confusion hill sum08

7/25/2019 Confusion Hill Sum08

1/726|ASPIRE, Summer 2008

An 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, cast-on-falsework concrete arch bridge that

sits 150 ft above the river.

profile SOUTH FORK EEL RIVER BRIDGES (CONFUSION HILL BRIDGES) / LEGGETT, CALIF.ENGINEER:California Department of Transportation, Sacramento, Calif.

PRIME CONTRACTOR:MCM Construction Inc., North Highlands, Calif.

CONTRACTORS 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.

by Kevin Harper, CaliforniaDepartmentof Transportation

AVOIDING

The relocation

of U.S. Route 101

required two high-

level bridges over

the Eel River

Route 101 is the primary route thatprovides direct access to Californiasnorth coast for commercial truckingand recreational traffic. North of theSan Francisco Bay area, this highwayis considered the lifeline of theCalifornias 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 re locat ingapproximately 1.5 miles of U.S. Route101 away from the active landslide. The

large ancient rockslide complex extends

L

ANDSLIDES

The emergency project

relocates a portion of U.S.

Route 101 away from a

large landslide requiring

two large bridges over

the South Fork Eel River.

The large landslide extends over

1000 ft above the river enveloping

the current highway. The slide is

approximately 3000 ft wide.

Photos: Caltrans.


2/7ASPIRE, Summer 2008|27

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 and

three-span, cast-on-falsework post-tensioned concrete two cell box girder with inclined piers

STRUCTURAL COMPONENTS:South Bridge: 68 concrete segments in spans of 348 ft, 571 ft, and 436 ft. North Bridge: Cast-in-place on falsework

with spans lengths of 175 ft, 229 ft, and 175 ft

BRIDGE CONSTRUCTION COST:South Bridge$37 million; North Bridge$9 million

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 configuration

actually 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, post-tensioned, 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 deck

width is 42.8 ft and the bottom slab

width 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 compress i ve s t rengthwas specified as 6100 psi for thesuperstructure and 5100 psi for thepiers. The concrete strength required attime of post-tensioning was 3600 psi.

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-in-

place, 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.

The south bridge, which is 275 ft abovethe river is a cast-in-place concrete

segmental box girder constructed by the

balanced cantilever method.

Photos: Jon Hirtz, Caltrans.

The north bridge, which is approximately 150 ft

above the river, was built as a cast-on-falsework post-

tensioned concrete box girder. The lower portion of

the falsework (table top) can still be seen below the

bridge as well as the temporary access trestle behindthe bridge.


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4/7ASPIRE, Summer 2008|29

For more information on this or other

projects, visit www.aspirebridge.org.

Protecting the Environment

The project is located at the northend of Mendocino County. It is withinthe majestic redwood forests nearStandish-Hickey State Recreation Area,approximately 10 miles north of the

town of Leggett where State Route 1terminates and joins U.S. Route 101.Several tourist attractions are nearbyincluding the adjacent Confusion HillMystery Spot from which the landslidegets its name.

The South Fork Eel River is federallydesignated as a Wild and Scenic River.Because of this designation, all bridge pierswere required to be outside of the ordinaryhigh water level of the river in order toexpedite the environmental process on this

emergency project. This requirement causedthe bridges to have relatively large centerspans over the river. Both bridges are three-span structures that carry two lanes of trafficand have see-through concrete barriersalong each edge of the deck.

CEB model. Consequently, a shrinkage-reducing admixture was added to themix to cut the concrete drying shrinkagein half. The shrinkage results of the mix

at the Devils Slide segmental bridgeswere similar (see article in Winter 2008ASPIRE), and a shrinkage-reducingadmixture had to be incorporated intothat mix as well.

The project is progressing on scheduleand within budget. The north bridgewas completed in January 2008.The contractor is currently truckingexcavated material from the large cutsbetween the two bridges over thenorth bridge. The south bridge pier 2

cantilever is expected to be completedin July, at which time the form travelerswill be moved over to pier 3. The closurebetween the cantilever tips is scheduledto be placed in early 2009.

_______________________

Kevin Harper is a senior bridge engineerwith the California Department ofTransportation, Sacramento, Calif.

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SOUTH FORK EEL RIVER BRIDGES/ CALIFORNIA

Photo: Caltrans.


6/7ASPIRE, Summer 2008|Web

SOUTH FORK EEL RIVER BRIDGES/ CALIFORNIA

Section near base of pier for

the south bridge. Corner

elements of the hollow pier

are heavily confined with

welded hoops to achieve

good ductility during a

seismic event

Photo: Caltrans.


7/7Web|ASPIRE, Summer 2008

Photo: Caltrans.

confusion hill sum08

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