Site Report: Sandy Guthrie reports from the new Washington DC Metro

By Sandy Guthrie09 December 2010

The New Austrian Tunnelling Method (NATM) is being used at Tysons Corner on the Dulles Corridor Metr

The New Austrian Tunnelling Method (NATM) is being used at Tysons Corner on the Dulles Corridor Metrorail Project.

Washington Dulles International Airport, situated just over 38 km outside the US capital, is one of three international airports for the Washington DC area and handles more than 20 million passengers a year - yet it has no rail link to the city.

This is changing with the $ 5.25 billion Dulles Corridor Metrorail Project in Northern Virginia, Phase 1 of which is well under way. It is being conducted by the Metropolitan Washington Airports Authority (MWAA), and the Metrorail service will be operated by the Washington Metropolitan Area Transit Authority (WMATA). The project is a joint venture involving Bechtel.

Bidding for Phase 2 of the project will take place in the middle of 2011, with a view to finishing in 2016, while Phase 1 covers the first 18.8 km (11.7 miles) of the extension to the existing Metrorail system.

The finished extension will run from East Falls Church to Dulles Airport and on to Ashburn. It will serve Tysons Corner, Virginia's largest employment centre, and the Reston Herndon area, which is the state's second largest employment concentration.

With 11 new stations, it will also link the airport with downtown Washington.

Phase 1 is planned for completion in 2013, and will run from East Falls Church to Wiehle Avenue on the eastern edge of Reston. It will include five stations, two ventilation buildings and wayside facilities. There are also two tunnels to be dug, along with the construction of five bridges and three aerial viaduct structures.

Outlining the scope of Phase 1, Dominic Cerulli, Bechtel's tunnel task manager, said there were more than 13 km of retaining walls, and numerous modifications to existing roadways. He said the major areas of construction could be grouped into four sections - aerial, civil, stations and tunnel. He estimated a total of 4.3 million direct-hire man hours and 2.2 million from subcontractors, for Phase 1.

A major section of Phase 1 is the construction of a twin-tube tunnel with a total length of 518 metres (1,700 ft), with 12.8 metres (42 ft) of excavation required a week.

The New Austrian Tunnelling Method (NATM) is being used - a technique which integrates the principles of the behaviour of rock masses under load, and monitoring the performance of underground construction during the work. Tunnel reinforcement, based on the ground conditions, requires steel girders to be installed every 91 cm (3 ft) for structural support.

"It's not long but it is tricky," said Mr Cerulli. "It is under the roadway so settlement of the road is not a good thing."

The area between the girders is coated with a special shotcrete mix to stabilise the tunnel. The shotcrete mix uses densified microsilica mixed with steel fibres to achieve high strength quickly. The mix has a slump of about 180mm which is said to ensure proper workability with the shotcrete pump while being sticky enough to avoid excessive rebound.

There is an initial liner, then a waterproof one, and a final finish "to make it look pretty for the public".

The project called for a mobile shotcrete plant with a small footprint at Tysons Corner. A Liebherr-Easymix 1.1 was chosen. With this, the mixing system, bins for the aggregates, the water tank and control room with the latest microprocessor controls are all integrated into the container.

The only parts to transport separately are the water and cement weighing module, and the cement feeding components. As the container stands on a steel foundation, no additional concrete foundations are needed.

The standard 1 m3 (1.3 yd3) ring-pan mixer from Liebherr has been upgraded in the Dulles Corridor Metrorail project with a rotary agitator. This adds considerably higher energy into the mix, to help ensure continuously high quality shotcrete. There is a 48 kW mixer motor.

Microsilica is stored and batched using a specialised silo, screw and weighing system. A vibration bowl with a conveyor belt delivers steel fibres directly into the pan mixer. With a batch cycle of 120 seconds, the production output is said to be about 30 m3/hr (40 yd3/hr), which Liebherr said easily matched the volume requirement for the project.

Litronic BCS II control, based on a Panel PC with microprocessor, and the Litronic FMS-II moisture control are also used to make sure the recipe is followed precisely.

The pan mixer discharges the shotcrete mix onto a concrete belt for loading special low-built truck mixers which move between batch plant and tunnel. The mix can be retarded for three hours, and then an accelerator is added.

Mr Cerulli said, "We are constantly checking the quality. Shotcrete is critical to the project and we have had no failures. It is very high quality, consistent shotcrete."

He said that the shotcrete strength development at 10 hours was the specified 1000 psi (6.9 MPa). At 24 hours, with a specified strength of 1800 psi (12.4 MPa) it reached 4132 psi (28.5 MPa), at seven days, specified 3500 psi (24.1 MPa), the actual figure was 6089 psi (42.0 MPa) and at 28 days when 5000 psi (34.5 MPa) had been specified the actual figure was 6692 psi (46.1 MPa).

The shotcrete plant had to be enclosed for winter protection.

"Last winter we had 3 ft (0.9 m) of snow," said Mr Cerulli, "but we only lost three days." Back-up plans for dry-mix equipment and materials, and an alternative local supply of shotcrete were in place but not needed.

He said Phase 1 of the Dulles Corridor Metrorail Project was benefiting from Bechtel's experience, and that there was NATM support from Austrian-based tunnelling and underground construction company Beton-und-Monierbau. On-site engineering support has been supplied by Gall Zeidler, a specialist in geotechnics, tunnel engineering, and tunnel construction management, which claims special expertise in transportation and infrastructure projects for major metropolitan settings.

Mr Cerulli said the project had seen good equipment supplier support. "You can't get a lot of people at the front," he said of the tunnelling work. "It gets too crowded. That's why equipment selection is so important."

The entire Dulles Corridor Metrorail Project is designed to offer a viable alternative to driving and to ease the notorious traffic congestion in the area. Phase 2 will run from Wiehle Avenue to Ashburn, and it will serve Reston Town Centre, Henron, Dulles Airport, Route 606 and Ashburn.

Preliminary engineering (PE) is currently underway for Phase 2 and is planned to be completed in 2011, ahead of design-built solicitation for the project.

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