PUMPS START SUBSTRUCTURES FOR CALIFORNIA SUSPENSION BRIDGE 2001
Concrete pours for the five substructure locations of the new Carquinez Bridge in the northern San Francisco Bay area are providing the foundation for what is described as the first major suspension bridge to be built in the U.S. in 35 years. At $189 million, the bridge contract is the largest award ever made by the California Department of Transportation (Caltrans).
The three-span, 3,465-ft-long structure features two twin-towers that carry the suspension cable between concrete anchorages at each end of the bridge, plus a concrete land pier at the south end . These five structural elements combine for 72,000 cu yds of concrete. While most of this yardage is poured, tower footings are pre-cast and barged to the site.
Spanning the Carquinez Strait between Vallejo and Crockett, the new structure will carry four lanes of west-bound traffic for I-80 on an 82-ft-wide deck. It replaces an existing bridge built in l927 that is being razed. Another existing bridge is being seismically retrofitted and will carry east-bound I-80 traffic. Both I-80 bridges are free-standing.
Work was started in March 2000 by FCI Constructors, Inc./Cleveland Bridge California, Inc., a joint venture. FCI is part of HBG Constructors, a holding company that also owns Flatiron Structures and two other construction firms. Flatiron serves as managing partner for the joint venture.
Early-stage pumping Concrete work was started at the tower and anchorage sites with small pours made by a series of Schwing concrete pumps from CF&T Concrete Pumping Inc., Hayward. Much of the work during the first ten months of construction has involved pile driving and rock socketing, excavation and shoring in establishing the foundations at all five substructure locations. Earlier this year the contractors completed driving all 12 of the 150-ft-long by l2-ft-dia piles for Tower 2, and were drilling down another 150 ft to install the rock socket.
Delays during rock-socket work at Tower 2 occured when ruins from an old pier were encountered. A crane-mounted oscillator cored through the debris and removed it. In addition, bedrock at this location was deemed "incompetent" by Caltrans. Contractors used an underreaming attachment for the drill to create a rock-socket larger in diameter than the driven pipe piling, then filled the void with concrete. The corrective work caused a 50-day delay. In the meantime, pile driving was resumed at Tower 3, where the bedrock was sound.
(The land pier to the south is designated Pier l; the 400-ft-high twin towers are designated Towers 2 and 3.)
Footings to be placed atop the piling are precast. At a nearby casting yard, a 42-meter Schwing pump delivers ready-mix for each twin-tower footing. According to the joint venture’s Curt Weltz, construction manager, the three-unit footing design consists of two 54x65-ft open-cell shells designed to float, plus a center connector tie.
"We’re casting four sets of footings, one for each of the twin tower locations," says Weltz. "They will be barged to the site, where we’ll sink the barge to allow the footing to float off the barge and over the piling. We’ll use another Schwing boom pump to pour 5,000 yds as each footing is sunk and grouted atop the piling."
The barge will then be refloated and returned to the casting yard for the next footing to repeat the process.
Concrete pumping at different site locations is usually scheduled for two seven-hour days each week. In late February, a seal-slab pour was started by a 42-meter Schwing pump for the north anchorage, which is founded on rock. The south anchorage will be built on piling. The 157x142-ft structure will start at 30 ft below grade and rise to 54 ft above grade. Schwing pumps are primed to deliver 20,000 cu yds for the two anchorages.
Sam Aiello is in charge of operations for CF&T Concrete Pumping, and helped create a scheme for delivering concrete for the Tower 3 piles. At the Vallejo abutment the pumper positioned a 32-meter Schwing that pumped the mix to another 32 on a barge, some 400 ft offshore. The piggy-back system saw the first unit pump downward from the abutment for 300 ft to a temporary work bridge, cross the bridge for another 380 ft, and finally downward again for 300 ft to the barge. Here, the second (non-pumping) Schwing 32 boomed the mix through flexible line to 200-ft-long tremies that extended to the full depth of the piles.
"With this system, we produced l00 yards an hour through nearly l,000 feet of system, while operating the Schwing 32 at a low pressure 80 b.a.r. That’s about one-third pump capacity, and it produced all the yardage the contractors wanted," says Aiello. "The Generation 3 Pump Kit on the 32 is fantastic. It enables us to get greater volume at lower pressures." On the footing pours, he adds, "we’ll do 130 yards an hour with the 42-meter Schwing."
Pier l pumped Pier l, between the south anchorage and the water, will rise 150 ft above grade. The pumper used a big 52-meter Schwing boom pump to complete the structure for l40 ft before it was topped off with a crane and bucket. The job was done in 14 separate pours of 54 yds for each 12-ft-thick lift. By late February, Pier l marked the most significant volume of concrete that had been placed above grade.
Ready-mix is supplied by RMC Pacific Materials, operating plants on both sides of the Carquinez Strait. From Vallejo on the north side, RMC’s central mix plant produces 260 yds/hr, while a portable plant has a l60-yd/hr output, and is just a short five-minute haul to the south bridge site. RMC’s general manager Jack Shade says the design for the piling concrete is a 6.81 bag with 1/2-in. crushed blue rock, flyash and a Delvo stabilizer. It cures at about 3,500 psi or 25 Mpa.
The superstructure of the Caltrans-designed Carquinez Bridge will feature steel orthotropic deck sections -- fabricated with the top, bottom and sides as a single unit. Built in Japan, the individual spans will be shipped in three ocean-going barges.