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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.
•
, ..," ...
.'
-
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.
'""'0'-.
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.
1/8" OIA. •U COlLS!FT 1ii
~~OG.
d!.> .. :.'~::'",(~~ .. :~. OG ~
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SELECTED ROCK ELlSANKMENTEIolBANKNfNT MATERIAL COv.PACTEO TO 9S\
TYPICAL SECTIoN
~IO ~O 0 0;:: l LAYERS,,::1 E.lIBANK.I,lENT OF r 1 ....
.... BATTER50ARO ••.• POLYSTYRENE EMBANKMENT •
o 20Til H UNTREATED 11. T COlIMERCLI.l r •.J a.F. PLA/iK l't CRADE COtlCRETE
Sil;no 8EDDING
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
1'/21%7±
-lS±- 6,/.-10%
+l'/a+l~a
+9±+S±+6%+6'/,
Horizon!.1
4S4430373918
789
101112
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-
~I
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, "
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'12191428It.,()~TlG
ZONE 12
ZONE 11
... ~'" S'.".,...
1 118 X n...,.., :'2
Horizontal and Vertical Diameter Changes asFunction of Overfill and Time After FillCompletion.
ZONE 9
WUU'I23'
ZONE 8
Figure 3.
HCJlIZO!'lUl;
-100 l)1OXl40~r.olO!w)'Il
t1tPTtl or fill l!'t'[R CJl)'tVtl(fl
Figure 4, Effective Densities at Time of Fill Completionand 31 Months Thereafter,
...
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A' .." C_.l..... 11 ....., •• A.... ~;"c_ ..
~.~CRI~t?0; ~3 U...-",,, ~,,~: ¥ ,..._.,
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tOIlE 9 - 155' OVERfiLL-- ....1i.".0I0t>~ .... _
~ ~.
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.
",/I
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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
.,,~
T
lON[ llOOll: $ l!:lOI[ • ZOhI( $ 20 ..[ •
'IT=' T
....
T
SlUE fUflNl$M[O ""1:
20000
T :
T
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~ ....
T
T
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H
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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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t
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.
'-0
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- all
.cocl=. ..
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ZOIl[ Z II-10-If lOlle, 11-10-&1[ ..U E .. ' rL[VATIG-H TTl .• fT [ ENT [L[VAnOIi 1T,., n'Ill HE lUll' l,lV(R e~C'''H 51.' " TILL He'\lllT QY[R (RO.... ,. , rT
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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.
.co..~. ..
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[ .."''' ....r .. ' [In.~ION TU' '7tI~L IIC:'''' !)"oCR eIlOW,. 11.1 n
lOIl[ I 1,-,0 - IT[""UU,r"T [l[VATION rlO.Z rT'Ill .. tit.' OV(~ (RO"" 51,1."
CONCRETE PIPE NEWS/91
•.
n. 11,
20Nf. 210;(I!llINf~NT II "VR1ll"f'.... 8"..0flU. rlf:l::;Hl l'V~lI Cl¥.·..l~ •.. 1<'.,()Hf. •. 8-1-fi:1
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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.
lON! y~
[,Mllf'H\I\lNl ELEwnl~ ... 1l~~
(IL:" Hf.IGHT CV(~, Cl1OI!N .•. l3Sl)'IIE•.• 6-1-68
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-
"" ,(I'lJfllirlM!NT ELtvRT 1~ ..•195filL HtH:{1t r.(1'1 t:MlI!l •.. 76l ... tE. .• 2·~-I;o
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,
,0"""103 ."
" . .~). >3
111' OV[/trll L20"'[ , _
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&I "H '9..... '''9.. II "OYT~S'Hf! ... ' '9......u9.
Rep (ZONES 1/:-6)
O·."
" ". ".."~J 47
COMPOSITE EFFECTIVE D~NSITIES-ME.THaD BCALIFORNIA DOT CULVERT ReSEARCH
D-~
, "-, ', . '. .., .
"
'0'", ..40 ~,," .
.. Ii ".
0'"
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IU' OVERFILL
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s·....,,"
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:,'. ..:.~ .. - ","UND LnER n
Figure 5. Effective densities - (based on unadjusted pressure)
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
o
50
40
K' 30
10
oo
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