ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine
ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine
AvoidingLANDSLIDESby Kevin Harper, CaliforniaDepartmentof TransportationThe emergency projectrelocates a portion of U.S.Route 101 away from alarge landslide requiringtwo large bridges overthe South Fork Eel River.The large landslide extends over1000 ft above the river envelopingthe current highway. The slide isapproximately 3000 ft wide.Photos: © Caltrans.The relocationof U.S. Route 101required two highlevelbridges overthe Eel RiverAn emergency project to relocate aportion of the Pacific Coast Highway(U.S. Route 101) away from a largelandslide in northern California requiredtwo large bridges to span the SouthFork Eel River. Although the bridges arewithin a quarter mile of each other, theyare dramatically different structures. Thedifference in the bridge types resultedfrom the vastly different terrains at thetwo crossings of the South Fork Eel Riverthat snaked along in the shape of agiant “U” with a bridge over each leg.The southern leg of the river, with itswide banks, required a 1355-ft-long,three-span, segmental concrete bridgethat was 275 ft above the river. Thenorthern leg of the river, which passedthrough a narrow rock walled canyon,required a 581-ft-long, three-span, caston-falseworkconcrete arch bridge thatsits 150 ft above the river.Route 101 is the primary route thatprovides direct access to California’snorth coast for commercial truckingand recreational traffic. North of theSan Francisco Bay area, this highwayis considered the “lifeline of theCalifornia’s north coast.” There has beena recurring problem of landslides aroundConfusion Hill over the last decade,resulting in frequent road closuresand high maintenance costs. When amajor landslide occurs that closes bothlanes, the traffic south of Confusion Hillmay have to backtrack and detour anadditional distance of 250 miles. Duringthe past 10 years, over $33 million hasbeen spent on slide repairs.The project involves relocatingapproximately 1.5 miles of U.S. Route101 away from the active landslide. Thelarge ancient rockslide complex extendsprofileSouth Fork Eel River Bridges (Confusion Hill Bridges) / Leggett, Calif.Engineer: California Department of Transportation, Sacramento, Calif.Prime Contractor: MCM Construction Inc., North Highlands, Calif.Contractor’s Segmental Engineer: Finley Engineering Group Inc., Tallahassee, Fla.Concrete Supplier: Mercer-Fraser Company, Eureka, Calif.Form Traveler Supplier: AVAR Construction Systems Inc., Campbell, Calif.Post-Tensioning Supplier: Schwager Davis Inc., San Jose, Calif.Reinforcing Steel Supplier: Fontana Steel, Stockton, Calif.26 | ASPIRE, Summer 2008
upwards from the river for more than1000 ft, enveloping the current highwaythat is benched into the mountain about240 ft above the river. The landslidearea is approximately 3000 ft wide.Geotechnical studies have concludedthat the slide is progressively losingstrength and there is a high probabilitythat the complex will continue to movein the future. This highway relocationproject is an emergency project that isfully financed with federal emergencyrelief funds. The expedited deliveryof this $65.7 million dollar projectonly took 28 months from the initialplanning study phase until award of theconstruction contract.The complete project includes twobridges, two tieback retaining walls, anda large cut between the two bridges.The bid for the structures work was$49.4 million. The contractor startedwork in June of 2006 and is expected tocomplete the project in 2009.North BridgeThe smaller north bridge is a concreteinclined leg frame arch with a 229-ftcenter span and 175-ft end spans. Thisbridge type fits this particular site wellby meeting the requirement to keepthe piers out of the ordinary high waterlimits of the river while still maintainingbalanced span ratios. This configurationactually kept the piers out of the higher100-year water level, which simplifiedthe environmental process even more.The river at this site was containedwithin a relatively narrow canyonwith steep rock walls, which providedsuitable foundation material to anchorthe inclined piers. The superstructureof the bridge is a cast-in-place, posttensioned,two-cell box girder withsloping exterior webs. The box varies indepth from 13.8 ft at the piers to 5.9ft at the ends and midspan. The deckwidth is 42.8 ft and the bottom slabwidth varies from 16.8 ft at the piersto 24.7 ft at the ends and midspan.The bridge was cast on falsework thatwas up to 140 ft tall. The solid concretetapered piers are anchored into themountain in 17.5-ft-wide by 6.9-ft-highby 80-ft-deep mined shafts to developthe full probable plastic moment ofthe piers during a seismic event. Theshafts were excavated through very hardrock that required blasting, as well asweathered and fractured rock regionsthat required rock bolting for stability.Concrete compressive strengthwas specified as 6100 psi for thesuperstructure and 5100 psi for thepiers. The concrete strength required attime of post-tensioning was 3600 psi.The south bridge, which is 275 ft abovethe river is a cast-in-place concretesegmental box girder constructed by thebalanced cantilever method.Photos: © Jon Hirtz, Caltrans.The north bridge, which is approximately 150 ftabove the river, was built as a cast-on-falsework posttensionedconcrete box girder. The lower portion ofthe falsework (table top) can still be seen below thebridge as well as the temporary access trestle behindthe bridge.The total post-tensioning jacking forceapplied at abutment 1 for the full-lengthtendons was 8776 kips. The tendonsconsisted of 0.6-in.-diameter, 270 ksi,low relaxation strands.South BridgeThe larger south bridge uses a cast-inplace,segmental concrete box girderwith normal weight concrete. Likethe north bridge, the piers were notonly outside the required ordinaryhigh water limits of the river, but alsothe 100-year flood levels. The southbridge was selected to be segmentalbecause its height above the river wastoo high for economical constructionusing falsework. The span lengths of thebridge are 348 ft, 571 ft, and 436 ft.Segmental cast-in-place concrete / California Department of Transportation, OwnerDrilling Subcontractor: Pacific Coast Drilling, Petaluma, Calif.Earth Work and Mined Shafts: Ladd & Associates, Redding, Calif.Bridge Descriptions: Three-span, segmental cast-in-place concrete single cell box girder constructed by the balanced cantilever method andthree-span, cast-on-falsework post-tensioned concrete two cell box girder with inclined piersStructural Components: South Bridge: 68 concrete segments in spans of 348 ft, 571 ft, and 436 ft. North Bridge: Cast-in-place on falseworkwith spans lengths of 175 ft, 229 ft, and 175 ftBridge Construction Cost: South Bridge—$37 million; North Bridge—$9 millionASPIRE, Summer 2008 | 27
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AvoidingLANDSLIDESby Kevin Harper, CaliforniaDepartmentof Transportation<strong>The</strong> emergency projectrelocates a portion of U.S.Route 101 away from alarge landslide requiringtwo large bridges overthe South Fork Eel River.<strong>The</strong> large landslide extends over1000 ft above the river envelopingthe current highway. <strong>The</strong> slide isapproximately 3000 ft wide.Photos: © Caltrans.<strong>The</strong> relocationof U.S. Route 101required two highlevelbridges overthe Eel RiverAn emergency project to relocate aportion of the Pacific Coast Highway(U.S. Route 101) away from a largelandslide in northern California requiredtwo large bridges to span the SouthFork Eel River. Although the bridges arewithin a quarter mile of each other, theyare dramatically different structures. <strong>The</strong>difference in the bridge types resultedfrom the vastly different terrains at thetwo crossings of the South Fork Eel Riverthat snaked along in the shape of agiant “U” with a bridge over each leg.<strong>The</strong> southern leg of the river, with itswide banks, required a 1355-ft-long,three-span, segmental concrete bridgethat was 275 ft above the river. <strong>The</strong>northern leg of the river, which passedthrough a narrow rock walled canyon,required a 581-ft-long, three-span, caston-falseworkconcrete arch bridge thatsits 150 ft above the river.Route 101 is the primary route thatprovides direct access to California’snorth coast for commercial truckingand recreational traffic. North of theSan Francisco Bay area, this highwayis considered the “lifeline of theCalifornia’s north coast.” <strong>The</strong>re has beena recurring problem of landslides aroundConfusion Hill over the last decade,resulting in frequent road closuresand high maintenance costs. When amajor landslide occurs that closes bothlanes, the traffic south of Confusion Hillmay have to backtrack and detour anadditional distance of 250 miles. Duringthe past 10 years, over $33 million hasbeen spent on slide repairs.<strong>The</strong> project involves relocatingapproximately 1.5 miles of U.S. Route101 away from the active landslide. <strong>The</strong>large ancient rockslide complex extendsprofileSouth Fork Eel River <strong>Bridge</strong>s (Confusion Hill <strong>Bridge</strong>s) / Leggett, Calif.Engineer: California Department of Transportation, Sacramento, Calif.Prime Contractor: MCM Construction Inc., North Highlands, Calif.Contractor’s Segmental Engineer: Finley Engineering Group Inc., Tallahassee, Fla.<strong>Concrete</strong> Supplier: Mercer-Fraser Company, Eureka, Calif.Form Traveler Supplier: AVAR Construction Systems Inc., Campbell, Calif.Post-Tensioning Supplier: Schwager Davis Inc., San Jose, Calif.Reinforcing Steel Supplier: Fontana Steel, Stockton, Calif.26 | <strong>ASPIRE</strong>, <strong>Summer</strong> 20<strong>08</strong>
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