differences between the two can aid in the design, selection, and installation of pipe materials, embedment materials, and safe depth of bury.

Rigid Pipe (Trench Load Condition)When rigid pipe, such as concrete, clay, and iron pipes which are stiffer than the surrounding soil is buried, the soil in the side prisms will compress further than the pipe will deflect. Friction in the soil in the trench above the pipe, between the three prisms results in an additional load on the pipe greater than just the central prism. This is due in part by settlement of the side prisms which begin to pull downward on the central prism as they settle. The result is that the pipe beings to support the soil and live loading of both the central prism, and the two adjacent side prisms.

Rigid Vs. Flexible Pipe Loading ConditionsThis phenomenon is often modeled by three springs, with the rigid pipe being the stiffest spring. When a load (the soil and surface loads) is applied to the trench, the three springs compress. Because one spring is stiffer than the other two, it absorbs more of the load. In this case, the rigid pipe being stiffer than the surrounding embedment material, and modeled by the stiffer spring, results in the pipe absorbing the most loading.

Fig. 6.4 Trench load conceptualization with rigid pipe.

NAPCO, a Westlake company 2801 Post Oak Blvd., Suite 600n Houston, Texas 77056

Tel 713.840.7473n Fax 713.552.0087

RigidVs.FlexiblePipeLoadingConditionsOverviewDifferencesinpipematerialswillleadtodifferentpipe-soilinteractionsoncethepipeisburied.Akeydifferenceinthepipe-soilinteractionistheflexibilityofthepipematerialinresponsetoearthandsurfaceloadinguponburial.FlexiblepipessuchasPVC,andHDPEaredefinedbytheirabilitytoyieldunderloadingwithoutfracturing.Rigidpipessuchasconcrete,clay,andductileironpipesarelimitedintheirabilitytoyieldunderloadwithoutsustainingdamage.Thiskeydifferencebetweenflexibleandrigidpipesleadstotwodistinctloadingconditionsimposedonthepipeonceitisburiedinthetrench.Thetrenchloadconditionappliesprimarilytorigidpipes,whiletheprismloadconditionappliesprimarilytoflexiblepipes.Understandingthedifferencesbetweenthetwocanaidinthedesign,selection,andinstallationofpipematerials,embedmentmaterials,andsafedepthofbury.

RigidPipe(TrenchLoadCondition)Whenrigidpipe,suchasconcrete,clay,andironpipeswhicharestifferthanthesurroundingsoilisburied,thesoilinthesideprismswillcompressfurtherthanthepipewilldeflect.Frictioninthesoilinthetrenchabovethepipe,betweenthethreeprismsresultsinanadditionalloadonthepipegreaterthanjustthecentralprism.Thisisdueinpartbysettlementofthesideprismswhichbegintopulldownwardonthecentralprismastheysettle.Theresultisthatthepipebeingstosupportthesoilandliveloadingofboththecentralprism,andthetwoadjacentsideprisms.

Thisphenomenonisoftenmodeledbythreesprings,withtherigidpipebeingthestiffestspring.Whenaload(thesoilandsurfaceloads)isappliedtothetrench,thethreespringscompress.Becauseonespringisstifferthantheothertwo,itabsorbsmoreoftheload.Inthiscase,therigidpipebeingstifferthanthesurroundingembedmentmaterial,andmodeledbythestifferspring,resultsinthepipeabsorbingthemostloading.

Thelongerthearrow,thegreatertheloadactinginthedirectionofthe

arrowatthatlocation.

FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipeDesignandConstruction5thEd.

FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipeDesignandConstruction5thEd.

Figure obtained from the PVC Pipe Association’s Handbook of PVC Pipe Design and Construction 5th Ed.

Figure obtained from the PVC Pipe Association’s Handbook of PVC Pipe Design and Construction 5th Ed.

Fig. 6.2 Downward shearing forces over rigid pipe.

The longer the arrow, the greater the load acting in the direction of the arrow at that location.

Shearingforcesincreasethe load

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Flexible Pipe (Prism Load Condition)

When a flexible pipe, such as a PVC, or HDPE pipe is buried, it is almost always less stiff than the surrounding soil and embedment material. The result is the opposite effect of a rigid pipe. The flexible pipe deflects more under loading than the surrounding soil. When this occurs, the central prism begins to settle more than the two side prisms and instead of the side prisms pulling downward on the central prism, like in rigid pipe burial, the side prisms now begin to support the central prism, and the load is shifted off of the pipe, and onto the surrounding soil. The flexible pipe is then supporting only the weight, or even slightly less than the full weight, of the central prism directly above the pipe. Because the pipe is now only supporting the prism of soil and live loading directly above it (the central prism) this loading condition is commonly referred to as “Prism Loading”.

Going back to the three springs model: With a flexible pipe, you can now see that the stiffer springs are represented not by the pipe, but by the surrounding embedment material. When the load is applied, the primary support is provided by the side prisms, and the load on the pipe is reduced.

Figure obtained from the PVC Pipe Association’s Handbook of PVC Pipe Design and Construction 5th Ed.

Fig. 6.5 Trench load conceptualization with flexible pipe.

NAPCO, a Westlake company 2801 Post Oak Blvd., Suite 600n Houston, Texas 77056

Tel 713.840.7473n Fax 713.552.0087

FlexiblePipe(PrismLoadCondition)

Whenaflexiblepipe,suchasaPVC,orHDPEpipeisburied,itisalmostalwayslessstiffthanthesurroundingsoilandembedmentmaterial.Theresultistheoppositeeffectofarigidpipe.Theflexiblepipedeflectsmoreunderloadingthanthesurroundingsoil.Whenthisoccurs,thecentralprismbeginstosettlemorethanthetwosideprismsandinsteadofthesideprismspullingdownwardonthecentralprism,likeinrigidpipeburial,thesideprismsnowbegintosupportthecentralprism,andtheloadisshiftedoffofthepipe,andontothesurroundingsoil.Theflexiblepipeisthensupportingonlytheweight,orevenslightlylessthanthefullweight,ofthecentralprismdirectlyabovethepipe.Becausethepipeisnowonlysupportingtheprismofsoilandliveloadingdirectlyaboveit(thecentralprism)thisloadingconditioniscommonlyreferredtoas“PrismLoading”.

Goingbacktothethreespringsmodel:Withaflexiblepipe,youcannowseethatthestifferspringsarerepresentednotbythepipe,butbythesurroundingembedmentmaterial.Whentheloadisapplied,theprimarysupportisprovidedbythesideprisms,andtheloadonthepipeisreduced.*Seeourtechnicalbulletintitled“BurialDepthGuidanceforGravitySewerPVCPipe”foradditionalinfo.

ReferencesPVCPipeAssociation.HandbookofPVCPipeDesignandConstruction.5thEd.Chapter6.

Thelongerthearrow,thegreatertheloadactinginthedirectionofthearrowatthat

location.FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipe

DesignandConstruction5thEd.

FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipeDesignandConstruction5thEd.

Comment [k1]: Canyoulinkthistotheothertechbulletinonthewebsite?

Figure obtained from the PVC Pipe Association’s Handbook of PVC Pipe Design and Construction 5th Ed.

Fig. 6.3 Upward shearing forces over flexible pipe.

NAPCO, a Westlake company 2801 Post Oak Blvd., Suite 600n Houston, Texas 77056

Tel 713.840.7473n Fax 713.552.0087

FlexiblePipe(PrismLoadCondition)

Whenaflexiblepipe,suchasaPVC,orHDPEpipeisburied,itisalmostalwayslessstiffthanthesurroundingsoilandembedmentmaterial.Theresultistheoppositeeffectofarigidpipe.Theflexiblepipedeflectsmoreunderloadingthanthesurroundingsoil.Whenthisoccurs,thecentralprismbeginstosettlemorethanthetwosideprismsandinsteadofthesideprismspullingdownwardonthecentralprism,likeinrigidpipeburial,thesideprismsnowbegintosupportthecentralprism,andtheloadisshiftedoffofthepipe,andontothesurroundingsoil.Theflexiblepipeisthensupportingonlytheweight,orevenslightlylessthanthefullweight,ofthecentralprismdirectlyabovethepipe.Becausethepipeisnowonlysupportingtheprismofsoilandliveloadingdirectlyaboveit(thecentralprism)thisloadingconditioniscommonlyreferredtoas“PrismLoading”.

Goingbacktothethreespringsmodel:Withaflexiblepipe,youcannowseethatthestifferspringsarerepresentednotbythepipe,butbythesurroundingembedmentmaterial.Whentheloadisapplied,theprimarysupportisprovidedbythesideprisms,andtheloadonthepipeisreduced.*Seeourtechnicalbulletintitled“BurialDepthGuidanceforGravitySewerPVCPipe”foradditionalinfo.

ReferencesPVCPipeAssociation.HandbookofPVCPipeDesignandConstruction.5thEd.Chapter6.

Thelongerthearrow,thegreatertheloadactinginthedirectionofthearrowatthat

location.FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipe

DesignandConstruction5thEd.

FigureobtainedfromthePVCPipeAssociation’sHandbookofPVCPipeDesignandConstruction5thEd.

Comment [k1]: Canyoulinkthistotheothertechbulletinonthewebsite?*See our technical bulletin titled “Burial Depth Guidance for

Gravity Sewer PVC Pipe” for additional info.

References:PVC Pipe Association. Handbook of PVC Pipe Design and Construction. 5th Ed. Chapter 6.

The longer the arrow, the greater the load acting in

the direction of the arrow at that

location.

Shearingforcesdecreasethe load