U.S. Capitol Visitor Center
- Location: Washington, DC
- Owner: Architect of the Capitol
- General Contractor: Centex Construction
- Subsurface Conditions: Pleistocene Terrace Deposits: mostly cohesive sand, stiff clay, and gravel;
Cretaceous Potomac Formation: Hard clay and dense sands.
- Technique(s) Utilized: Diaphragm Wall,
Jet Grouting
- Approximate Key Quantities: Diaphragm Wall - 130,000 square feet, Jet Grout Wall - 380 linear feet, Temporary Anchors - 500
The number of people visiting the US Capitol has grown extensively over the years, from approximately one million annually in the 1970s to more than three million annually in more recent years.
Visitors from across the nation and around the world have waited in line on the Capitol's east stairs, often facing extended waiting periods with no protection from inclement weather, to tour what is one of the few public "people's houses" of government in the world.
Congress had been in the process of identifying possible solutions for a more comfortable and visitor-friendly waiting area for three decades. The initial meetings and plans for a visitor center began in the 1970s, in an effort to manage the first real boom in tourism at the Capitol, to maintain the Capitol's safety in light of recent civilian-related attacks, and to address the desire of visitors to see the government's Legislative Branch in action. The idea finally came to fruition after a gunman with a history of mental illness shot and killed two US Capitol Police officers in July of 1998.
In March of 1999, the Architect of the Capitol was granted the funds required to revalidate a 1995 design study of a visitor's center. On June 20, 2000, congressional leaders representing both political parties gathered for the groundbreaking of the official Capitol Visitor Center (CVC.)
PROJECT BACKGROUND
The design for the new CVC called for a massive, 580,000-square-foot, subterranean structure with three levels, built directly beneath the US Capitol building. The CVC, which would be built 50-feet below current grade, would comfortably house up to 4,000 Capitol visitors at a time.
Construction started on the new CVC in the spring of 2002. The first sequence of the project included site demolition, excavation of soil, construction of the foundation and walls, installation of load-bearing elements, portions of site utility work and the completion of the roof plate for the center.
THE WORK
Nicholson was contracted to construct a 130,000-square-foot reinforced diaphragm wall support of excavation system. The diaphragm wall was the main technique utilized to construct the perimeter wall of the support of excavation. Jet grouting was used as a compliment technique.
As part of the contract, 380 linear feet of "panel" jet grout wall was installed at the perimeter of the Senate tunnel and central stairs for the extension of a 75-ft deep water cutoff around and below existing structures to seal the permeable strata below the design bottom of the excavation. At these locations, a structural connection was not required. However, the continuous water cut off of the system around the perimeter excavation needed to be maintained. Jet grout columns were installed for structural tie-in connections between the diaphragm wall and the building foundation structures at the Senate tunnel and central stairs for support of excavation and ground water control. At these locations, access to the large equipment for the diaphragm wall installation was difficult. The utilization of smaller drills was used to complete the installation of the cut-off around the perimeter.
During the mass excavation of the site, which was three levels deep and the equivalent of five football fields long, Nicholson installed over 500 temporary anchors, with capacities of up to 375 kips. The anchors were installed using the duplex drilling method to minimize ground disturbances and loss under the building's current foundations. Due to the design footprint of the building, the anchors were installed in a precise horizontal and vertical weaving, to avoid reciprocal interferences. Internal steel bracing was also used in areas where the anchors were not feasible.
The Capitol is founded on spread footings placed on dense, water-bearing granular and cohesive soils. The continuous cut off created by the concrete diaphragm wall and jet grouting surrounding the entire perimeter of the addition was selected to limit settlement from dewatering during excavation and control movements of the building's foundations.
THE RESULT
The new Capitol Visitor Center opened in December of 2002.
Massive in scale, the footprint of the new CVC spans approximately five acres (196,000 square feet) and actually has a larger footprint than that of the Capitol (175,000 square feet.)
The facility serves as a welcoming point of entry for visitors, and offers an educational environment in which they can learn more about the Capitol, the House, the Senate and the legislative process. The CVC also offers its guests several amenities, including a 530-seat restaurant, two gift shops, an exhibition hall and 26 restrooms.
The CVC is the largest project in the Capitol's 200-year history and the most extensive addition since the Civil War.
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