Non-Disruptive Alternatives for Grade Separation Projects in Urban Settings Using Very Shallow Conventional Tunneling

Synopsis:

The construction of the approximately 520 meter long, twin soft ground single-track Metrorail guideway tunnels at Tysons Corner, Virginia is substantially complete at the end of 2012. The tunnels are part of Phase I of the Metropolitan Washington Airports Authority’s (MWAA) Dulles Corridor Metrorail Project (DCMP), which will ultimately extend the Washington Metropolitan Area Trans-portation Authority’s (WMATA) Metrorail system 37 kilometers to Ashburn, Virginia and will include a station at Washington Dulles International Airport. Excavation of the tunnels proceeded through urban Tysons Corner at very shallow depths using the Conventional Tunneling Method.

Conventional tunneling was primarily chosen due to the soft ground and very shallow nature of the tunnel alignment as the only alternative to provide a grade separated underground solution without disrupting the important traffic arteries at the project location. Tunneling occurred partially at overburden depths of only 2.5 meters. Grade separation represents an important solution to construction projects in highly urbanized environments where disruption of traffic or other transportation infrastructure is not a viable option. Facing even greater challenges is the currently active CQ039 segment of the East Side Access (ESA) Project in New York involving tunneling under Northern Boulevard. This grade separation consists of a very large cross section, soft ground conventional tunnel in combination with ground freezing for pre-support, ground water cut-off, and underpinning of an existing Metro Tunnel, above lying road, and viaduct.

Successful execution of such projects supports the use of conventional tunneling with pre-support and deformation controlling technologies for tunneling at shallow depths and through road and railroad embankments. This paper addresses the range of design, risk, and quality considerations associated with each project leading to the success of implementing conventional tunneling and thus provides experience that can be applied for future, shallow grade separation projects.

Non-Disruptive Alternatives for Grade Separation Projects in Urban Settings Using Very Shallow Conventional Tunneling
2013 Underground Construction Conference Proceedings
April 22-24, 2013
Prague, Czech Republic