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The Kuparuk River on the NorthSlope of Alaska is a typical northernstream with water flows of less than5,000 cubic feet per second (cfs) formost of the year, but large springbreak up floods that can exceed140,000 cfs extending over a 2 milewide flood plain. In addition to thistremendous amount of water, large 5thick, fresh water ice floes also occurduring spring floods. To supportdevelopment of the new Kuparuk oilfield, an access road was built acrossthe Kuparuk River in the late 1970sat a location where the flood plain isapproximately 10,000 wide.

Economically bridging this floodplain for the spring break up flood

Modern Steel Construction / July 2000

Kuparuk River

Submersible BridgeNorth Slope of Alaska

Special Award: Special Purpose

Jurors Comments

Bold application of steel...a structure that must

endure extremely heavy vehicle loads and face

one of natures most hostile environments:

arctic cold, large-scale ice flows and

overtopping spring floods.

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has presented a design challengesince the original construction of thisaccess road.

For nineteen years, the gravel road atthe east and west channels of theKuparuk River was breached annuallyand allowed to wash out during springbreakup, resulting in a six to eight weekroad closure interrupting access to theKuparuk oil field. Historically, the cost ofpermanent bridges to provide access tothe field were found to be cost prohibi-tive due to extreme environmental condi-tions, gross vehicle loads of up to4,000,000 pounds and the riverhydraulics in which large water flowsmust be passed during the spring floods.The innovative solution consists of sub-mersible bridges in combination withpaved roadway sections that are allowedto overtop during peak spring break-upfloods.

The new Kuparuk River east (210long) and west (150 feet long) channelsubmersible bridges, completed in 1999,reduce the closure period of this criticalroad link to a maximum of one week peryear and eliminate the need to annuallyreconstruct the road. The cost savings ofmore than three million dollars per yearin oil field operational and advancedmaterial purchase costs will pay for theproject cost i n four years.

The crossings pass the peak springbreakup flows (typically more than seventimes greater than summer flows)

through and across the existing SpineRoad by using a combination of weldedsteel submersible bridges and paved lowwater roadways. The short-span, stout,welded steel structures are elegant intheir simplicity and are a practical, cost-effective answer to permanently crossinglarge, dynamic arctic coastal plain rivers.

Design and Construction

The bridges are designed to supportany oil field vehicle currently in opera-tion on Alaskas north slope. The largest

of these vehicles weighs approximately3.8 million pounds while others havemaximum wheel loads that exceed 370kips (185 tons). The entire load carryingcapacity of the bridge is provided by thewelded steel structure. The concrete deckwas provided as a driving surface, for lat-eral buckling support of girders, and toensure composite action for horizontalloads and lateral ice loads.

Environmental loads for the bridgeconsist of wind, seismic, river current,buoyant and river ice loading. For thisdesign, wind, seismic, current and buoy-ant forces were insignificant when com-pared to the ice loading. Design ice

Modern Steel Construction / July 2000

thickness was 52 of hard structural i ce(5 total nominal ice thickness) that iscapable of imposing tremendous loads onthe bridge deck and ice breakers.

The bridge substructure consists oflarge diameter, heavy wall and weldedsteel pi pe pil esthe onl y practicalmethod to support the massive vehicleloads. Each pile bent, spaced at 30 con-sists of four vertical 36 diameter, 1 wallAPI 5LX-52 pipe piles driven to 80 pen-etration to support the heavy vehicleloads and provide lateral support for iceloads. Required vertical capacity (designload) of each pile was 750 kips (375 tons)which was easily achieved using aDelmag D-62 diesel impact hammer,rated at 165,000 foot-pounds of energy.Each in stream pile bent has a steel icebreaking pipe installed at 45 degrees onthe upstream side. When impacted by anice floe, this design will fail the ice sheetin bending, rather than crushing, sub-stantially reducing the lateral force on asingle ice breaker from approximately750 kips to 320 kips. This unique designsaved time and cost by reducing the iceloads to the structure, providing reason-able tolerances for field fit-up and utiliz-

ing a vertical pile in lieu of more expen-sive and impractical (for installation inhard permafrost) batter pi les.

The pipe piles were slotted at cut offto receive the steel box pile cap. The slot-ted pile to pile cap connection providedboth a full moment connection for lateralload resistance and the load transfer forvertical loads, eliminating the need forbearing sti ffeners. Cost saving 1 interi-or bearing stiffeners attached to the topflange of the pile cap also eliminated theneed for bearing stiffeners in the girders.

The bridge abutment design utilizesopen sheet pile cellular structures, a

rugged and proven innovation utilizedthroughout the Pacific Northwest. Theseabutments are constructed in a circulararc in which the sheet pile cell remainsunclosed beneath the roadway. As isfound in closed cellular structures, hoopstresses are generated between the sheetpiles, but with the open cell abutmentsthe hoop stress is resisted solely by soilfriction along at the tail ends of the struc-ture. For these submerged structures, thetail ends of the central cell (tailwalls)were protected from scour by inclusion ofshort wingwalls on the upstream anddownstream sides. Wingwalls were con-

structed in a circular pattern and behavein the same manner as the central sheetpile cell. This abutment type is not scoursensitive since sheet pile embedment atthe streamside face is not required forthe abutment stability.

The submersible bridge design utilizes30 spans that allow an extremely shal-low, high-capacity deck and girder sys-tem. The superstructure consists ofeleven 22 deep steel plate girders at 3spacing encased in cast-in-place structur-al concrete. The girders were constructedusing ASTM A572, Grade 50 material

meeting Charpy V-notch impact criteriaof 15/12 (avg./min.) foot-pounds at -50F for cold weather performance.

The steel structure also served asformwork for the superstructure concreteby eliminating the need for falsework andminimizing on-site construction time andcosts. A 26 diameter fabri cated steelhalf-pipe deck nose is welded to the edgeof the upstream girder to provide a roundsurface that further limits ice crushingloads on the deck. The deck nose is fittedwith guardrail support pipes that arewelded to the outside girders. A rolled

plate welded to the downstream girder

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Project Team

OwnerPhillips Alaska, Inc.

DesignerPeratrovich, Nottingham, &Drage

Steel FabricatorsJesse Engineering Company

General ContractorAlaska InterstateConstruction

provided the concrete formwork on theother side of the bridge. Steel plateplaced on the bottom flange of the plategirders and attached with intermittent fi l-let welds served as the underside con-crete form for the bridge.

The bridge design required an easilyremovable guardrail for the crossing ofoilfield vehicles up to 60 wide. Weldedsteel pipe sleeves at the edges of the deckprovide support for the removableguardrails. The identical sections ofguardrail, each fourteen feet long, areeasily removed and replaced as the pipelegs slide into the pipe supports in thebridge deck. This system has receivedmuch praise for its ease of use, especiallyin extremely cold weather.

Fabrication

Fabrication of welded steel bridge

components was a critical element to thesuccess of this project. All of the compo-nents of the bridge were shop fabricatedinto sections that were easily transportedto the site to minimize costs. Both thedesign engineer and general contractorworked carefully with the fabricator toassure that all fabricated pieces would beeasily erected in the field. Careful matchmarking of each fabricated member andthe fabricators careful attention to detailallowed the structure to be erected intemperatures averaging -30 F and windspeeds of up to 20 mph without requiringfield modifi cations.

Conclusion

By utilizing a combination of durable,submersible bridges and paved low waterroadways an innovative solution wasdeveloped to provide reliable access tothe Kuparuk oil field for 51 weeks out ofthe year. The unique design kept the costof the project within reasonable limits forcrossing two river channels in a floodplain over two miles wide. The projectwas designed and constructed on timewith a tremendous cost savings over tra-

ditional, elevated bridge designs. Thissuccessful design promises to serve as amodel for future expansion of the infra-structure on the North Slope of Alaskaand in other climates around the world.