Design-Construction Feature

Giant ,segmental Precast

Prestressed Concrete Culverts

R. (Ray) H. Hebden, PESenior EngineerT. M. Thomson & Associates Ltd.Consulting Engineers and ForestersVictoria, British Columbia

A Canada

Three creek crossings of a new free-way in southwestern British Co-

lumbia (B.C.) have recently been corn-pleted using precast prestressed con-crete to form buried arch culverts (Fig.1). The concept of precast concrete cul-verts is not new, but these structures arenovel in terms of size and constructiondetails.

The location of the project is shown inFig. 2. The new highway was con-structed to provide a better route fromthe coast to central B.C. and the rest ofCanada than provided by the previousHighway 1 through the Fraser Canyon.

Note: This article is based on a paper presented atthe International Conference on Concrete in Trans-portation, Vancouver, British Columbia, September1986, cosponsored by the American Concrete in-stitute and the Canadian Portland Cement Associa-tion,

It was constructed on an acceleratedschedule to be open in the spring of1986 for EXPO 86 in Vancouver.

The particular section of highway onwhich these crossings are located isroughly in the middle of the CascadeMountain Range as it extends into theCoast Range in B.C. The elevations ofthe crossings are between 750 to 900 m(2400 to 3000 ft) above sea level. Thespecial physical characteristics of thetopography, the logistics of working inthis somewhat remote site and the con-struction schedule all influenced thedesign of the structure for the crossings.

A profile of the arch culverts that wereconstructed is shown in Fig. 3. At 20 m(65 ft) span, the opening width is ap-proximately 2 to (6.5 ft) wider thanwhat is claimed to be the world's largestARMCO Superspan multiplate recentlycompleted in Ontario. The opening

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Describes three highway creek crossings recentlycompleted in British Columbia using large spansegmental precast post-tensioned concrete culverts.Includes an overview of project constraints, designand construction aspects, and cost factors, withcomments on the circumstances where it may bebeneficial for engineers to consider large buriedstructures as alternatives to short span bridges.

height is 6 in (20 ft), and the length ofeach culvert is generally 65 m (213 ft).The structures of the three crossings areidentical, with the exception of a slightvariation in length to suit differing skewangles.

The reason for the unusually largedimensions of the buried structures wasa compromise between conflicting re-quirements and constraints. T.M.

Thomson & Associates Ltd. (THOM-SON) was under assignment to the B.C.Ministry of Transportation and High-ways (MOTH) for the design of this 8 km(5 mile) section of highway in the Bos-ton Bar Creek Valley.

Initially, most design considerationspointed to a conventional multiplateculvert to direct the creek under thehighway. Hydrological assessment indi-

Fig. 1. Completed arch culvert with vehicular traffic.

PCI JOURNALNovember-December 1986 61

cated the design flood discharge of thecreek could be handled by a 7 to 8 m (24to 27 ft) diameter full circle pipe. Also, itwas very desirable to avoid bridge decksat this mountainous location. The highpotential for icing caused the MOTH toimpose restrictions of no curves or spi-rals as well as minimum grades to bedesigned for bridge decks.

There was considerable difficulty infinding a satisfactory alignment in thenarrow creek valley that net thesecriteria for bridges. In addition, the ob-vious advantages of lower maintenancestrongly favored the selection of buriedstructures for the crossings. A furtherand very strong factor was also the factthat a multiplate culvert can usually beconstructed at a cost considerabl y lessthan a bridge.

Environment factors played an im-portant role in the planning stage anddesign process. Boston Bar Creek is aprime habitat for trout, and fisheriesauthorities considered even a 7 m (23 ft)

diameter pipe to be a "dark hole" bar-rier to migrating fish when the lengthwas as long as 65 in (213 ft). In addition,there are some active debris torrentpaths and avalanches intersecting thecreek immediately upstream of thecrossing sites, and large stumps and logswere seen in the creek bed. Fisheriesconcerns and the ominous piles of de-bris in the creek tended to steer the se-lection of structures for the crossing tobridges.

The concept of remaining with aburied structure but extending its size topass the expected debris and to allowsufficient light for the fish was agreed toby the MOTH. The 20 m (65 ft) span wasselected to allow passage of logs atflood. With a circular arch shape givingan opening height of 6 n (20 ft),fisheries' approval was received basedon a limiting length of65 m (213 ft).

The first concept for such a large cul-vert was an enlarged version of the cor-rugated multiplate, open bottomed cul-

COQUIHALLA HIGHWABOSTON BAR CREEK SECTION

e

Fig. 2. Map showing location of project.

ME

62

WAY SURFACE-

I I I ^ I ^HIGHWAY SURFACE - MINI I OV R

ice -PRECAST CONCRETE ARCljCULVER SEGMEIJT

CO CRETE EAOWALL ANDWING WA L

/CREEK BERM$NMENT n/ il,, ..'

CONCRETEt ^ FOUNDATION

10317 _i_/ 10

Fig. 3. Profile of the arch culverts. Note: 1 m = 3.28 ft.

vert. However, there were several diffi-culties with this idea. First, there wasno precedent of such a large multiplateever being constructed.

After some experience with previousdifficulties in erecting large multiplatearches, the MOTH, the geotechnicalconsultant and THOMSON felt thatsuch a corrugated steel arch would becritically sensitive to the care taken inbackfill, the quality of the backfill, andwould require a thick cover over thecrest for live loads. These large, flexiblearches essentially achieve their strengthfrom a thick soil arch. This would re-quire extremely close supervision andpossibly a great quantity of importedbackfill.

The second objection was doubt as tothe ability of a supplier to meet the de-livery schedule for three large struc-tures. Finally, to construct a large mul-tiplate steel culvert, it would have beennecessary to provide closely spacedtemporary supports under the platesuntil a minimum layer of backfill wasplaced. This was impractical, since

there was insufficient space in the val-ley to relocate the creek around the con-struction site.

A buried bridge type of structure, on astraight span covered with a layer of fill,as well as a reinforced steel plate arch,were also briefly investigated but wereconsidered uneconomical.

An initial investigation of the use ofcast-in-place concrete for the arch cul-verts was carried out. The points in favorof cast-in-place concrete were the con-stant cross section being suitable for atravelling form and the relative freedomof slab thickness. However, this free-dom was not really a great advantage,since a high quality, dense concrete wasdesired, in any case, for durability.Cast-in-place concrete was eventuallyrejected due to the fact that most of thecasting work would have had to takeplace in the worst winter weather con-ditions, and the sand available locallywas felt to be too coarse for a highstrength, dense concrete.

Precast concrete was finally chosenfor its ability to guarantee excellent

PCI JOURNAL/November-December 1986 63

quality durable concrete and the factthat production could take place in con-trolled plant conditions during thewinter. There appeared to he a betterprobability of achieving the secheduledcompletion date with precast arches asopposed to cast-in-place concrete. Gooddimensional control and close super-

vision were also advantages foreseen forprecast concrete. A preliminary costcomparison showed precast concrete tobe slightly more economical, but as thedegree of confidence in the unit priceswas not high for these fairly unusualstructures, the recommendation for pre-cast concrete was based on technical

PRECAST CONCRETE ARCHCULVERT SEGMENT

DRAIN---- - \ ,-SHIM AS REQUIRED FOR ALIGNMENT

--GROUT FOLLOWING SEGMENTERECTION AND ALIGNMENT

OYWIDAG THREADED ANCHOR BAR

GALVANIZED ANCHOR SLEEVE

CONCRETE FILL PLACED IMMEDIATELYPRIOR TO PRECAST ERECTION

Fig. 4. Arch footings showing base connection of precast segments.

Fig. 5. Finite element mesh model of soil-culvert interaction.

M

W

ZX 100 E

EZ Y

-100 W20

LIVE LOAD g

IiiU

0 5000LL E

-J 2a_y 3000Xa

I

Z EW2 E

O y2

Fig. 6. Design moments and axial forces due to backfill and live load.

considerations.The size of the arch dictated that the

arch be segmented transversely as wellas longitudinally. When the arch was di-vided into halves with a joint at the top,each half arch segment would almostexactly fit a standard flat bed trailer forlegal loads.

Design ConsiderationsDue to the tight time constraint, it was

necessary to proceed with the footingsin the spring of 1984, before the detaileddesign of the arch slabs was completed.The width of a solid cast-in-place stripfooting was designed for the verticalloading. The depth was dictated bymaking the underside of the base at alevel below the expected scour depth. Adiagram of a typical footing is shown inFig. 4.

A large portion of the length of eachfooting was to be on solid rock but someareas required bearing on granular ma-terial. For these areas, a bearing pres-sure of 950 kPa (10 tons per sq ft) wasused,

A rigid connection between the arch

slabs and the footings was employed.This was accomplished by setting large55 mm (#18) diameter Dywidag, Grade400 (60 ksi) thread bars in voids cast intothe footing, as shown in Fig. 4. Initially,it was anticipated that the base of theslabs would hear on a strip of elas-tomeric bearing material, creating es-sentially a pinned base connection.However, subsequent analysis showedthat with the relatively thin slab sectiondesired for precast economy, base rota-tions were quite high.

Joint rotation in this case is caused bymoments built up as backfill is addedand compacted from the footing to thecrest. The rotation and a high axial loadresulting from the arch effect made de-sign of a suitable elastomeric bearingimpractical. A steel rocker arrangementwas rejected as being costly and too dif-ficult to protect against corrosion, so afixed base arrangement was adopted.

Analysis of the arch slabs was doneusing a finite element computer simula-tion to model the interaction of soil andstructure, The mesh of elements isshown in Fig. 5, with the heavy line tothe left indicating one special case in-

PCI JOURNAL^November-December 1986 B5

ALL VOIDS GROUTED WHENSTEEL IN PLACE

is

DYWIDAG THREADEDBAR CONNECTION —

PRECAST CONCRETE ARCUt.VERT SEGMENT

00

20 GAP

740

00

0 e

STEEL PIPE SLEEVE

POST TENSION DUCTS

NEOPRENE RUBBER STRIP

Fig. 7. Crest connection of arch culvert.

vestigated with a rock boundary near thestructure. The particular program usedhad the capability to account for inelas-tic behavior so limit state designmethods were used. Load factors wereapplied to soil density and live loadthat were generally consistent withCanadian bridge design codes. Assis-tance with the computer analysis as wellas in the appropriate assignment of soilparameters was received from the CivilEngineering Department of the Univer-sity of British Columbia.

A diagram of moments and axial forcesresulting from the analysis is shown inFig. 6. The somewhat unusual momentpattern seemed to develop as backfilllayers were brought up in a staggeredfashion. A 1 m (3.28 R) difference in levelwas used as a pessimistic estimate ofgrade control, Once the backfill clearedthe crest, the moment pattern stayednearly constant and the axial force in-creased with successive layers of fill.This agreed fairly closely with the clas-sic compression arch under uniformload theory.

The values for moments and axialforces are for the worst condition of thethree crossing sites with 9 m (30 ft) of fillover the crest. The other two site condi-tions were for a shallow cover of 2 m (6.5ft) over the crest which was simply anintermediate case in the analysis.

It was anticipated that during con-

struction there would be a need to haulfill material over two of the sites. To ac-commodate this, a live load of a two-axle 93,000 kg (100 ton) off-road haultruck was checked during the computersimilation, when the fill cover was 2 m(6.5 ft). The moment curve on the bot-tom of the figure is the maximum influ-ence of such a truck.

By comparison, it can be seen that themoments incurred by the heavy axleloads are much less significant than theeffects of the rising backfill. In fact, atthe crest the negative or hogging mo-ment does not change sign under thelive load so a crest joint with top steelonly, as shown in Fig. 7, was employed.The transverse connection bars at thecrest were also 55 m (#18) diameterDywidag thread bars at the same spac-ing as the base/footing connection. Thetop bars were grouted into pipe sleevesand were set but not stressed.

The slabs themselves were designedas plain reinforced sections under axialload with a ductility ratio of not less thanthree.

As can he seen in Fig. 7, the crestflange-type joint formed a significantridge beam when all of the arch pairswere grouted together. This ridge beamwas post-tensioned with four 15 mm (0.6in) diameter strands in each of eightducts. This post-tensioning served adual purpose. When cover was shallow,

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the stressed crest beam acted to spreadconcentrated axle loads parallel to theaxis of the culvert, thus assisting in thestructure's ability to handle concen-trated loads.

After the depth of fill increased to theextent that concentrated loads are dis-tributed by the fill cover, the longitudi-nal stressing served to secure the pairsof arch segments together against thespreading forces of soil friction. It wasdifficult to quantify the latter effect sothe stressing was sized on the basis ofthe beam action under the concentratedlive loads mentioned above.

All joints between precast arch slabsegments, and between segments andthe footing, were grouted except for thetransverse joints running between thefooting and edge of the ridge beam. Aplastic foam sealing strip was used inthis area. it was felt that there would bean advantage in keeping the whole cul-vert structure as torsionally soft as pos-sible for the instance that the end of onestrip footing went from a rock support togranular, and some differential twistingof the structure resulted.

Construction AspectsAs mentioned above, the strip footings

were constructed in the fall of 1984,when the arch slab design was beingfinalized and precast bids arranged. Theexcavation, footing construction and rip-rap work was performed by the roadgrade contractor under the direction ofthe MOTH. Riprap was initially used tocontain the stream with dykes betweenthe footing locations, to allow dewater-ing the excavation. When the footingswere cast, the riprap was moved to formscour protection. Fig. 8, below, showsthe completed footings and riprap scourprotection.

Casting of the arch slab segments wascarried out at a precast plant in Langley,B.C. in the Fraser Valley, approximately150 km (100 miles) from the site. The 148precast segments were cast from Januaryto March 1985. In order to meet theschedule required, three forms wereused on a one-day cycle for each. Theforms consisted of concrete archesthemselves, with steel side forms andsteel end block forms. The half archsegments could he cast "on the flat"

,r rig.

Fig. 8. Footing and scour protection for culvert.

PCI JOURNALI November-December 1986 67

since the base angle of each piece wasapproximately 30 deg to the horizontal.The low slump concrete mix used wouldrest at that angle. The forms are shownin Fig. 9.

Each segment was 12.2 m (40 ft) longon the chord of the arc, and 2.5 m (8.2 if)wide. The thickness was generally 250mm (10 in.) except at the base, wherethe thickness was flared to 350 mm (14in.) over approximately one quarter ofthe arc for the high negative moment inthis area. At 25 tonnes (27 tons), eachsegment would just fit onto a standardflat bed truck at legal load.

Specified concrete strength for theprecast segments was 50 MPa (7250 psi)at 28 days, but the actual averagestrength achieved by the precast con-tractor, APS (Architectural PrecastStructures Ltd.), was considerably higherthan this. The generous strength was re-quired for early handling capability,

Reinforcement was welded wire meshin equal size at both surfaces. This wasdone to bracket the close spaced wavesin the moment curves shown in Fig. 5,which were actually shifting slightly in

position with alternating backfill layers.The welded mesh also gave the advan-tage of short development length forthese moment waves and better reten-tion than bars for pulling out of tensionsteel on the inside surface of the curve.Supplemental bars were used for thehigh moment area at the footing.

The erection scheme worked out bythe precast contractor was quite ingeni-ous and effective. A steel truss framewas fabricated to span between insideedges of the footings with a width of onesegment. The truss, as can be seen inFig. 10, was designed to support the topcorners of a pair of arch segments andhad positions for two hydraulic handjacks on each side at the top. A 181 tonne(200 ton) capacity mobile crane was sta-tioned to the side of one footing andswung the precast arch pieces from thetruck to position on the footings and thetruss, as shown in Fig. 11.

When properly aligned, the hydraulicjacks supporting the crest edge of theunits were lowered, causing the verticalabutting surfaces of each pair to comeinto contact. The arch pair were then

Fig. 9. Plant casting of arch segments.

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Fig. 10. Erection truss used for mounting arch segment.

Fig. 11. Partially erected arch culvert.

PCI JOURNAL/November-December 1986 69

Fig. 12. Placement of arch base dowels.

self supporting. Following placement ofone pair of segments, the truss was ad-vanced along the footing to the next po-sition.

Erection started in April 1985 and wascompleted in June 1985. Initially, theerection crew could only achieve two orthree segments per day. Once familiar-ity with the sequence was gained, theyquickly advanced to an average of eightto ten segments per day (five pairs), witha maximum of eleven per day. The lim-itation on erection speed became theability to deliver precast segments to thesite on the busy construction road.

The precast segments were loweredover the base connection dowels asshown in Fig. 12. The large Dywidagbars were set in plastic concrete in 2(X)mm (8 in.) diameter voids immediatelyprior to placing the segment so that thebars could be shifted slightly to accom-modate tolerances. Steel shims wereused under the precast base and be-tween the base and the thrust surface onthe footings to control alignment. Whena pair of arches were in place and selfsupporting, the top connection bars

were installed.After all precast arch pairs were

erected, and prior to the grouting of jointsurfaces, strand groups for the post-ten-sioned tendons in the ridge beam weredrawn through interconnected ducts.Joint grout was excluded from theducts by neoprene foam pads glued tothe precast element at the plant in asimilar manner as strips set to seal thebottom and sides of grouted volumes.After several days curing of the jointgrout the strands were stressed andgrouted.

While the precast contractor wascompleting the finishing work andstressing the culverts, the grading con-tractor returned to complete the head-walls. Following curing of the headwallconcrete, the backfill operation was ini-tiated by the grading contractor.

Specifications for backfill under thegrading contract called for a 1.0 m (3.28ft) minimum thickness of graded, selectstructural backfill around the culvert.This layer, shown in Figs. 13 and l4, wasprimarily for protection of the surface ofthe concrete rather than a structural soilarch.

The backfill was placed and com-pacted in 460 mm (18 in.) lifts evenly onboth sides of the culvert. This is thesane procedure as generally used with alarge corrugated plate structure. The re-sultant stresses induced in the arch areprobably somewhat less than those pre-dicted by the 1.0 in (3.28 ft) difference inelevation side-to-side as anticipated inthe computer analysis. The completedgrade over one of the culverts is shownin Figs. 15 and l.

Concluding RemarksFrom the experience of this project,

large span precast segmental arch cul-verts are feasible and economicallycompetitive with bridges and othertypes of construction. Advantages of aburied structure such as virtual absenceof maintenance, no deck icing problems

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Fig. 13. Beginning of backfilling culvert.

Fig. 14. Later stage of backfilling culvert.

PCI JOURNAL November-December 1986 71

{ ' s .O.

3*--

Fig. 15. Completed culvert with vehicular traffic.

Fig. 16. Another view of completed culvert with vehicular traffic.

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and greater freedom of road alignment,compared to bridges, can he achievedeven in fairly remote areas.

Final costs of the Coquihalla culvertsversus the estimated costs for bridgesindicate that in shallower fill conditions,such as creek to grade separations of 6 to8 m (19.7 to 26.2 ft), the cost of a seg-mental precast culvert is approximatelythe same as for a bridge. However, forhigher fill situations such as creek tograde separations of greater than 8 m(26.2 ft), a segmental precast culvert canoffer significant savings over a bridge.The highest Ievel culvert of the three onthe Coquihalla project, with streambedto grade separation of 15 m (49.2 ft),was completed for approximately one-half the estimated bridge cost at theproject site,

The fixed contract (1984 prices) forthe supply and erection of the precastconcrete arches was $966,160 (Cana-dian) for all three culverts. This hidprice converts to a unit cost of $5222 perlineal meter. The approximate cost ofthe footings for all three culverts was$750,000. The structural backfill layeramounted to $90,000 with an additional$50,000 for general backfill.

The precast segmental culverts areinherently tolerant of high live loads incombination with shallow cover depth.They would be well suited to railwaycrossings and off-highway haul roadcrossings. Compared to corrugated steelplate culverts, the precast concrete cul-verts are much more tolerant of backfillquality and compaction, and are ex-pected to have a considerably longerservice life due to the dense high

strength concrete possible with precastsegments.

This project won a Special Jury Awardin the 1986 PCI Professional DesignAwards Program.

CreditsProject Name:

(1) Boston Bar Creek Culvert,Station 363 (Boulder site).

(2) Boston Bar Creek Culvert,Station 389 (Miranda site).

(3) Boston Bar Creek Culvert,Station 426 (Box Canyon site).

Owner:Province of British Columbia, Ministry

of Transportation and Highways,Victoria, British Columbia.T. R . Johnson, PE, Deputy Minister.

Design Engineer: T. M. Thomson & As-sociates Ltd., Consulting Engineersand Foresters, Victoria, British Co-lumbia.Structural Design: R. H. Hebden, PE.

Overall Project Manager: 'V. D. Cripps,PE.

Construction Supervisor: J. G. McCall,PE.

Precast Construction: APS (Architec-tural Precast Structures Ltd.),Langley, British Columbia.

Geotechnical Consultant: R. A. SpenceEngineering Inc., Vancouver, BritishColumbia.

Specialist Computer Assistance: Dr.D. Anderson, PE; Dr. N. Nathan, PE;Dr. P. Byrne, PE. All Professors ofCivil Engineering, University ofBritish Columbia, Vancouver, BritishColumbia.

PCI JOURNAL/November-December 1986 73