AEG/PGS: Conventional Tunneling at Tyson’s Corner

BWH Section of the Association of Environmental &

Engineering Geologists and the Potomac Geophysical Society

Date: Thursday May 21, 2015


Baltimore MD – Washington DC – Harrisburg PA

The Meeting will be held at the Amphora Restaurant, 377 Maple Ave., Vienna VA, 6:00-8:30pm. Reservations are necessary,  we need a head count for the restaurant, so, if you wish to attend dinner ($40, members; $45, non-members), please inform Bob Fraser by May 15th at 703-624-3965 (takes both voice and cell text messages) or via E-mail at Non-members and guests are welcome but must have a reservation. Let’s make this joint meeting a success: make a reservation!

TOPIC: Conventional Tunneling (SEM) at: Tysons Corner, Virginia

PRESENTER: Vojtech Gall, Ph.D., P.E. Gall Zeidler Consultants, Ashburn, VA

BIO: Dr. Gall is a Principal of Gall Zeidler Consultants, a tunnel engineering and consulting company headquartered in Northern Virginia. He has over twenty-five years of experience in the design, construction, and construction management of tunnels and underground structures. He frequently serves on expert review panels dedicated to the investigation and improvement of tunneling schemes ranging from initial inception stages through final design development at bid document level. His services relate to a wide variety of facilities ranging from hydro-power schemes in remote settings to complex transit structures at shallow overburden in difficult, over-built urban settings. He has led tunneling efforts on large infrastructure project such as Dulles Metrorail in DC, the Caldecott 4-th bore tunnel in San Francisco and the East Side Access project in New York City. He holds M.Sc. and B.Sc. degrees in Civil Engineering from the RWTH University in Aachen, Germany, a M.Sc. in Mineral Engineering from the University of Alabama and a Ph.D. in Civil Engineering from the University of Maryland. He is a registered Professional Engineer in 18 states and has authored over 40 technical publications on the subjects of tunneling.

ABSTRACT: The Metropolitan Washington Airports Authority (MWAA) is constructing the Dulles Corridor Metrorail Project (DCMP), which will extend Washington Metropolitan Area Transit Authority’s (WMATA) rail services from the Metrorail Orange Line in Fairfax County, Virginia to Route 772 near Ashburn in eastern Loudoun County, Virginia. The extension will be known as the Silver Line and once completed will add 23 miles to the existing Washington Metro System. The project was subdivided in two phases. Phase I segment is 12 miles long and involves 5 stations (two at grade and three elevated) and was opened to public in 2014. Phase II will extend the rail by another 11 miles with a station at Dulles International Airport and a terminus station in Ashburn.

Both phases use the design-build (DB) delivery method. Phase I was constructed by Dulles Transit Partners (DTP) a Joint Venture of Bechtel and URS and included twin 1,700 feet long single track, 22 feet diameter conventional tunnels (excavated by the Sequential Excavation Method or SEM) that are situated in the urban setting of Tysons Corner.

The presentation will address design and construction of these soft ground tunnels that emerged as the most feasible alternative to practically all other tunneling methods including open and closed face TBM drives and cut-and-cover techniques. Tunneling was accomplished with shallow overburden of as low as about 7 feet from the tunnel crown. Because of the shallow depth, soft ground conditions, and the need to control settlements, the tunnel design included use of a double row grouted pipe arch canopy as pre-support for the first 300 feet of excavation and a single row pipe arch canopy for the remaining length of tunnels thereafter. A real-time monitoring of the surface when tunneling the first 90 m under International Drive was implemented to assure VDOT of the tunneling performance.

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