A major phase of the $750 million Lower Monongahela River Navigation Project, construction of the new Braddock Dam is scheduled for completion this month. The project, which involves massive precast sections, marks the first time in U.S. Corps of Engineers history that a dam has been built using “in-the-wet” techniques. To modernize navigation of the lower Monongahela River in part by replacing a century-old fixed crest dam with a gated dam 100 yards upstream, the Corps adopted a cost-saving construction scheme involving off-site fabrication of concrete segments designed for flotation from the casting plant to the dam location. Accordingly, dam segments were produced in Leetsdale, Pa., as the dam foundation was constructed concurrently from a floating plant at Braddock, eliminating the time and expense of using traditional, temporary cofferdams to provide a dry work area.

CASTING SEGMENTS

At a temporary casting facility in Leetsdale, two hollow segments resembling barges were assembled using both precast and conventional cast-in-place concrete. The segments comprise the overflow surfaces of the dam, the gate sills, part of the stilling basin, and a small portion of the pier bases. Precast panels — 438 in all, some weighing 80 tons — were vertically placed and interconnected to form a grid pattern for each segment. A monolithic, post-tensioned concrete base slab served to tie the panels together, creating multiple watertight compartments enabling the two colossal segments to float.

Dam Segment One was the largest of the two, weighing in at 11,600 tons (before grouting and the addition of concrete upon setdown) and measuring 333 ft. ∣ 104 ft., about the size of a football field. At 9,600 tons, Dam Segment Two measured 265 ft. long ∣ 104 ft. wide ∣ 43 ft. high. The post-tensioned concrete base slab of each segment was cast around a grid of corrugated metal “cans” positioned to align with 77 78-in.-diameter drilled shafts in the foundation system. Reaction beams in the bottom slab were designed to transfer the dead weight of the segment to another 12 setdown shafts. Flat jacks and steel pistons were also cast into the bottom slab to allow leveling of the segment after setdown.

CASTING OFF

Flooding of the casting basin for the first segment began in July 9, 2001. Thirty hours later the structure was floated to a parallel launch basin from which it was deployed onto the Ohio River, where it was towed 14.7 miles to the center of Pittsburgh before entering the Monongahela River and continuing upriver another 12.8 miles to Duquesne, Pa. On its journey, the segment passed through Dashields and Emsworth Locks on the Ohio River — nearly filling the lock chambers as it passed with less than 12-in. clearance on the bottom and sides — in addition to Lock 2 on the Monongahela River. The 15-hour, 27.5-mile trip terminated at an outfitting pier in Duquesne where the structure was prepared for sinking onto the drilled shaft foundation at the Braddock project site.

DOCKING AND LOCKING IN

Prior to positioning and submerging onto the foundation, each segment had to be outfitted with additional bulkheads, work platforms, ballast piping and equipment. To ensure a height above water level after setdown, 24 vertical feet were added to the pier bases of each structure. A seven-winch system was installed on the three piers of the first segment to position it for setdown. A water-ballast system to pump water into the cells formed by the precast panels was also engineered to facilitate placement on the foundation.

In early December, Dam Segment One was towed about 1.5 miles to the project site on the Monongahela River at Braddock. After steel cables from winches attached to the segment were run to six mooring piles, the winch-cable system was used to precisely position the unit — within one-inch of specified design values for horizontal and vertical alignment — over a grid of foundation drilled shafts. Water was pumped into the hollow compartments to slowly lower the segment into position as alignment was maintained by the cables and mooring piles.

Reinforced concrete drilled shafts were constructed to carry the weight of the dam and operating loads into bedrock. Each of the 77 foundation shafts comprising a grid is 78 in. in diameter and about 40 ft. long, with 15 to 20 ft. of the length drilled into bedrock. In addition, 12 set-down shafts supported the segments' load upon placement as the weight of each unit was distributed by its six hydraulic jacks onto six set-down shafts.

The drilled shafts and a tailrace H-pile foundation were built only after significant premlinary measures: pre-excavation providing a 140-ft.-wide, 650-ft.-long foundation footprint; installation of steel sheet piling to serve both as cutoff walls restricting flow paths beneath the new dam and retaining walls during subsequent stages of the project; dredging of the area between the cutoff walls to final grade; and, covering the area with a graded gravel base. A total of 400,000 cu. yd. of dredged material was used to help restore a nearby brownfield, providing fill at the former Duquesne mill site on which the City Center of Duquesne was built.

Following setdown, grout was applied in the space between the graded gravel base and the bottom slab of the segment to prevent any water flow below the dam. After underbase grouting, the interior compartments were filled with a combination of concretes. Initial placement of a highly flowable “tremie” concrete designed for underwater applications was followed by dewatering of each compartment and a concrete fill placed in the dry. Additional grout was used to lock the dam segment onto each segment shaft.

The dam tailrace was also built “in-the-wet” using 31 interlocking precast panels, each more than 30 ft. long and between 10 to 20 ft. in width. Besides interlocking with the adjoining tailrace panel, each unit was anchored by means of a special groove cast into the downstream edge of the dam segments. The 70-ton panels — match cast and steam cured — were installed from cranes mounted on a floating plant using a guide frame. The area beneath each tailrace panel was filled with tremie concrete to create a tailrace section supported by the H-pile system.

PLUGGING LEAKS

On June 17, 2002, Segment Two reached the Braddock Dam site, traveling the same course as Segment One. Crews maneuvered it into place by mid-afternoon and were sinking it the following day when leaks were discovered. According to Army Corps of Engineers spokesman Richard Dowling, divers sent initially to repair the seals underwater determined that such measures would be insufficient; the segment would have to be refloated.

As crews began to slowly sink the segment by filling its hollow compartments with water, the structure was kept on a level plane by manipulating water levels in the various enclosures. In dropping the segment 25 feet to the river bottom, maintaining an even level was essential to set the unit properly on the grid of 44 drilled shafts comprising its foundation. Further, alignment with the first segment had to be exact.

Although the leaks presented no threat to the integrity of the dam, failing seals between the tanks would make it difficult to keep the segment level during setdown. Consequently, crews began pumping water immediately out of the segment to refloat it. Three seals were found in need of repair or tightening.

BECOMING OPERATIONAL

After placement of Segment Two, work was continued above water using conventional construction techniques. The upper 40 ft. of the five dam piers, the 110-ft.-wide dam tainter gates and operating machinery, footbridges, and equipment buildings will be completed by floating plant. By the end of the project, the existing fixed-crest concrete dam about 600 ft. downstream will have been dismantled down to the riverbed. Materials from the demolition will be placed in downstream locations for fish habitat.

Constructing a gated dam at Braddock enables the Corps' Pittsburgh District to replace old and inefficient locks at Charleroi and completely eliminate the 100-year-old locks and Dam Three at Elizabeth, Pa. Project Manager Henry Edwardo anticipates completion of the Braddock Dam by the end of the year and a Spring dedication.
— adapted from information supplied by the U.S. Army Corps of Engineers, Traylor Bros., Inc., and Pittsburgh's Post-Gazette