www.cpaa.asn.auConcrete Pipe Associationof Australasia

1980 Seminar on theBacher/Davis Investigations

NewConceptsof Loads onUnderground Structures

,,The history of concrete pipe

installation in Australia is closelyrelated to investigation and practiceoverseas.

So the Australian Standard CA33­1962 takes into account the investi­gations of M.G. Spangler andW.]. Schlick. This standard iscurrently under review by the Stand­ards Association.

In furthering the understandingof soil/structure interaction ofburied concrete pipelines, theAmerican Concrete Pipe Associationsponsored investigations at NorthWestern University about whichDr. R.A. Parmelee reported at ourfirst National Seminar.

Another investigation of soi 1­structure interaction has been spon­sored by the American Department

of Transport. It has been conductedby the California Department ofTransportation, Division of High­ways, under the direction of twoof its senior bridge engineers,A.E. Bacher and R.E. Davis.

It is our opinion that this workgives every indication of being amajor breakthrough in our under­standing of the forces acting onburied pipes under differingconditions of bedding and fill depth,and that it will be advantageous tothe committee revising CA33.

This paper is an up-to-~ate reportof the Bacher/Davis field studies. It ilhas been reprinted with"the per­mission of the American ConcretePipe Association from their period-ical Concrete Pipe News and formsthe basis of Mr. Bacher's lecture.

•

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-

Rigid Culvert Tests - Mountainhouse CreekPart I

byRaymond E. Davis

Senior Bridge EngineerCalifornia Department of Transportation

andAlfred E. Bacher

Senior Bridge EngineerCalifornia Department of Transportation

Field studies of concrete pipeculvert behavior recently con­

cluded at Mountainhouse Creek,in California, dramatically dem­onstrated the significant influenceof the surrounding embankmenton pipe behavior and the benefi­cial results that may be derivedfrom soil-structure interaction,even with relatively rigid pipes.

A dummy, 84-inch, pipe proto­type was designed for a nominal1,0000 load rating, but three-edgebearing tests indicated a loadcapacity closer to 1,5000. Califor­nia specifications traditionallyhave barred 1,0000 pipes fromhighway embankments and haveallowed only 16 feet of overfill inemhankments where they could

be used. With proper construc­tion methods and inspection, thedummy culvert at MountainhouseCreek sustained an overfill of 45feet before the first hairline crackappeared and a total of 136 feetwithout significant distress. Other,less rigorous construction proce­dures resulted in gross failure ofthe pipe.

CONCRETE PIPE NEWS/63

Access Pipe

200 ft. 72 in. ~ R.C.P. 30000

'"

·····················c·······················a.G.

o.

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240 ft. 84 in. 4> R.C.P. 10000 -t-'''''-::----<-j+-­

109 ft.~129 ft.~136 ft.~133 ft.

Zone 12, Zone 11 Zone 10. Zone 9 ' Zone 8

1+-------- Dummy Pipe

Selected Rock Embankment .-J

Timber Bulkhead

-. "" .... '" .....~.... ~ ..

Figure 1. Longitudinal Section Through Prototype Culvert,Showing Test Zones and Maximum Overfills.

,

Figure 2. Pipe Dimensions and Instrumentation and Beddingand Backfilling Parameters.

TYPE 2 BACKFilLEMBANKMENT

grammetrically, are plotted in Fig­ure 3, effective densities acting onthe pipe in Figure 4. Observationsand conclusions have been exten­sively discussed in the referenceslisted in the bibliography, and willonly be briefly summarized here­In.

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TYPICAL SECTIoN

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pacted (95%), well graded, con­crete aggregate. In the remainingzones, ordinary embankment ma­terial was placed by excavatingmachinery around the pipe andcompacted with hand tampers.

Diameter changes, measuredwith an extensometer and photo-

The dummy culvert, which ter­minated within the embankmentat a straw-buffered, timber bulk­head, was divided into six, 40­foot long zones, comprising five,8-foot long pipe segments, each.A longitudinal section throughthe culvert and embankment isshown in Figure 1, bedding andbackfilling parameters in Figure 2.

Two installation conditionswere examined: two Zones, 7 and8 were embedded in as-footdeep, vertical sided trench ex­cavated after the embankmenthad been constructed and com­pacted to the proper level, the re­maining Zones, 9 through 12,were installed as "positive pro­jecting."

The bedding condition wasvaried between zones and allzones were underlaid by a rockfilled trench 3 to 8 feet deep toproduce an essentially unyieldingfoundation. Shaped bedding wasused in Zones 7 and 9; Zone 8 ona 6-inch layer of dry sand; Zone10 on a 60-degree, Class C. con­crete bedding; Zone 11 on a6-inch by 24-inch polystyreneplank; and Zone 12 on line bear­ing.

Backfill material in Zones 7 and8 was comprised of well com-

64/CONCRETE PIPE NEWS

Verlical

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Overfill .II Diameter Changeslsi Hairline I (:::;"~<::h'::.::) ---j

(rollck 1-Zone (feel)

The overfills at which hairlinecracks appeared first, and ulti­mate changes in diameters aretabulated below:

Differences in behavior of Zones7 and 8 were inappreciable ex­cept that the widest cracks atZone 7 (O,OSS-inch) ultimatelywere about half the size of thosein Zone 8. Average cracking inthe two zones was similar, indi­cating that shaping of the beddingwas of no significant value, Witha little patching, to prevent rein­forcement corrosion, both zonescould become functional.

The very significant differencesin behavior of Zones 7 and 9,both with shaped bedding, clear­ly illustrated the value of en­trenchment and provision of ade­quate lateral support with well­compacted backfill. Zones 9through 12 exhibited continuousbreaks and compression' spallingat the ends of the horizontaldiameter. "Bowstringing," theseparation of shells of concreteoutside the inner and outer rein­forcing cages was particularlysevere in Zone 9. Shaped beddingin this zone proved to be detri­mental-crushing of the invert atZone 12, on line bearing, permit­ted sufficient vertical movementto foster arching in the soil so thatthe pipe in Zone 12 was ulti­mately in superior condition tothe better supported pipe inZone 9.

Settlement of the pipe at Zone11 into the polystyrene plank alsocreated a soil arch, and the piperemained in better-than-averagecondition up to the point of ulti-

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Figure 3.

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Figure 4, Effective Densities at Time of Fill Completionand 31 Months Thereafter,

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ZONE I: - 1o,' OVERFilL

CONCRETE PIPE NEWS/65

ACKNOWLEDGEMENTSThe research work described

herein was sponsored and par­tially financed by the UnitedStates Department of Transporta­tion, Federal Highway Administra­tion. The contents of this paperreflect the views of the writerswho are responsible for the factsand the accuracy of the data pre­sented herein. The contents donot necessarily reflect the officialviews or policies of the CaliforniaDepartment of Transportation orthe Federal Highway Administra­tion. This paper does not consti­tute a standard, specification, orregulation.

Appendix - R~ferences1. Davis, R. E., Bacher, A. E., and

Obermuller, J. c. "Structural Be­havior of a Concrete Pipe Culvert-Mountainhouse Creek - (Part1}," California Division of High­ways Report R&D 4-71, Apr.,1971.

2. Davis, R. E., Bacher, A. E., andObermuller, J. C. "Concrete PipeCulvert Behavior-Part 1," Journalof the Struclural Division, ASCE,Mar., 1974 (Proceedings Separate10404).

3. Davis, R. E., Bacher, A. E. "Con­crete Pipe Culvert Behavior--Part2," Journal of the Structural Divi­sion, ASCE, Mar', 1974 (Proceed­ing Separate 10405).

mate compaction of the poly­styrene at 80 feet of overfill, afterwhich the pipe deteriorated rap­

idly.Effective density distributions at

60 feet and 100 feet of overfill re­inforce the above qualitative ob­servations of distress. High aver­age densities at the upper octantpoints of Zones 7 through 10 at60 feet of overfill reflect the rela­tive rigidity of the soil-pipe sys­tem, while sub-hydrostatic upperdensities a1 Zones 11 and 12 dem­onstrate the arching in the soil asthe pipe settled: 1. into the softpolystyrene at Zone 11; and, 2.due to invert crushing at Zone 12,respectively.

High lateral densities at Zone 8reilect the early buildup of pas­sive soil stress components as thepipe tried to flatten and expandhorizontally against the restraintof the well-compacted backfill,fostering a uniformity of soilstress around the pipe periphery,with small consequent momentsand shears in the pipe wall. Suchpassive components were alsoultimately generated at Zones 9,11 and 12 but only ailer sufficientextension of the horizontal diam­eter to produce severe distress.

The decreases in several effec­tive densities at Zone 10 betweenthe 60 feet and 100 feet overfillsresulted from pipe breakage andmovement of the wall away fromthe soil.

The increase of densities from60 feet to 100 feet overfill at Zone11 occurred as the settlement ofthe pipe into the polystyreneplank ceased and no further soilarching occurred.

Based on relative distress ob­served, the quality of bedding andbackfill parameters may be ratedfrom best to worst as follows:

1. Entrenched pipe: a. Zone 7,shaped bedding; b. Zone 8, fineaggregate (unshaped) bedding(actually nearly a tie).2. Positive projecting pipe: c.Zone 11, on 6-inch polystyrene

66/CONCRETE PIPE NEWS

plank; d. Zone 12, line bearingon hard earth; e. Zone 9,shaped bedding; and, f. Zone10, 60-degree concrete bed­ding.On the basis of unmodified

economics, the bedding andbackfilling parameters may belisted in the follOWing order-thetotal cost per linear foot of trench­ing, bedding, placing pipe andbackfilling is also listed for eachlone:

1. Zone 12, $11.332 Zone 8, $13.563. Zone 7, $14.014. Zone 9, $15.345. Zone 11, $16.676. Zone 10, $18.26Results of the experiments

demonstrated the potential forgreatly modifying current tablesof overfills for concrete pipe pro­vided that proper constructionprocedures, which induce soil­structure interaction, are em­ployed. Results of the Mountain­house Creek tests have been in­corporated into tests of a grosslyunderdesigned pipe at Cross Can­yon, in Southern California, toaugment the information alreadyobtained. Field tests at Cross Can­yon are essentially complete, andanalysis of data is progressing.

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Rigid Culvert Tests - Mountainhouse CreekPart II

byRaymond E. Davis

Senior Bridge EngineerCalifornia Department of Transportation

andAlfred E. Bacher

Senior Bridge EngineerCalifornia Department of Transportation

This article is the second of a serieson an experimental installation ofprecast concrete pipe by theCalifornia Department of Transport­ation. Full reports of the researchare available through the NationalInformation Service.

Previously described field test­ing of a 1,0000, reinforced

concrete, dummy culvert, atMountainhouse Creek (August,1978), was accompanied by testsof a functional culvert, about 60feet from the dummy pipe, in thesame embankment.

Standard, 3-edge bearing testsperformed on 3 segments of thepipe, nominally designed for a4,0000 load rating, yielded an ac­tual range of 4,300 to 4.7000.with an average rating of 4.5000.

Figure 1 depicts a longitudinalsection through the pipe and em­bankment. Figure 2 shows pipedimensions and bedding andbackfilling parameters in six test

zones. A segment near the centerof each zone was instrumentedwith soil stressmeters and SR-4strain gauges as shown in Figure 2.All instrumented segments, withthe exception of that in Zone 1,were surmounted by a 9-foot­wide by 7-foot-deep layer ofbaled straw (Method B Backfill).

Pipe segments were placed in a5-foot-deep, vertical-sided trenchexcavated in a previously con­structed embankment pad. Struc­ture backfill surrounding the pipecomprised single-sized ('/o-inch±)pea gravel in Zones 1 to 3, a uni­formly graded, well-compactedconcrete agg·. egate in Zones 4to 6.

CONCRETE PIPE NEWS/89

'",",

lONES 3&4

.,,~

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Figure 2. Bedding and backfill parameter for thesix zones of the embankment section.

Figure 1. Mountainhouse Creek embankment section.

ZONES I a 6

"

2000 0

lONES 2 a.5t~=§~~!~t=J'-'

----_ .... - l~ ....

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Pipe regments in Zones 1 and 6rested on shaped beddings ex­cavated in undisturbed embank­ment material, those in Zones 2and 5 on shaped beddings re­formed from embankment mate­rial after the trench had been ex­cavated to the level of the bottomof the pipe. Zones 3 and 4 restedon line bearing on a 6-inch-deeplayer of sand.

Instruments were read at 10­foot increments of fill height dur­ing construction. Soil stresseswere converted to /leffective den­sities," i.e., densities required toproduce measured soil stressesunder corresponding overfills.Strain readings from SR-4 gaugeson the inner and ouler peripheriesof the reinforcing steel cages wereconverted to stresses and used todetermine wall bending momentsand thrusts.

Soil stressmeters, purchased un­der a bidding procedure in con­formance with California's StateContract Act, may have heen ofquestionable quality, ,ome indi­cating lower soil stresses at maxi­mum overfill than those measuredat the datum, 10-foot-overfilllevel! Not to be outdone, the SR-4gauge strain readings indicatedthat a significant number of pipecross-sections manifested meantensile thrusts at fill completion,by comparison with those at thesame datum plane.

either of these anomalies canbe positively attributed to instru­menial error, the first possiblyevidencing the great effectivenessof the low modulus inclusion insoil stress reduction; the secondbeing a possible consequence of"reverse shrinkage" in concretepipe segments which had beenstored in the sun for 7 monthsprior to embedment in the moistsubterranean environment. '

Profiles of unadjusted, meas­ured soil stresses are shown inFigure 3 for overfills close to 50feet. Adjusted soil stress pro­files, made symmetrical by aver-

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1

gO/CONCRETE PIPE NEWS

aging about the vertical diameter,and by combining what appearedto be the most valid data fromZones 2 and 5, and 3 and 4, areshown in Figure 4. Effective densi­ties calculated from soil stressesmeasured at the time of fill com­pletion and one year later areshown in Figure 5.

For purposes of comparing be­havior of the various zones, ex­perimental bending moments andthrusts have been listed in Tables1 and 2, respectively. These valueswere calculated from strain cross­sections for overfills of 50 to 60feet and reduced linearly to anoverfill of 50 feet.

No surface cracking was visibleduring embankment construction.An examination of concrete corestaken for testing of two soil stress­meters, after wetting, revealed in­cipient hairline cracking in theplanes of the reinforcement-thisphenomenon is typical and resultsfrom shearing failures as the pipesplits into three separate shells re­sulting in "bowstringing" inweaker pipes.

A comparison of soil stress pro­files for Zone 1, with Method ABackfill, and Zone 6, with MethodB (baled straw) 8ackfill, demon­strates that almost all perirheralstresses acting on the culvert havebeen appreciably decreased byplacement of the layer of strawover the culvert. From Table 1 it isclear that most of the wall bend­ing moments were also reduced .Compressive thrusts at locations5, 6, 7 and 8 were appreciablygreater at Zone 1 than at Zone 6.

Wall bending moments at Zone2, with shaped bedding, wereabout the same as, or larger than,those at Zone 3, on line bearing.Moments at Zone 4, on line bear­ing, were inappreciably differentfrom those in Zone 5, on shapedbedding. 80th facts indicate, aswas noted in Phase 1 of the proj­ect, that shaped bedding providesno structural advantage over linebearing on a layer of sand.

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Table 1. Comparisons of experimental momentsreduced to 50-foot overfill linearly.

1 • 11.1 8.2 - 9.5 - 6.7 8.0 - 5.2

2 5.5 6.9 * 9.5 2.4 - 0.9 7.8

3 6.8 5.2 7.1 2.5 1.2 • 9.9

4 - 11.0 •- 13.7 - 9.2 -4.9 - 6.9 -4.6

5 * 15.8 - 2.8 - 1.6 3.1 6.7 3.0

6 - 0.9 * 20.4 7.1 2.2 5.0 2.2

7 •- 17.0 4.5 5.2 1.6 1.9 4.5

8 * - 7.1 4.7 - 7.1 - 4.5 - 1.6 - 3.2

*Highest moment this position

Table 2. Comparisons of experimental thrustsreduced to 50-foot overfill linearly.

1 22.1 * - 35.3 1.9 2.2 0.8 - 17.4

2 25.7 9.5 10.8 29.0 27.7 * - 4.7

3 24.8 •- 32.0 - 16.4 -17.9 -16.3 - 10.7

4 - 8.5 •- 47.1 - 18.0 - 24.0 -·14.1 - 13.0

5 -46.3 - 5.9 - 46.2 - 39.1 •- 54.1 -43.2

6 - 23.7 * - 43.6 3.1 - 28.5 - 8.9 - 1.3

7 * -49.4 - 20.9 4.4 - 29.0 - 10.9 - 12.0

8 * - 32.6 - 2.6 - 19.2 - 10.0 - 14.4 - 22.1

*Highest compressive thrust this position

".

Figure 3. Rigid concrete pipe culvert research, radial soil pressure (psi)at fill heights at 50 foot (approximately) level.

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CONCRETE PIPE NEWS/91

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Wall moments at Zone 2 aregenerally higher than those inZone 5; those in Zone 3, thanthose in Zone 4. This fact suggeststhilt the pea gravel backfill, whichpossesses no cohesive qualities,may tend to shift around duringembankment construction, leav­ing the structure incompletelysupported and reducing the soilstructure interaction. Since thedifferences in moments were notgreat, this statement does notnecessarily imply that the materialis wholly unsatisfactory, but that itis not as good as the well-gradedbackfi II.

At Zones 5 and 6, the differencein average wall moments is inap­preciable, suggesting that shapingof the bedding has about thesame effect whether performed inundisturbed material or by shap­ing replaced material in an over­excavated trench.

Figure 4. Rigid concrete pipe culvert research, radial soil pressure(psi) at completion of fill.

CONCLUSIONS

Method B Backfill, employing alayer of compressible materialsurmounting a rigid pipe, will pro­duce lower overall soil stressesbut more intense stress gradientsthan are produced by Method ABackfill. Results from Part 1 of thisresearch project clearly demon­strated the advantages of placingthe pipe in the trench to providelateral Support. This second partof the project provides the addi­tional information that the back­fill material used between thepipe walls and trench should bewell-compacted, well-graded ma­terial, and that this material is bet­ter than a single-sized materialsuch as pea gravel. Both projectsindicate that shaping of the bed­ding provided little advantageover placing the pipe on a flatbedding of fine aggregate. If ma­terial is to be shaped, it is of littleconsequence whether this shap­Ing takes place in over-excavatedand reformed foundation materialor undisturbed embankment.

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The choice of an untried soilstressmeter for tests of the 4,0000pipe by the Department was un­fortunate, and the significant pro­portion of large negative soilstresses and meter failures threwa cloud upon the credibility of aneven larger number of small posi­tive and negative soil stresseswhich may have been valid. Lackof credibility of the soil stresses,coupled with the significant pro­portion of strain readings whichwere indicative of tensile hoopstresses in the pipe walls underthe full embankment load, pro­vided a very low confidence levelfor much of the project's experi­mental data.

In spite of these facts, however,the above-stated conclusions,based on certain valid compo­nents of the strain data and corre­lations of these data with quasi-

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theoretical results based on soilstress data, can b'e consideredvalid.

RECOMMENDATIONSFOR IMPLEMENTATION

1. Previous California practicerequired shape¢ bedding for pipes',42 inches to 72 inches in diam­eter for overfillt' exceeding 100feet, and for all pipes over 72inches in diameter. Results of thisresearch indicate that, from apractical standpoint, a flat bed­ding comprising a layer of sand 6inches in depth produces struc­tural behavior as satisfactory asthat produced by shaped beddingfor pipes as large as 84 inches indiameter.

2. All previous California cul­vert research has clearly demon­strated that the placement of a

92/CONCRETE PIPE NEWS

4,0000 pipe, the authors are ofthe opinion that it was conserva­tively designed for the construc­tion procedures employed, Thisfact is further emphasized by therelatively good condition of Zones7 and 8 at the time of embank­ment completion in the neighbor­hood dummy culvert, which wasdesigned with a 10000 load rat­ing and placed under Method ABackfill. These two zones wereentrenched, placed on shapedand flat sand beddings, respec­tively, and surrounded with well­compacted, well-graded concreteaggregate.

Concrete pipe segments with arange of D-Ioadings are being ex­amined under a number of pa­rametric conditions of beddingaod backfilling at Cross Canyon .Establishment of final curves ofrequired pipe strength for givenoverfills will be deferred untilanalytical work is complete onthat project,

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COMPOSITE EFFECTIVE D~NSITIES-ME.THaD BCALIFORNIA DOT CULVERT ReSEARCH

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layer of soft material surmountinga culvert will produce significantdecreases in the soil stresses' act­ing vertically on the culvert. In thecase of the arch culvert, such ver­tical stress reduction has beenshown to be decidedly detrimen­tal to the culvert's structural be­havior. Results of this researchsuggest that placement of thestraw layer over an entrenched.round, reinforced concrete pipereduced overall soil stresses butdid not, in all cases, reduce wallbending moments because oflarger stress gradients. Inasmuchas a third study of reinforced con­crete pipe is currently being per­formed (at Cross Canyon), inwhich structural effects of a num­ber of different types of soft in­clusions are being assessed, a (inaldecision as to the optimummethod of effecting stress reduc-

tion will be deferred until com­pletion of that project.

3, Structural backfill materialshould comprise a well-graded,well-compacted material contain­ing a range of grain sizes, in lieuof a poorly graded pea gravel.Rigid construction inspection pro­cedures are required to ascertainthat the material is placed andcompacted in shallow layers toavoid the inclusion of voids in thevicinity of the pipe walls and toassure adequate compaction.

4. The relatively low effectivedensities acting laterally on, thepipe. ranging from 28 to 45 per­cent of the vertical densities un­der Method A Backfill, suggest asa plausible effective .density dis­tribution 140 pef:42 pef (vertical:horizontal) for round pipes.

5. Considering the almost totallack of observed distress in the

ACKNOWLEDGEMENTS

The research work describedherein was sponsored and par­tially financed by the UnitedStates Department of Transporta..tion, Federal Highway Administra­tion. The contents of this paperreflect tbe views of tbe writerswho are responsible for the factsand accuracy of the data pre­sented herein. The contents donot necessarily reflect the officialviews or policies of the CaliforniaDepartment of Transportation orthe Federal Highway Administra­tion. This paper does not consti­tute a standard, specification, orregulation.

Appendix - References

1. Davis, R. E., Bacher, A. E.,"Structural Behavior of a Con­crete Pipe Culvert-Mountain­house Creek-(Part 2)/' Cali­fornia Division of Highways Re­port FHWA-CA-ST-4121-75-S;September. 1975.

CONCRETE PIPE NEWS/93

­,

••I,

. ,

Rigid Culvert Tests - Mountainhouse CreekPart III

This is the final article of a series onan experimental installation of pre·cast concrete pipe by the CaliforniaDepartment of Transportation. Fullreports of the research are availablethrough the National Technical/n­formation Service.

byAlfred E. Bacher

Senior Bridge EngineerCalifornia Department of Transportation

andAlbert N. Banke

Associate Bridge EngineerCalifornia Department of Transportation

Based on the results of Moun-tainhouse Creek Reinforced

Concrete Pipe Research, Part 1and Part 2, various changes havebeen made in the design criteriaand the specifications for RCP cul­verts by CALTRANS as discussedin the following paragraphs.

,;,

Load Factor DesignLoad factor design is presently

specified for all special reinforcedconcrete pipe designs by theCALTRANS Bridge Planning andDesign Manual. Volume 1 of thismantlal states in the table of Fac­tors for Service Load Design, Serv­ice Load Design is "not applicablefor culvert design. Use Load Fac­tor Design."

CONCRETE PIPE NEWS/l07

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o

o

,MOUNTAINHOUSE CREEK RCP - ZONE II

I METER NUMBER 12I OPRESSURES AT SOIL- CONCRETE INTERFAC~,I I,

II I 0

II, bI I, I, ,I 0 II I,

0 ,, ,,1

I~I

~ ~I

0 '~,

0::1 01UI 0:: ,,0::, "","', ,i!), !;;I, I-,I :],-" -,;;:, "-,,

'1

X' )(,"', "',:;, ::E,

....' ""....' !2'

BO 100 120 6 12

I DEPTH OF MONTHS AFTERFILL AT COMPLETION OFll. ROADWAr EMBANKMENT

Figure 1. Soil Pressure Meter at Zone 1, Mountainhouse Creek, Part II.

Figure 2. Comparison of Experimental and Theoretical Moments,Mountainous Creek, Part II.

EXPERIMENTALMOMENT

+ EXPERIMENTAL MOMENTBASED ON SR-4 STRAINGAGE.

EIlEXPERIMENTAL MOMENT­AVERAGED.

-THEORETICAL MOMENTBASED ON OBSERVED SOILPRESSURES. NEUTRAL POINTANALYISIS.

THEORETICAL MOMENT

EXPERiMENTAL AND THEORETICAL MOMENTSMOUNTAINH OUSE CREEKZONE 1-77' OVERFILL

TOB/CONCRETE PIPE NEWS

Two Bands of loadingTwo bands of loading are spe­

cified for the design of RCP cul­verts. Portions of CALTRANS De­sign Specifications read as fol­lows:

"Earth pressures or loads onculverts shall be computed or­dinarily as the weight of earthdirectly above the structure.

Earth pressures on rigid culvertsare to be computed for the fol­lowing two conditions:1. Earth load (vertical) of 140

pounds per cubic foot andan equivalent fluid pressure(lateral) of 42 pounds percubic foot; and,

2. Earth load (vertical) of 140pounds per cubic foot andequivalent fluid pressure (lat­eral) of 140 pounds per cubicfoot.

An approved analytical tech­nique based on the principlesof soil mechanics and soil-struc­ture interaction may be used inlieu of the above specificationsfor rigid and flexible culverts."Variations in effective densities

are illustrated in Parts 1 and 2 of

~I

BACKFILL

NOTE t: 30~ MINIMUM UP TO45' 0 0, THEN 2/30, BUTNO MORE THAN 60"REQUIRED,

SOIL CEMENT BEDDING

0.0'

SOIL CEMENT BEDDING

EXCAVATION BACKFILL

METHOD 2 - RCPSO'MAXIMUM OVERFILL

EXCAVATION BACKFILL

EXCAVATION

GRADING PLANE

0.3'

SAND BEDDING

IN TRENCH

EMBAr~KMENT CONSTRUCTEDPRIOR TO EXCAVATION

2'

SAND BEDDING

IN EMBANKMENT

EXCAVATION B~CKFILL

IN TRENCH

METHOD 3 - R C P80' MAXIMUM OVERFILL

IN EMBANKMENT

SHAPED BEDDING

2'

EXCAVATION BACKFILL EXCAVATION IBACKFILL

ORIGINAL GROUND OR GRADING PLANE

METHOD 1- RCP20' MAXIMUM OVERFILL

EXCAVATION BACKFILL

Figure 3. Concrete Details, Concrete culverts, CAL TRANS Standard Plans.

EXCAVATION I BACKFilLk (jRi'3INAl GROUND OR

;:1.i'f.i=7777m "

EXCAVATION

these papers. Zone 1 more nearlvconforms with the 140V:42H load­ing, while Zones 7 and 8 havehigher lateral pressures that ap­proximate the 140V:140H loading.It is apparent that CALTRA S'specified bands of loading pro­vide a range of design loadingsthat essentially satisfy the ob­served variations in effective den­sity about the RCP culvert periph­ery.

Also of consequence is the es­sential linearity in loading on RCPculverts, using Method A. A typi­cal fill height versus pressure plotat Zone 1 is shown (Figure 1). Theloading is essefllially linear fromzero to the completion of the fillat the centerline of freeway. Theinstrumentation which includedSR-4 strain gages and soil pres­su re meters, revealed good cor­relation between the internalstrains and external strains-de­spite problems encountered in theinstrumentation (Figure 2). Asshown in Zone 1, the experimen­tal moments based on SR-4 straingages, and the theoretical mo­ments, derived from soil pres­sures, have excelle?t correlation.

A key issue in all of our culvertresearch has been effective den­sity increases subsequent to fillcompletion. It is a significant fac­tor in flexible culvert design.However, no such increase hasbeen witnessed on these RCP re­search project5. All zones indicatinsignificant ch'j.J1ges in effectivedensity from 12 to 31 monthsafter fill completion.

There was also asymmetry ofloading on all zones similar tothat experienced on all of ourother culvert research projects.It is apparent shear forces are act­ing on the culvert-soil interface.Our ongoing RCP research projectat Cross Canyon includes Cam­bridge meters which have the ca-

--------- .J pability of measuring these shearvalues as well as the normal pres­sures.

CONCRETE PIPE NEWS/lOg

Method A, Three Types ofBedding and Backfill

Method 1 permits usage ofhigher strength concrete pipewith roadway embankment back­fill (Figure 3).

Method 2 permits placing thereinforced concrete pipe on a flatbedding. Embankment is placed aminimum of 'hd or 5-foot maxi­mum depth prior to excavation.The structure backfill is thenplaced to this same depth. Road­way embankment backfill isplaced over the upper portion ofthe reinforced concrete pipe.

Method 3 represents a modifi­cation of previous CALTRANSbedding and backfill require­ments. Three types of beddingand backfill are shown. Inappre­ciable differences between be­havior of the pipe with shaped

bedding, or bedding on a layer offine sand, indicate the advisabilityof offering these two options asalternatives. The recent success­ful application of a soil cementbedding makes this technique avalid third option. A maximum49-foot overfill formerly allowedfor 36000 pipe, has now beenincreased to an 80-foot overfillusing this method of installation.

It should be emphasized thatnot all of these methods may nec­essarily be included on a specificculvert. The selection of culvertalternatives for a given site is de­termined by the design engineer.

Method B BackfillResearch on reinforced con­

crete arches and steel structuralplate pipes, has conclusivelyshown that soft inclusions should

not be applied to such culverts.CALTRANS at one time permittedMethod 8 for reinforced concretearches, reinforced concrete boxes,and reinforced concrete pipes.Based on experience, and re­search, we now advocate its ap­plication to RCPs only (Figure 4).

The maximum peripheral effectivedensity on Method B RCP testzone was half of that observed on

Method A test zones (Figur" 5,Part 2). Significant reductions inall peripheral effective densities

were effected by placement of

baled straw over the pipe. Por­tions of CALTRANS StandardSpecial Provisions read as follows:

"At locations shown on theplans, baled straw backfillingoperations shall consist of plac­

ing a layer or layers of baled

EXCAVATION BACKFILL

IN EMBANKMENT

METHOD B

Figure 4. Delineation of Method B Backfill.

TABLE -(-851.6

STHENGTtl AND uses IJF REnJFORCED CONCRETEPIPE FOR DIAMETERS FROM 12" TO 103"

I. Conforms,to AASHTO Designr:tion M-170.2. S~eci(J1 str·.:;ngih-cracking D-Iond shall be used in ordering

specifying pipe

3. Interpolaled or extrapolated.

4. Cover heights exceeding 100' shall be considtHed a

speciol desIgn.

5. For slflgre pipe installations only - see Standard

Speciol ProviSions for Method B Backfill

requirements.

NOTES.

•~Ma~imum Height of C.over (Ft.)ii: -

..?-

~- . Method Bcb _b Mel hod A Bac:kfillBackfill {51· - 0 o 0

~ 0 E 0· ~~ -~ Method Method Method-" uo 30u I 2 3

" 1,000 1,500 6 16 26 39IN 1,350 2,000 8 20 32 48

1,700 (2) 2,500 (3) 10 25 38 57

IV 2,000 3,000 12 28 45 682,500 l2l 3,000 (3) ,4 35 56 84

V 3,000 3,750 17 42 68 100

3,600 f21 4,200 131 20 50 80 (NOTE 4 )

Figure 5. Reinforced Concrete Pipe Overfill Tables takenfrom CAL TRANS Highway Design Manual.

IN TRENCH

EXCAVATION BACKFill

PIPE LAYERSDIAMETERS OF STRAW

TO 3' I

3' - 6' 26' AND GREATER 3

110/CONCRETE PIPE NEWS

straw over double cage rein­forced concrete pipe. Themet~od of installation shall beas follows:1. Install the pipe and backfill

to 0.5-foot over the top of

the pipe.2. Place baled straw over the

pipe to a width at least equalto the outside diameter ofthe pipe and to a depth, inlayers, as follows:a. One layer for pipe diam­

eter less than 3 feet.b. Two layers for pipe diam­

eters 3 feet to 6 feet.c. Three layers for pipe diam­

eters 6 feet and greater.

3. Place roadway embankment,in accordance with the provi­sions in Section 19-6, "Em_bankment Construction:' ofthe Standard Specifications,on each side of the straw tothe level of the top of thestraw.

4. Complete the roadway em­bankment.

The binders on the bales ofstraw shall not be cut."

Linear Foot PaymentA portion of CALTRANS Stand­

ard Special Provisions spells outthe linear foot payment proce­du res:

"The quantities of structure ex­cavation and structure backfillinvolved in excavating andbackfilling culverts will not bepaid for as separate items. Fullcompensation for structure ex­cavation and structure backfill,for controlling and removingwater from excavations and forfurnishing and installing or con­structing all cofferdams and allother facilities necessary to theoperations 'and their subse­quent removal, if required bythe E~gineer, shall be con­sidered as included in the con­tract prices paid per linear footfor the various sizes shapes, ,and kinds of pipe culverts or

per cubic yard for the concreteand per pound for the bar rein­forcing steel involved in con­structing reinforced concretebox and arch culverts, which­ever applies, and no separatepayment will be made there­fore."

Revised Rep Overfill TablesThe CALTRANS Highway De­

sign Manual has recently been re­vised. Allowable overfills for RCPhave been increased approxi­mately 60 percent. Portions of thesection relating to usage of rein­forced concrete pipe now reads:

"7-851.6 Strength Requirementsfor Reinforced Concrete Pipe."Table 7-851.6 gives the maxi-

mum height of overfill for rein­forced concrete pipe, up to andincluding 108" diameter, usingthe backfill method specified inStandard Specifications Section,Structure Backfill, and is referredto as Method A Backfill. MethodB Backfill requirements are in­cluded in the Standard SpecialProvisions and provides for plac­ing baled straw over a pipe thathas been backfilled up to 0.5-footover the top of the pipe as speci­fied in Standard SpecificationsSection, Structu re Backfi II. MethodB backfill method is for use onsingle pipe installations only. Anyplan to utilize Method B backfillor any other culvert backfillmethod that varies from the speci­fied Method A backfill shall besubmitted to the Office of Struc­tures Design for an evaluation ofthe structural adequacy of theproposed installation. The D-Ioadrequirement for maximum heightof overfill is based on the crack­ing D-Ioad producing an O.Ol-inchcrack 12 inches long under thethree-edge bearing test called forin AASHTO Designation M-170.Pipe shall be ordered by thecracking D-Ioad shown in thetable.

The designer should be awareof the premises on which the

table is computed as well as itslimitations. Table 7-851.6 presup­poses:o That bedding and backfill sat­

isfy the terms of the StandardSpecifications, the conditions ofcover and pipe size required bythe plans.

o That a small amount of settle­ment will occur under the cul­vert equal in magnitude to thatof the adjoining material out­side the trench.

o Subexcavation and backfill asrequired by the Standard Spe­cifications where unyieldingfoundation material is encoun­tered.Table 7-85;1'6 gives height of

cover based on conditions nor­mallyencountered.

AcknowledgementsThe initiation of the RCP re­

search at Mountainhouse Creekand the determination of designparameters to be considered waswith the full cooperation and par­ticipation by the California Pre­cast Concrete Pipe Association. Itwas a mutual involvement by bothCPCPA and CALTRANS.

Walt Creasman, Ameron, andJoe Zicaro, HydrcJ Conduit Cor­poration, in particular, contrib­uted significantly to the researchproposal. More recently, ErnieRogers, Managing Engineer, Cali­fornia Precast Concrete Pipe As­sociation, and Tom Breitfuss,Hydro Conduit Corporation, havebeen instruriiental in the impld­mentation aspEkts.

Appendix-References

1. CALTRANS Bridge Planning andDesign Manual, Volume 1

2. CALTRANS Standard Specifications,january 197B

3. CALTRANS Standard Plans, March1977

4. CALTRANS Standard Special Provi-sions

10.1S, 1-3-7B Method B Backfill19.10, 4-10-78 Culvert Excavationand Culvert Backfill

S. CALTRANS Highway Design Man­ual, April 197B

CONCRETE PIPE NEWS/111