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NOVEMBER 2011 FEATURE STORY:
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Steel Fiber Reinforced Concrete Segments on Brightwater
The population of the Puget Sound region has more than doubled in the last 40 years and projections of the population growth indicated that the existing King County wastewater treatment infrastructure would not be able to cope. The new Brightwater wastewater treatment facility will serve portions of King and Snohomish counties in the greater Seattle area. The construction started in 2006 and includes the construction of a treatment plant, 20 km of pipes and pumps taking wastewater to and from the plant and a marine outfall. The highly variable glaciated geology changes rapidly between firm clays to unstable sand. This variability is problematic for tunneling contractors, who also had to cope with high water and ground pressures at depths of up to 120 m underground. Solution The contractor, a joint venture between Jay Dee and Frank Coluccio Construction Co., is completing the remaining 3 km of the BT-3 tunnel from the Ballinger Portal to the North Kenmore Portal at NE 195th Street and 80th Avenue NE. Jay Dee-Coluccio had successfully completed 6.5 km of a different section of the project. The ground conditions on this new section required the Lovat earth pressure balance tunnel boring machine to be modified to cope with pressures that reached up to 7 times atmosphere. The 4-m final internal diameter tunnel is being lined with precast concrete segments. The project was designed with a performance requirement for the steel fiber reinforced concrete based upon the flexural toughness test ASTM C1609. Maccaferri Inc., the U.S. division of the global Officine Maccaferri Group, worked with the precast concrete segment manufacturer and tunneling contractor to optimize the steel fiber type and dosage to achieve the performance requirement. After trials of the Wirand FF3 it was shown that a dosage of 40 kg/m3 met the performance specifications. To ensure the correct dosage of fibers was delivered into the concrete mix time after time, Maccaferri supplied its DOSO machines to the precast plant. This equipment was fully integrated into the precast facility using Maccaferri’s production specialists. On this project, due to the short concrete mixing times, the vibrating table system operated in combination with the fiber feeder. This was synchronized with the batch cycle timing so that the fibers were introduced to the hopper at the same moment that the aggregates were added. By creating this “preblend mix,” distribution of the fibers was improved at the concrete mixer, avoiding fiber agglomeration that can be associated with short concrete-mixing times. Maccaferri is rapidly making a name for itself as a provider of innovative and cost-effective solutions for tunneling works. Working closely with the project team enables mutual benefits to be realized. Steel fiber reinforced concrete precast segments provide benefits compared to traditionally reinforced precast segments:
Unlike macro-synthetic fibers, steel fibers are proven to offer structural benefits to concrete; on some major infrastructure and metro projects, Wirand Steel Fiber Reinforcement has replaced all the traditional steel reinforcement within tunnel segments. The high Young’s Modulus, very low creep and high temperature resistance of steel fibers, as well as the shear and flexural toughness, compared to macro-synthetic fibers are keys to this structural capability. Project at a Glance Project: Brightwater BT-3 |
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