The Metro Rail Red Line tunnels, completed by the joint-venture company of Traylor Brothers/Frontier-Kemper Constructors in late 1998, are parallel twin portals that now link Hollywood to Universal City. The tunnels are part of a 6-1/2-mile-long rail line that starts in North Hollywood and will end in downtown Los Angeles.
The contractor selected large-diameter tunnel boring machines to carve out the railway path through the base rock of the Santa Monica Mountains.
Most of the railway was stable enough for use of a standard 4000-psi mix design for a cast-in-place tunnel lining, but about 2,500 feet of the tunnel's path was located in a stratum of friable sandstone. Engineers also had to also answer local environmental concerns about potential groundwater contamination because of this strata's excessively porous nature. When engineers considered both of these long-life requirements, they specified a concrete liner.
Construction plans called for a tunnel excavation of 20 feet 8 inches in diameter. Engineers then specified a 2-foot-10-inch-thick concrete liner for a final tunnel diameter of 17 feet 10 inches. The bid contract called for a standard 4000-psi concrete mix design in portions where the host rock was structurally sound. But where the host rock was sandstone, engineers specified a 6000-psi mix design with a 0.40% shrinkage requirement.
For the selected cast-in-place technique to work, concrete for the tunnel lining's circumference would have to be placed as soon as possible after excavation so as to prevent any ground movement and additional wall erosion. Boring machines, also called moles, were to burrow on parallel paths and were scheduled to tunnel 50 to 75 feet per day, 7 days a week. Planners scheduled a work crew each day to position concrete forms in each tunnel's recently excavated path.
Along with these design and scheduling considerations was the problem of supplying crews with fresh concrete. Managers estimated that at the tunnel's greatest length delivery could take more than 3 hours. This estimate didn't include the drive from its plant, nor the discharge period.
While joint-venture operations managers had employed set-retarding admixtures in other tunnel lining operations, they had never encountered the demanding low-shrinkage requirement set by the design engineers. It was time to call the special-effects gurus.
Rick Hyden, technical director for Southdown Corp. worked closely with Don Fretz, the local Master Builders field sales representative. Hyden's team split the mix-design problem into three elements. "Our challenge was to develop a mix that would provide high 28-day strength yet quick strength development to allow for same-day stripping, a high degree of workability with an extended working time, and low shrinkage," said Hyden.
Fretz suggested a polycarboxylate type of high-range water reducer to meet the strength and workability challenges of daily liner movement. Adjusted to the proper dosage, the recommended admixture, Rheobuild 3000FC, provided an accelerated set time 1/2 hour to 1-1/2 hours faster than other superplasticizers used on other tunnel projects.
The extended working time presented another challenge. The design team also selected a hydration-control admixture, Delvo, to stabilize the fresh concrete. The batchman was told the estimated total transport time and adjusted the dosage rates each day.