ASCE Panel Recommends Tunneling through Tysons Corner

Traffic Situation & Metro Extension

'The Washington DC region is the third most traffic congested area in the nation and the problem is only worsening due to urban sprawling occurring in Northern Virginia.  Currently, the Washington DC METRO system only goes as far west as the Vienna stop, which leaves a gaping hole between western communities such as Tysons Corner, Reston, the Dulles Airport, and Ashburn. 

The concept for extending the metrorail line to Dulles Airport has been in existence since the 1950’s, which was during the planning stages for the construction of the Dulles Airport.  Now, almost 60 years later, the construction of the extension is underway with an approved 23.1 mile metrorail line extension.  The majority of the extension will be constructed in the median of the Dulles Airport Access Highway and Dulles Connector Road (Toll Road); however, an aerial alignment is planned to pass through Tysons Corner, a fast growing, economically vital area, densely packed with retail and office buildings. 

Proposed Tunnel Alternative

To avoid the lifetime environmental impact of the aerial alignment, DSC worked with Dragados to develop a tunnel alternative.  The tunnel solution utilizes a large bore TBM, which is based on a project being constructed in Barcelona, Spain. The tunnel is approximately 42 ft in diameter, is about 3.4 miles long,  houses four stations to service the Tysons Corner area, and will be constructed using an EPBM machine. 

The tunnel provides advantages such as flexibility of station locations, train storage capacity, a separate fast track line to airport, and community enhancement, all at a cost similar to that of the aerial alignment.

The tunnel alternative was widely accepted by the public and the project Stakeholders and an independent panel of ASCE engineers was developed to determine the feasibility of the tunnel alternative.  DSC and Dragados made several presentations at the request of the ASCE panel and provided them with valuable information that facilitated the review process.  The presentations addressed questions regarding site geology and tunnel constructability, especially those concerning the proposed NATM mining techniques for adits and cross passages and mixed face TBM conditions.     

ASCE Panel Review and Conclusion

The ASCE panel had 90 days to review the project information and determined the tunnel option through Tysons Corner is feasible.  Not only did they find the tunnel feasible, they made the following statement, “With more attractive development in place and with more development density allowed by the tunnel alternative, it is reasonable to assume that the tunnel option would provide greater positive long-term economic and business impacts than the aerial option.”   

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