Frequently Asked Questions about Building the Silver Line Guideway
Guideway completed last week in a major milestone for Silver Line.
The new Silver Line celebrated a major milestone Tuesday: completion of the aerial guideway that will take the trains from East Falls Church to Reston.
Here is a reprint of the Frequently Asked Questions of the project that was distributed by Dulles Metro Rail. McLean Patch is running the following verbatim and unedited.
"Q. What is a guideway?
A. A guideway is a structure that supports trains or other vehicles that ride over it. This project features both at-grade and aerial guideways."
"Q. Where were aerial guideways built?
A. Aerial guideway begins where the new line splits off from the existing WMATA Orange Line tracks just before the West Falls Church Metrorail stationand yard. It flies over the westbound lanes of I-66 and descends to the median of the Dulles Connector Road (Route 267).
Aerial guideway resumes at the Connector Road near the Chain Bridge Road overpass, then crosses over the eastbound lanes of the Connector Road on to the northwest side of Route 123, and into the Tysons East station. The rail continues at elevated level, then flies over I-495 andinto the Tysons Central 123 station where the rail line briefly goes underground.
The tunnel portion resurfaces in the median of Route 7, at the Tysons Central 7 station. From there, the rail line goes aerial again along the median of Route 7 to the Dulles Toll Road. One more flyover takes the guideway westward from Route 7, and the guideway then begins its descent to the median of the Dulles International Airport Access Highway, reaching ground level before Wolf Trap Farm Park."
"Q. Who built the aerial guideway?
A. The aerial guideway was built by Bechtel-led Dulles Transit Partners, the design-build contractor for the Dulles Corridor Metrorail Project. The task manager in charge of constructing the aerial guideway handpicked a team of foremen, superintendents, and field engineers, who, combined, brought over 100 years of large aerial bridge infrastructure experience to the project. The team, in turn, hired on local ironworkers, equipment operators, electricians, and laborers and provided additional training and expertise wherever it was needed."
" Length of Aerial Guideway: 6 miles
(3 miles inbound and outbound)Length of Truss: 360 feet"
"Q. How many miles of this project are aerial guideway?
A. Of the entire Phase 1 alignment of 11.7 miles, three miles of inbound and outbound guideway will be aerial —or a total of six miles of aerial track."
"Q. What is the average height of the guideway (from ground level to the base of the guideway)?
A. The average above-ground height is 35.7 feet."
"Q. What is the tallest point of the guideway?
A. Approximately 55 feet, located above the southbound entrance from Route 123 to I-495."
"Q. How were the aerial structures built?
A. The guideways were constructed by connecting more than 2,700 concrete segments, each weighing about 25 to 40 tons, using massive pieces of equipment called trusses. A truss is also known as a launching gantry, or horizontal crane."
"Q. What is a truss? How much do they weigh? How big are they?
A. A truss, or launching gantry, is a large, rigid structure that erects and pieces together the segments of the aerial guideway. They weigh in excess of 365 tons and are approximately 360 feet long."
"Q. How many different trusses were needed for this project?
A. Three. Each took approximately 4-5 weeks to assemble and 4-5 weeks to disassemble. The trusses were procured from Rizzani de Eccher, headquartered in Italy, who also provided technical support for each time the trusses needed to be moved."
"Q. How did they move along the alignment?
A. Trusses are self-propelling mechanisms that erected spans between each pier. They were assembled at the locations where each major segment erection began."
"Q. What is a segment? How big are they?
A. A ”segment” is a large, unique piece of reinforced precast concrete designed to interlock with other segments to form a “span” between two piers. The segments are held together with steel cables, called post-tensioning strands. Spans make up the guideway."
"Q. Where were the segments made? How many were needed; what was the process?
A. The segments were all fabricated off site at a precast facility located in a remote area of Dulles Airport. More than 2,700 segments were be constructed and trucked to sites along the alignment. Each segment was custom engineered to fit a specific location along the alignment, and was “match casted” to fit its immediate neighboring segments."
"Q. How were they connected?
A. The truss’ lifting mechanisms hoisted the segments into place in the alignment, where they were sealed with epoxy, joined and aligned. The spans have six post tensioned tendons (each approximately six inches thick and made up of up to 19 strands of high strength twisted steel) that were threaded through the interior of each segment. The cables were then anchored into steel blocks at the end of each span and pulled tight (post tensioned) with approximately 4.5 million pounds of force (for an average 130 foot span), locking the individual segments into a single structural unit."
"Q. How big are the piers that support the guideway?
A. The segmental box is roughly 7 feet, 6 inches wide by 8 feet tall, with a top slab of roughly 16 feet wide cast integrally with the segment through the standard guideway. However, these change to 7 feet wide by 5 feet tall with a top slab of 16 feet through the stations, where the spans are shorter by roughly one half. Guideway spans have slightly thinner walls and slabs at 9 inches thick, while station spans are 10 inches thick."
"Q. When did this work begin and where?
A. Construction of the aerial guideway began in 2010 and completed in July 2012. Construction of caissons completed in April 2011, and piers and pier caps to support the aerial guideway followed. The first superstructure began in the median of the Dulles Connector Road over Old Chain Bridge Road in the spring of 2010. That first truss built the flyover on to Route 123, over Scotts Crossing Road, through the Tysons East Station, and over Capital One Drive. When this guideway was completed, just east of the Beltway, that truss was taken down and reassembled in the median of the Dulles International Airport Access Highway (DIAAH) to construct the flyover to/from Route 7.
The second truss originally constructed the guideway portion from just south of the Beltway, over the Westpark connector bridge, and over Tysons Boulevard. When that portion was complete, the truss was dismantled and reassembled in the median of Route 7, where it westward from approximately Westpark Drive/Gosnell Road toward the Tysons West Station at Spring Hill Road."
For more information, visit the Dulles Metro Rail