Traylor Shea Ghazi Precast built a high-capacity plant in the desert highlands above Los Angeles County for one specialty job, knowing other opportunities might arise. Three years and two contracts later, the producer has now set a North American record for output from a carousel-style plant, where precast tunnel segments are fabricated in track-mounted forms that circulate from cage placement, to mix filling, to finishing stations, then on to steam curing chambers before repeating the cycle. A typical tunnel of 10-ft. or greater diameter will have 4-ft. or 5-ft. long rings, each consisting of six segments. Other carousel operations in the past 15 years have served sewer outfall or rail tunnel projects in Boston; Sarnia, Mich.; and Minneapolis, Minn.

Since 2001, TSG's Littlerock, Calif., dual-line plant has produced more than 100,000 segments for two City of Los Angeles sewer tunnels. TSG has moved on to a third contract requiring 51,000 segments to line two supply tunnels within the Metropolitan Water District of Southern California's Inland Feeder project. Seismic conditions along the 44-mile stretch call for the tunnels to maintain watertightness under static earth loading; exposure to an 8.0 magnitude earthquake; and against aquifer pressure equivalent to 470 lbs. per inch.

Like other automated precasting methods, the carousel originated in Europe. As part of acceptance testing leading up to approval for the first contract, TSG brought City of Los Angeles officials to France to see the method first hand. The producer's subsequent success on the contracts, where segment output can reach as many as 288 pieces per day on the dual carousel line, now has Europeans looking to the U.S. example. “Producers and engineers have travelled from France, The Netherlands, Spain and the U.K. to see how we maintain quality control, and have been able to ship more than 10,000 loads without a rejected segment,” notes TSG Plant Manager Mehdi Ghazi. “We create a manifest documenting reinforcing steel, mix characteristics, production time and curing time and temperature for each segment.”

Augmenting quality control, he adds, are high intensity mixers, precision aggregate weighing and moisture monitoring, moisture compensation adjustment, and automated mix handling and form charging.

Carousel casting economics begin with a tunnel job of 10,000 ft., according to Ghazi, who heads a Portland, Ore., construction company bearing his name, and oversees TSG on behalf of Evansville, Ind.-based Traylor Bros. and Walnut, Calif.-based J.F. Shea Co. Under separate contracts, Traylor and Shea have been part of joint venture contractors handling construction of the three jobs TSG has supplied. Production for the Inland Feeder contract is scheduled to continue through August 2005, primarily on one of the two carousel lines. The second line is geared to cast rail tunnel segments for Los Angeles' Metro Gold Line extension, construction of which is projected to begin next year. Over the next three to four years, TSG estimates that up to $500 million in segment production contracts could be let on environmental and transportation jobs in the U.S.