Design/build team works to complete $20 million Pentagon renovation project.
As part of the project, a new heating and refrigeration plant (H&RP) was constructed in 1995 that required a substantially greater amount of condenser water than the existing system could provide. Ultimately, it was decided to install a new condenser water system to the H&RP plant. The 600-foot intake water line of the system supplies condensed water to the Pentagon air-conditioning chillers from the Boundary Channel Lagoon.
Just as a need existed to bring water into the system, eventually that water would have to cycle back out. For this, plans called for replacing the existing 60-inch diameter condenser outfall pipeline that would run from the plant through Arlington County, VA, to the nearby Roaches Run Waterfowl Sanctuary.
When the intake and outfall lines for the Pentagon's H&RP was put out for bid, the design-build team of Modern Continental Construction (general contractor) and Jacobs (design) was awarded the $20 million design/build contract in October 2001 for Phase 1 of the project.
Major structural components under Phase 1 of the project included construction of the intake tunnel, the intake structure at Boundary Channel Lagoon, and a portion of the screen house at the H&RP.
The design/build contract award adopted by the Pentagon for the renovation allows design/build to operate as a single entity under one contact. This is in contrast to the standard government contracting approach of design-bid-build, which often results in conflict between the designer, construction contractor and owner.
Funding for Phase 2, which included the remaining work on the screen house and the entire 60-inch outfall line and structure for the project was funded in November 2002. When the project was put out for bid the contract was won by Modern/Jacobs, with plans for completion in April 2004.
Phase I construction
At the time of the Phase 1 contract award, Pentagon officials said the winning design-build team showed innovative ways the project could be accomplished while saving the government money.
Steve Kramer, P.E., Jacobs Civil Inc., principal of Trenchless & Water Resources Group, said, "One of the most significant decisions of the design-build team was to change the eight-foot diameter intake line from a conventional tunnel with a segmental liner to an installation using microtunneling and large diameter centrifugally cast fiberglass reinforced polymer mortar pipe manufactured by Hobas Pipe USA.
"Keep in mind," Kramer emphasized, "tackling any portion of the Pentagon renovation posed special challenges. The Pentagon is no ordinary building. Massive in size, it covers 29 acres of land and encompasses 6.6 million square-feet of office space--three times the floor space in the Empire State Building. Furthermore, in the 58 years since construction of the military headquarters of the United States, none of the Pentagon's original major building systems had ever been replaced or significantly upgraded."
He explained that while an earlier design was produced for the condenser intake/outfall lines when the new H&RP was built in 1997, they were never constructed. When the project was re-bid, the earlier design, calling for a conventional tunnel with a concrete pipe casing, was made available to qualified bidders.
The re-bid documents also requested that construction actions be provided to collectively address the aging building's many problems. It also required the design-build team to present creative solutions to address the following challenges:
* Working in soft ground conditions and a complex geology beneath the water table;
* Crossing beneath low grade installations;
* Coordinating with multiple outside agencies; and
* Fast track design and construction using the design/build process.
"Once we reviewed the earlier design and began evaluating ways to meet these criteria," Kramer said, "the team concluded that the installation of the intake line to draw water from the Boundary Channel Lagoon, which is fed by the Potomac River, could be vastly improved through the use of micro-tunneling and large diameter Hobas pipe that offered a smooth I.D. and superior hydraulic properties."
By introducing this and other innovative changes, Kramer said the team's approach provided the following benefits:
* Mitigated construction risk;
* Reduced capital and life cycle costs;
* Reduced likelihood of cost overruns;
* Improved work safety; and
* Simplified construction.
Some of the challenges that construction of the intake tunnel presented included difficult ground conditions and a complex geology. The tunnel location itself is also deep, located 70 feet below grade.
Kramer indicated that one advantage the earlier design offered was in the route of the planned intake line. Since the existing tunnel followed such a serpentine path, a decision was made early on to make the route of the new tunnel much straighter and relatively short, only 600 feet.
Work on the project actually got under way in October 2001, which was just three weeks after 9/11 and within days following the start of the massive demolition and rebuilding to restore the Pentagon back as quickly as possible.
Kramer said that from the outset the project was on a fast track schedule. The Jacobs team was required to produce drawings very quickly. "Often, changes were requested by Modern Continental for either the good of the project or to save money," added Kramer.
Within 120 days of receiving the notice to proceed, Jacobs had delivered 70 construction documents and 70 specifications to the Pentagon Renovation Team. "This demanding schedule was conducted in order to allow construction to proceed quickly because of concerns as to the long-term viability of the existing intake structure," he explained.
Initial job activity
In preparation for intake tunnel construction, some of the first work on the project involved the installation of sheet pilings around the perimeter of the site and across the entrance to the lagoon to form a cofferdam to permit construction of the intake structure in the dry.
Philip Colton, project manager for Modern Continental, said the cofferdam construction provided a watertight environment for crews to construct the massive intake structure.
Construction of intake structure
The intake structure was constructed at the edge of the Boundary Channel Lagoon (east of the Pentagon H&RP) and required steel sheet pile for support of the excavation, The excavation was approximately 20 feet deep, and extended into the lagoon 10 feet. The sheeting was installed with two levels of bracing with a phased removal as construction progressed upward in the structure. A proposal was accepted and implemented as a field change to eliminate much of the lowest level of bracing by doubling the size of the original concrete mud mat from 150mm (6-inch) thick to a 300mm (12-inch) thick concrete mud mat. This approach saved money, reduced the time to install the bracing and substantially increased productivity in placing the slab forms, rebar and concrete.
The intake structure walls are greater than three feet thick to prevent uplift when the structure is dewatered. The concrete placements took place from July through October 2002, with ambient air temperature often exceeding 100 degrees F. A combination of early morning concrete placements and the use of chilled water and ice to batch the concrete, maintained the temperature of the concrete below the 90 degrees F specified temperature.
The mechanical and electrical equipment in the intake structure consists of eight sluice gates, two trash racks and rakes, and ancillary equipment to minimize the potential of larger debris from the lagoon obstructing flow or damaging the equipment. The sluice gates will be installed before the roof system is installed, and the trash racks and rakes will be lowered through the roof access hatches.
The intake pipe was installed using a microtunneling process that required constructing a concrete launch and retrieving shaft. A 30-inch thick slurry wall shaft was used to support the excavation for both shafts. The shafts were excavated with a rectangular hydraulic clamshell mounted to a crawler crane. The clamshell was equipped with a rotating hydraulic head and jack controls to swing and position the bucket. The opening was maintained with a bentonite slurry mixture with a specific gravity of approximately 1.20 gm/ml after desanding. The 30-foot diameter launch shaft was made up of six arching segments, and the 24-foot diameter receiving shaft was made up of five arching segments. Each arch segment of the slurry wall was excavated using three overlapping bites of the clamshell. W30 steel H beams were used as end-stops, with the flanges running tangential with the face of the slurry wall. The area between the flanges of the end-stops, on the unexcavated side, were filled with planks of styrofoam to prevent concrete from migrating around the end-stop and fill the area between the flanges with concrete.
After completing excavation on each arch segment, the rebar was lowered into place and concrete was placed using 12-inch tremie pipes. The tremies were progressively raised to ensure the concrete was discharged at least five feet below the top surface of the rising concrete level. The rising concrete displaced the bentonite slurry, which was desanded using a separating plant capable of removing solids as small as 0.0008-inch. The upper 25 feet of the softer soil was excavated with a hydraulic excavator. The lower 50 feet of soil consisted of a very stiff to hard sandy clay material that was excavated with a clamshell bucket. A mini-track excavator was required to break the harder material. The shaft was plugged with a thick reinforced concrete plug measuring 7.87 feet that was cast in the bottom of the excavated shaft.
Ground control outside the shaft, for the micro tunnel boring machine (MTBM) launch and retrieval, consisted of jet-grouted columns constructed in a pattern 20-feet wide by 15-feet long at the launch shaft and 20-feet wide by 10-feet long at the receiving shaft. Geotechnical monitoring was performed to ensure that primary settlement did not damage adjacent structures. The vertical extensometers and horizontal inclinometers installed along the tunnel alignment were used to control a compensation grouting process, which consisted of horizontal grout tubes installed in the receiving shaft to prevent movement of the existing structures. A pre-construction survey was performed on the existing structures to provide data for analyzing the structures for damage from construction.
The microtunneling machine was a Herrenknecht AVN2000D, equipped with slurry pressure balance to minimize potential of over mining solids. An interjack station was installed 60 feet behind the MTBM, and was capable of providing a jacking force of 500 tons. The primary jack system was equipped with four jacks capable of providing a combined jacking load of 1,100 tons.
The maximum jacking load measured was 588 tons. After 240 feet of pipe was installed, the interjack station was used to advance the MTBM and lead the 60 feet of pipe. Bentonite slurry was filtered through the same separating plant used for the slurry walls. To maintain a specific gravity of approximately 1.20, the separating plant was equipped with two centrifuges for clay particle removal. The tunnel pipe was a centrifugally cast fiberglass reinforced polymer mortar pipe manufactured by Hobas Pipe USA Inc. The MTBM operation used two 10-hour shifts per day except when the MTBM was under adjacent structures, then 24-hour shifts were used.
Screen house construction
A new screen house is being constructed to house two traveling water screens, equipped with spray wash pumps for each screen to prevent clogging of the screens. The support of excavation will use a steel sheet pile system similar to the intake structure and will require dewatering efforts to allow the internal support system to be minimized to two levels of bracing. The new screen house structure will be tied into two existing concrete pipes to provide flow to the Heating and Refrigeration Plant for the Pentagon and ancillary structures.
Outfall pipe construction
A new outfall will be installed to discharge the flow from the H&RP. The new outfall pipe includes several challenging features, including crossing lands controlled by local government agencies, state government agencies, and other federal government agencies. The pipe is being installed using open trench operations, but will also include a segment that is direct jacked.
Now that the project is nearing its extended April 2004 completion date, Colton concludes, "That the right combination of technology, equipment and personnel can make a huge difference in the success of a project of this magnitude. Also, the leadership of the Pentagon officials on this unique project should serve as an excellent example of how future design/build government projects can be successfully completed while providing considerable cost savings to the government."
* The $1.2 billion cost figure was the original cost cap prior to Sept. 11,2001 for construction costs only.
The author acknowledges the assistance of the following that contributed to this article: Rachel Decker, communications specialist, Pentagon Renovation Program; Steven R. Kramer, P.E., Principle of Trenchless & Water Resources Group, Jacobs; Jerry Nystrom, P.E., construction manager, Jacobs; Philip B. Colton, project manager, Modern Continental; and Jim Gerace, microtunneling superintendent, Modern Continental.
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|Date:||May 1, 2003|
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