WIXLIB

99Transportation Research Record stems--areSUMMARYThe primary use of prestressed pipe to date has beenfor pressure pipe installations.In Caltrans, ithas been limited to special drainage designs and hasbeen considered a semirigid design. The prestressedconcrete pipe research reported in this paper,coupled with the reinforced concrete pipe researchby Hydro-Conduit using pipes that share common dimension ratios and Ameron prestressed concrete pipedesigns, gives further support to the dimensionratio concept.Prestressed concrete pipe continues to offer anacceptable alternative for special drainage des igns--i. e., where there is an unstable soil condition in a potential slide area. Under high fills itis not recommended, since there is admittedly theadverse effect of the earth loads being added to theprestressing forces.ACKNOWLEDGMENTThe Caltrans Culvert Committee and Ameron cooperatedand participated fully in the initiation of the prestressed concrete pipe culvert research at CrossCanyon and the selection of the design parameters tobe considered. Walt Creasmon of Ameron contributedsignificantly to the initiation of the prestressedconcrete pipe research proposal and, most recently,to this paper on design summary and implementation.REFERENCESl .2.R.E. Davis,F.M. Semans,D.W. Spannagel,andA.E. Bacher. Rigid Pipe Proof Testing Under Excess Overfills with Varying Backfill Parameters.California Department of Transportation,Sacramento, Oct. 1978 to July 1982.Standard Specifications forHighway Bridges,12th ed. AASHTO, Washington, DC, 1977.Publication of this paper sponsored by Committee on Culverts and HydraulicStructures.Effect of Heavy Loads on Buried CorrugatedPolyethylene PipeREYNOLD K. WATKINS, RONALD C. REEVE, AND JAMES B. GODDARDCorrugated polyethylene pipe, developed originally in 4- to 12-in diameters forland drainage, is now manufactured in larger diameters for other uses of buriedconduits such as culverts, air ducts, and service conduits. Tests were conductedon pipes with 15-, 18-, and 24-in inner diameter to investigate the structural performance and performance limits of these larger-diameter pipes when subjectedto external soil pressures. For pipes in typical native soil backfill, compactedby typical methods to greater than 80 percent standard density (AASHTOT-99), less than 1 ft of soil cover (called minimum cover) was found to be adequate protection against H-20 (32-kip/axle) loads and up to 54-kip/axle"super-loads". The soil envelope does not have to be select material. At lessthan minimum cover, the performance limit is either (a) excessive pipe deflection or (b) localized reversal of curvature directly under the wheel load. Underhigh soil cover, both the performance and performance limit are pipe deflection(out-of-roundness), which is a function of the total vertical soil pressure and isequal to or slightly less than vertical -soil strain in the backfill material on bothsides of the pipe, herein referred to as sidefill. In compacted soil backfill, pipedeflection is less than 10 percent for either H-20 loads on minimum soil coveror vertical pressures up to 2500 psf under high soil cover. Pipe stiffness isroughly equal to steel and is greater than aluminum in 16-gage, 2-2/3 x 1/2 corrugations.Corrugated plastic pipe is one of the leading pipematerials used for land drainage in the UnitedStates. It was introduced in the late 1960s, beginning with small inner diameters (ID) (3 and 4 in),which were used primarily for agricultural landdrainage. During the 1970s the uses for corrugatedplastic pipe greatly increased.Sizes up to 15-indiameter were developed, and applications were extended to highway drainage and to various residential and commercial construction uses, includingfoundation drainage, home sewage disposal, and grainaeration.With the introduction of 18- and 24-inpipe in 1981, the uses for corrugated plastic pipeagain expanded to a still wider range of applications, including mining, culverts for roads anddriveways, and other types of entrance and ditchcrossing applications.In 1979, field loading tests were conducted atHamilton, Ohio, to evaluate the structural performance of 12-in corrugated polyethylene pipe forvarious types of culvert installations Ill· With therecent development of the larger pipe diameters (18and 24 in), the following additional questions arise.Are there any structural limitations pertainingto these larger-diameter pipes when they are subjected to heavy external soil pressures? Subdrainage pipe is often backfilled with gravel or similarmaterial that provides a filter for the inflow ofground water but also provides radial support forthe pipe.If the pipe is used for purposes otherthan subdrainage (i.e., culvert), a select backfillmaterial is not needed as a filter. But is it neededas support for the pipe?Most drainage pipe isinstalled in trenches and at shallow depths that canbe excavated by a backhoe or wheel trencher orplowed in with a drainage plow. The most criticalpipe loadings are surface wheel loads usually noheavier than H-20 truck loads, the highest legalhighway wheel loadings.Can buried polyethylenepipe, with minimum cover, resist the super heavywheel loads of construction equipment, off-highwaytrucks, etc.? Can corrugated polyethylene pipe beburied under very high embankments or in very deeptrenches? What about pipe stiffness?To answer these questions, field tests were conducted at London, Ohio, by Utah State University(USU) in cooperation with Advanced Drainage Systems,Inc., to investigate the effect of heavy loads passing over shallow buried corrugated polyethylenepipe.An additional series of tests conducted atUSU used a pressure soil test cell to simulate theeffect of great depths of cover, which in some casescan result in very high soil pressures. Comparative

100Transportation Research Record 903NOTATIONbackfilled to the original soil level (as shown inFigure 1) to provide a continuously decreasingheight of cover from one end of each pipe run to theother.The height of cover was determined fromelevations taken along the pipe before and afterbackfilling.The following notation is used in this paper:Materialstests were conducted by the Wadsworth Testing Laboratory of Canton, Ohio, and hy usu on the At i ffnPAAof functionally equivalent corrugated pipes ofsteel, aluminum, and polyethylene.D nominal pipe diameter inside diameter(in) Id depth of corrugation (in),Dm mean diameter of pipe D d (in),B width and height of select soil envelope,usually gravel (in) (in this study, widthand height were equal) ,H height of cover above the top of the pipe(in) IH/D ratio of height of cover to nominal pipediameter (dimensionless) ,6y vertical pipe deflection (in),6y/D a ratio of decrease in vertical diameterto the original circular diameter(dimensionless),y soil density based on AASHTO T99, appliedto native soil in situ, or after compactionof soil backfill (pcf) ,P vertical soil pressure at the top of thepipe (psf) ,c vertical soil strain (in/in), andF/6y pipe stiffness (lb/in of length inof deflection) MINIMUM COVER TESTSProcedureA test course of seven pipe runs was set up at asite near London, Ohio, as shown in Figure 1. Eachpipe run consisted of two 20-ft sections coupledtogether at the midpoint (the midpoint was not ameasurement point).The objective of the testingprogram was to determine, for buried corrugatedpolyethylene pipe, the relation of pipe deflectionto height of soil cover under large wheel loads atvarious backfill densities.A single-axle H-20 load, 16 kips/dual wheel, wasused as the basic load, but "super-loads" up to 27kips/wheel, as might be applied by heavy off-highwayequipment, were also investigated.The standardtruck H-20 rear axle load of 32 kips was simulatedby use of a John Deere model 762 scraper. The frontwheels of the loaded scraper (16 kips each) werecentered directly over the pipe as shown in Figure 2.DeflectionThe pipe deflection under each wheel was measured byusing a spring-loaded, direct-reading deflectometer(see Figure 3).Deflection was measured to thenearest 1/16 in at five loading positions at 3-ftspacings on each side of the midpoint of each 40-ftpipe run, which made a total of 10 measuringpoints/pipe. As the testing proceeded, it becameapparent that deflections were less than 5 percenteven when the height of cover was less than 12 in.To obtain a wider range of deflections, it was decided to remove cover from the shallow end of eachpipe run and to test the pipe with the H-20 load atzero cover.The pipes wereinstalledinslopingtrenchesandThree sizes of corrugated polyethylene pipe weretested:15-, 18-, and 24-in diameters.The recently developed 18- and 24-in-diameter pipes aremanufactured with a slightly angled helical corrugation of approximately 2 . These sizes were comparedwith the annularly corrugated 15-in-diameter pipemanufactured by the continuous extrusion and corrugating process. The pipes used in the tests wererepresentative of standard production material. Thehigh-density polyethylene resins used in the manufacture of the pipes complied with the requirementsof Type III, Class C, category 5 as defined anddescribed in ASTM Dl248. The pipe stiffness valuesrecorded on the test specimens exceeded the proposedminimum pipe stiffness requirements of large-diameter pipe of 40 psi at 5 percent deflection and 30psi at 10 percent deflection.As shown in Figure l, the testing was designedfor three soil densities: 75, 85, and 95 percentAmerican Association of State Highway and Transportation Officials (AASHTO) standard density. Thenative soil at the site was used as backfill on sixof the seven pipe runs. Uncompacted gravel backfill(AASHTO coarse aggregate 57, uncompacted) was usedon run 3. The backfill soil was a mixture of twostrata: a 2. 5-ft stratum ot sandy clay silt (Unified Soil Classification CL) and a sandy silty clayfrom below that depth (CL) The soil backfill around the pipe was compactedby mechanical power tampers as shown in Figure 4.For pipe runs 1, 2, 5, 6, and 7, the soil was compacted in successive lifts of about 6-10 in each,depending on the desired degree of soil density.More lifts and more passes achieved greater density.The soil backfill for pipe run 4 (75 percent specified density) was dumped on and around the pipe witha loader and spread with a dozer blade7 there was nocompaction except the slight compaction due to theD-6 crawler dozer passing over the completed backfill with a track pressure less than or equal to 3psi.A specified density of 75 percent usuallyindicates uncompacted or very lightly compactedsoil. The density of the soil was checked with aTroxler nuclear densitometer at several stationsalong the pipe and at various levels as the backfilling progressed.ResultsThe results of the tests for minimum cover on 24-indiameter pipe subjected to H-20 loads are shown inFigures 5 and 6, where 6y/D is plotted as a function of both height of soil cover in inches and thedimensionless soil cover term H/D. The dashed curvesare power curves of the form (y bxm) that represent the best-fit curves of the data for the 24-inpipe at the three average soil densities of thefield tests--95.8, 91.8, and 75.0 percent. Becausethe soil densities attained on pipes 5 and 7 werenearly equal, the data were combined for analysis.For purposes of design, plots of 95 percent confidence levels were evaluated by using the 24-inpipe data for soil densities of 75, 80, 85, 90, and95 percent standard AASHTO density. These are shownin Figure 6. The 95 percent confidence level plotis that plot below which 95 percent of all test datafall. The test data for pipe 3, uncompacted gravel