DESIGN GUIDE - Snap-Tite

PEXYZZ, the format of the thermoplasticmaterial designation code for PE pipeWhere:PE indicates polyethyleneX is the characteristic density range forthe compound used to make the pipe asdefined within ASTM D3350Y is the characteristic slow crack growthresistance range for the compound usedto make the pipe as defined within ASTMD3350ZZ is the long-term hydrostatic stress at23O F, expressed in hundreds of psiHistorically, the market for PE pipe wasdominated by essentially two primarythermoplastic material designation codes.These were PE2406 and PE3408. In 2005,changes were made to ASTM D3350 toallow for the identification and integration ofmuch higher levels of technical performancein PE piping materials within the NorthAmerican standards system. This resulted ina temporary proliferation of PE thermoplasticpiping material designation codes. Today, westill have a fairly broad selection of materialdesignation codes for PE piping systemsthroughout the marketplace. However, forall practical purposes, the market for PEpipe is characterized by the three commonthermoplastics materials designation codes.research on polymer performance in PEpiping and offers the designer or end-userexceptional levels of pipe system performance.For example, the PE4710 piping productssupport a higher long-term hydrostatic stressrating making the pressure rating for a givenwall thickness of pipe 25% higher than acomparable PE3608 piping product. By thesame token, these piping products exhibit asignificantly higher resistance to slow crackgrowth. Given the exceedingly high technicalperformance of the PE4710 piping products,it is no surprise that they meet or exceed all ofthe technical requirements of the PE3408 orPE3608 piping products. For this reason it isnot uncommon to see these piping productsdual labeled as PE3408/PE4710 or even triplelabeled as PE3408/PE3608/PE4710.Snap-Tite High-Density Polyethylene (HDPE) PipeSo the thermoplastic piping materialdesignation code follows the form below.PE2708 – This piping product is producedfrom a medium density compound as definedin the current version of D3350 and is widelyused in natural gas distribution and somespecialty applicationsPE3608 - This piping product is the legacyproduct resulting from the old PE3408thermoplastic piping material designationcode that was so widely specified and usedprior to 2005. It is not uncommon today to seethese piping products dual labeled PE3408/PE3608.PE4710 - This piping product designationrepresents the culmination of years of technicalWWW.CULVERT-REHAB.COM (800) CULVERT10

Snap-Tite High-Density Polyethylene (HDPE) PipeTable 2-1 provides a summary of the different ASTM D3350 cell classifications for each of thesematerials based on these three primary thermoplastic piping material designation codes.Table 2-1Typical Cell Classificationby Current Thermoplastic Piping Material Designation tyD 1505Melt IndexFlexuralModulusTensileStrengthResistanceto SlowCrackGrowthHydrostaticDesignBasis, ellNumberTypicalValueCellNumberTypicalValueGR/CM32 0.9250.9403 0.9400.9474 0.9470.955D 1238GR/10MIN3 0.4-0.154 0.154 0.15D 790PSI3D 638PSI3D 1479HOURS7500Minimum6100Minimum7500MinimumD 2387PSI312504160041600D 1603%EColoredwith UVStabilizerC2% MinCarbonBlackC2% MinCarbonBlack40,000- 80,0002600 300054110,000 180,0003000 350054110,000 180,0003000 – 3500Notes:1. The density provided is base resin density (without the influence of carbon black). Typical PE4710 HDPE pipe has a densityof 0.956 to 0.964 with carbon black.2. To be designated a PE4710, the pipe resin must meet certain supplementary requirements established by the HydrostaticStress Board (HSB) of the Plastics Pipe Institute (PPI).11WWW.CULVERT-REHAB.COM (800) CULVERT

Table 2-2Representative Minimum Cell Classificationby Thermoplastic Piping Material Designation CodeThermoplastic Piping MaterialDesignation CodeMinimum Cell Classification PerASTM D3350PE2708233373EPE3608345464CPE4710445474CIt should be noted that other PE thermoplastics piping material designation codes do existand may be encountered in the different markets. However, the three primary PE thermoplasticpiping material designations codes of Tables 2-1 and 2-2 represent the principle PE pipingproducts in the market today. For the culvert lining market, HDPE resins with a PE3608 orPE4710 are commonly used for solid-wall piping systems.The selected thermoplastic piping material designation code and minimum cell classification isthen combined with the appropriate production and installation standards to effectively specify atough, durable PE piping system. A model specification is available in Chapter 13 and availableat culvert-rehab.com or by contacting your local Snap-Tite representative.WWW.CULVERT-REHAB.COM (800) CULVERTSnap-Tite High-Density Polyethylene (HDPE) PipeTable 2-2 below provides a simplification of Table 2-1 and illustrates the relative ease with whichPE piping products may be specified. Using this approach allows the designer or specifier toaccurately designate the appropriate PE piping product through the use of a single thermoplasticpiping material designation code and a relatively simple text string that establishes the physicalproperty requirements for seven key performance properties.12

WWW.CULVERT-REHAB.COM (800) CULVERTHydraulics.Chapter 3.Hydraulics14

Hydraulics3-1 Flow in CulvertsMuch of the information in this chapter is asummary of information presented by theFederal Highway Administration’s (FHWA)Hydraulic Design Series (HDS) No. 4 andNo. 5 publications. HDS 4 is titled Introductionto Highway Hydraulics (FHWA 2008a-FourthEdition) and deals with culverts as “closedconduits” in chapters 7-9. HDS 5 is titledHydraulic Design of Highway Culverts (FHWA2012-Third Edition) and is a comprehensiveculvert design publication, developed tostandardize procedures and simplify analysisof culvert flow. As stated in both documents,flow conditions depend not on just the culvert(or the inserted liner) but also the interactionwith upstream and downstream conditions.The choice of a culvert lining or replacementcannot be made merely along the potentialflow capacity of a pipe, but also on hydrology(climatological and watershed characteristics),site data, aquatic ecology concerns,maintenance issues and overall economics.Ideally, culverts are designed to transportwater with minimal headwater buildup.Headwater is the water surface elevation onthe upstream side of a culvert and providesthe energy to force water through a culvert.When a channel waterway is constricted likethat of a culvert through an embankment,conveyance capacity is typically reduced.However, the choice of a culvert (andits relining option) is one that can beeconomically justified, short term and manytimes long term, against the costs to meetthe entire flow criterion. It is not uncommonto accept some increase in upstream waterlevel, or design headwater, as long as itstays below allowable headwater depth orspecific distance below the roadway shoulderelevation to prevent overtopping of anembankment and/or roadway.have water over the top of the inlet, so lessthan full or gravity full-flow conditions wouldbe expected.When Snap-Tite is inserted into anotherpipe with a higher Manning’s “n” factor,it is not uncommon for the same flow tobe maintained. For example, when a 36”corrugated metal pipe (CMP) is lined with a30” Snap-Tite liner, the calculations showan increase in flow of 35% using Manning’sequation. If the existing culvert is notundersized for current hydrological demands,then flow after lining with Snap-Tite will notbe dramatically affected since the capacity ofthe lined culvert is near that of the old culvert.3-2 The Manning’s EquationThe Manning’s equation is used to determinethe gravity full-flow condition in storm drainsystems.Q .0006136 x (d8/3 S1/2)/nWhere:Q Discharge/flow, cu ft per secd pipe inside diameter, in inchesS culvert barrel slope (feet/foot) (h1 – h2)/LL pipe length, in feeth1 Entry Culvert elevation in feeth2 Exit Culvert elevation in feetn Manning’s factorHistorically, a simple approach for comparingculverts was accomplished with a flow ratecomparison using Manning’s equation. WithManning’s equation, the capacity for a pipe asopen channel flow (less than full) and gravityfull-flow conditions can be approximated.Many existing culverts were designed for100-year flood conditions and infrequently15WWW.CULVERT-REHAB.COM (800) CULVERT

Snap-Tite is made using solid-wall HDPEpipe. The extrusion of HDPE resin createsa very smooth pipe. The Snap-Tite jointallows sections of pipe to be joined togethermechanically in the field without increasingthe OD (outside diameter) or increasing/decreasing the ID (inside diameter) of the linerat the joint. Snap-Tite Culvert Liner has beentested in full-flow conditions to determine theManning’s factor. Utah State Water ResearchLaboratory tests determine that an “n” factorof 0.00914 is valid for Snap-Tite in full-flowconditions, which is consistent for industrystandards for the pipe.Solid-wall thermoplastic pipes appear toremain smooth throughout their lifetime. Thesurface of some materials may change withtime. Based on industry practice,WWW.CULVERT-REHAB.COM (800) CULVERTSnap-Tite and other solid-wall materials aresmooth throughout their useful life and theManning’s “n” factor is considered constant.In low-flow situations, it is possible forsediment to accumulate in a culvert. Sticks,rocks and other debris may collect inside.The condition of a liner is sometimes notthe same over its expected life, as cracked,corroded and even deformed liners will affectthe smoothness of a pipe and will impactflow. These situations may influence theselection of a different “n” factor. While inmany applications the measured “n” factor isnormally used, the final selection of the “n” isthe owner or engineer’s choice based on theirexperience.Hydraulics3-3 Selection of Manning’s Factor16

Hydraulics17Table 3-1Comparative Flow Rates for Corrugated Metal Pipe (CMP) lined with Snap-Tite Based on Manning’s equation n .00914 for Snap-Tite , n .024 for CMP, s .001 ft/ftSnap-Tite CMPCulvert SizeID (in)Flow(gpm)Flow(cfs)OutsideDia. (in)DR 32.5 Av. ID(in)Flow(gpm)Flow(cfs)% ERT-REHAB.COM (800) CULVERT

Snap-Tite ConcreteCulvert SizeID (in)Flow(gpm)Flow(cfs)OutsideDia. (in)DR 32.5 Av. ID(in)Flow(gpm)Flow(cfs)% 024111.564%WWW.CULVERT-REHAB.COM (800) CULVERTHydraulicsTable 3-2Comparative Flow Rates for Concrete Pipe lined with Snap-Tite Based on Manning’s equation n .00914 for Snap-Tite , n .015 for Concrete, s .001 ft/ft18

Existing ConcreteBox SizeManning’s n FactorSnap-Tite Liner Size3 ft. x 3 263"18611160%0.01563"14911175%4 ft. x 4 ft.5 ft. x 5 ft.6 ft. x 6 ft.3-4 VelocityVelocity is the speed at which water flowsthrough a culvert. When the velocity exceeds3 feet per second (fps), sediment is normallyentrained in the flow, and the culvert isconsidered self cleaning. If the velocity is lessthan 3 fps, sediment will usually buildup inthe culvert. In evaluating sediment potential,factors such as particle size, specific gravity,cohesiveness, flow velocity and roughness ofthe pipe must also be considered.Once the flow rate is determined usingManning’s equation, then the velocity, V (ft/sec), can be approximated by using theequation below:V Q/AWhere:Q flow, cu ft per secA area, ft sqAs the velocity increases, sediment is nolonger a problem in most situations. It isconsidered high when velocities are over 12feet per second. Solid-wall HDPE pipe hasbeen used in slurry and dredging applicationsat velocities approaching 18 to 20 fps, withexcellent wear resistance compared to mostother materials. Short-term exposure to highvelocity may cause long-term damage. Aslarge rocks and debris strike the Snap-Tite liner, damage can occur. Damage and wear ismore likely at higher velocities.When the velocity is known to be high,streambed scour and bank erosion mayoccur at the discharge of the outlet pipe. The19Box fullflow cfsSnap-Tite Flow cfs% of Flowhigh velocity and flow condition can erodea channel. An apron of formed concrete orriprap under the discharge is commonly usedto prevent erosion and scour at the discharge.High velocity in a liner can cause separationon the liner joints when the liner is not groutedin place. Grouting of the liner into the hostculvert will solve separation concerns fromvelocity.It should be noted that one of the anomaliesassociated with flow in circular pipes is thata partially full pipe will have higher dischargeflow rates than a full pipe can carry, due to theincreased friction along the wetted perimeter(Figure 3-1). Flow rates above 80% full willbe higher than a pipe with full pipe flow, witha peak at 93%. Velocities above 50% will behigher than full pipe velocities with a peak atapproximately 80% full mark.Figure 3-1FULLRatio of Depth of Flow to Diameter d/D, %HydraulicsTable 3-3Comparative Flow Rates for Concrete Box Culvert lined with Snap-Tite Based on Manning’s equation n .00914 for Snap-Tite , s .001ft/ft9080Area706050DischargeHydraulic Radius403020Velocity100.1.2.3.4.5.6.7.8.91.01.11.2 1.3Hydraulic Elements in Terms of Hydraulic for Full Section(V/Vfull, Q/Qfull, A/Afull, R/Rfull)Chart courtesy of Introduction to Highway Culverts.WWW.CULVERT-REHAB.COM (800) CULVERT

Snap-Tite is made using low-pressure HDPEpipe. The Snap-Tite joint is designed for usein gravity flow applications and to meet therequirements for ASTM D3212. Pressure fromheadwater or tailwater conditions should notharm the liner or the joint. Snap-Tite is notdesigned for long-term pressure applications.Table 3-4Factors Influencing Culvert DesignInletControlOutletControlHeadwaterXXInlet ConfigurationXXAreaXX3-6 Types of Flow ControlShapeXXOnce a pipe exceeds the point of gravityfull-flow, culvert operation is ruled at all timesby one of two conditions: inlet control oroutlet control. When lining culverts, both inletand outlet control must be considered. Thehydraulic capacity of a culvert depends upona combination of factors that influence eachtype of control, identified in Table 3-4. Theslope of a culvert, that is barrel slope, is theprimary factor influencing whether or not aculvert will be in inlet or outlet control.Barrel SlopeXXBarrel Roughness-XBarrel Length-XTallwater-XFactorHydraulics3-5 Pressure ConsiderationsNote: For inlet control, the area and shape factorsrelate to the inlet area and shape. For outlet controlthey relate to the barrel area and shape.Figure 3-2Outlet ControlFigure from U. S. Department of Transportation Federal Highway Administration Hydraulic Design of Highway CulvertsWWW.CULVERT-REHAB.COM (800) CULVERT20

HydraulicsOutlet control occurs when flow through theculvert barrel or tailwater can not accept ashigh a flow as the inlet opening will accept.Tailwater is the water surface elevation on thedownstream side of a culvert as measuredfrom the invert at the culvert exit. Hightailwater alone can make a culvert operateunder inlet control, but long culverts withrough interiors or slightly sloping culverts areother factors with outlet control. Figure 3-2depicts a couple of instances when outletcontrol governs flow.Most culverts relined with Snap-Tite operatein inlet control since high tailwater waterconditions are not as common inwell-designed drainage systems. Additionally,the smooth barrel will typically allow morewater than the inlet, so the inlet becomes thecontrolling section of the system.Inlet control means the discharge capacity ofa culvert is controlled at the culvert entranceby depth of headwater, inlet factors likeentrance type, barrel/inlet area and inletshape, and, in rare cases, barrel slope. Inletshape is typically the same as culvert barrelexcept when enlarged with tapered inletsand flow enhancement devices at the barrelentrance. In inlet control, the roughness,length of culvert or outlet conditions (includingtailwater depth) are not factors in determiningculvert capacity.The entrance type is a major factor for inletcontrol performance. Commonly foundentrance types include square edge withheadwall, end mitered to the slope, projectingbarrel, and beveled entrance. Relined culvertinlets are likely to also utilize wingwalls (seechapter 6) placed at an angle

corrugated metal pipe (CMP) replacement. In most cases, Snap-Tite actually out-performs the concrete and corrugated metal it rehabilitates. Even at smaller diameters than the original pipe, Snap-Tite allows for better throughput than concrete or corrugated metal due to the smooth wall interior of the pipe. Furthermore, the Snap-Tite .