IN THE WET CONSTRUCTION POSES BENEFITS FOR DAM CONSTRUCTION
In the Summer of 2002, the US Army Corps of Engineers, Pittsburg District, reached the most critical phase in the modernization of the Braddock Dam. The joint venture effort of lead contractor J.A. Jones Construction Company, Charlotte, NC, and Traylor Bros., Inc., Evansville, IN, successfully tackled the high-profile “In-The-Wet” construction challenge.
Located in the lower Monongahela River in Braddock, PA, the original Braddock Dam was completed in 1906. Addressing the serious deterioration and structural problems of Locks and Dams 2, 3 and 4 has been an extensive on-going project since January of 1992. The $750 million construction is scheduled for operational completion in 2003, with several miscellaneous projects to be completed by 2008.
With pre-construction, engineering and design activities wrapping up in September 1994, modifications to the chamber floodway bulkhead, upper guard wall work and the construction of new abutments were completed in 1998. J.A. Jones and Traylor Bros. were awarded the contract to complete “In-The-Wet” construction for part of the Braddock Dam modernization in July of 1999. Since the start of construction in Summer 2000, the contractors have made headlines as the first to attempt a project of this caliber.
“In-The-Wet” construction is the use of off-site prefabrication combined with lift- or float-in of large precast segments onto pre-installed foundations. These segments are locked into the foundations by grouting and infilling of the segments with tremie concrete. Because a significant amount of the work is transferred from the river environment to on land construction sites, the use of this technology has several advantages over the conventional fixed cofferdam method and the time and cost-consuming process of digging out the river bottom and building the dam from the ground up. “In-The-Wet” technology has the solutions that provide cost savings, shorten construction duration, reduce environmental impact and enhance construction safety.
For the Braddock Dam Project, J.A. Jones and Traylor Bros. were commissioned to construct a 600-foot long gated dam with four gate bays to replace the 100-year old fixed crest dam. The contractors used the “In-The-Wet construction method to create and transport two giant concrete segments using precast panels interconnected in a grid pattern.
The vertically placed panels were assembled for each segment and tied together with a monolithic concrete base slab, creating multiple watertight compartments. The combination of the interconnected panels and the post-tensioned base slabs created two huge concrete floating segments.
The first segment measured 333’ by 106’ and weighed in at 23.5 million pounds. Flooding of the casting basin for the segment began on July 9, 2001. Within 30 hours, the segment had maintained its buoyancy and was floated to a parallel launch basin where it was prepared for deployment into the Ohio River. On July 26, the segment began its 15-hour, 29-mile journey from Traylor Bros.’ precast yard and launch basin in Leetsdale, PA, to the outfitting pier in Duquesne. At certain points along the way, the segment was required to float in locks with less than 12” of clearance on the bottom and the sides of the chambers. At the outfitting pier, 24 vertical feet was added. The additional construction was necessary to increase the height of the segment for setdown. A seven-winch system was installed on the three piers for set down positioning. A water-ballast system was installed to pump water into the cells created by the precast panels. The water was pumped into the cells until the segment reached is final destination on the drilled shaft foundation system on December 6. 2001.
In February 2002, the second, smaller segment completed its first leg of the journey to Duquesne. Weather deterred contractors in Spring of 2002, creating flood conditions on the Monongahela River but the transporting and installation process of segment two continued to make progress. On June 17, 2002, the second segment began its journey from the outfitting pier to Braddock. Measuring 265 feet stern to bow and 104 feet port to starboard, the smaller segment weighed in at 9,600 tons.
The bottoms of the segments are recessed for 77 pre-installed foundation shafts, which match the landing caissions for mounting at the site. Each segment is equipped with six hydraulic jacks to balance the segment’s load onto the six set-down shafts.
When completed, the first half of the overall structure consisted of hundreds of individual thin-walled, hollow-core components fabricated with reinforced concrete. The dry launching basin, which had served as the construction site for several months, was then flooded with water. Crews held their breath, waiting for the 11,000-ton concrete structure to float in the Leetsdale-based dock 27 miles from the dam. July 26, 2001, the section stayed maintained its buoyancy and was towed to the dam.
Once in position, the cavities of the structure were flooded with water. The segment was manipulated into position and was anchored on the pre-installed drill shafts. The pumping contractor used Tremie concrete to complete underbase grouting and infilling of the segment, which anchored the piece into its place on the dam.
One day later, the second section of the 82-foot high dam sprung several leaks, causing concern and speculation over whether or not this new technology is logical. But the contractors stand by the method and continue to emphasize the advantages of in-the-wet construction over cofferdam implementation.
The final phase of the project, outfitting the dam with piers and walkways will take approximately one year.