Tunnel keeps transit station environmentally sound

Portland’s new Westside MAX (Metropolitan Area Express) adds 18 miles to the region’s existing light rail line, providing modern rail access from Gresham, east of the city, to Hillsboro on its west. “We now have 33 miles of light rail connecting the convention center, Blazer Arena, civic stadium and other major destinations,” says Tuck Wilson, project director for the Tri-County Metropolitan Transportation District. “Within two years, the line will also connect with the Portland Airport.”

The $963 million Westside extension includes a 3-mile twin-bore tunnel under Washington Park, the busiest tourist attraction in the Portland area; one underground station; 19 surface stations; four multimodal transit centers; and nine park-and-ride lots. Parsons Brinckerhoff, New York, served as the lead civil engineering consultant for most of the project and the construction management consultant for all of it.

The twin-bore tunnel segment was selected over an aboveground alignment to preserve an environmentally sensitive canyon west of Portland. It incorporates Washington Park Station which, at 260 feet below the surface, is the deepest underground transit station in the United States.

The tunnel had to be cut through three types of rock: dense Columbia River basalt; a hard, clay-like silt known as Sandy River mudstone; and Boring lava, a locally derived basalt. At the western end of the tunnel, where rock conditions changed continuously, workers excavated via drill-and-blast, while a laser-guided tunnel-boring machine was used to cut through Columbia River basalt on the eastern end.

The boring machine was custom-built with a smoother-than-usual face to prevent the cutters from becoming plugged with basalt nuggets. Noise barriers were erected around the tunnel portals to protect the surrounding neighborhood during blasting, and homes near the east portal were soundproofed with double-glazed windows and air conditioning – a first for a transit project.

Belying its subterranean location, Washington Park Station is open and spacious thanks to 34-foot-high platform caverns, generous lighting and the constant stream of fresh air that flows naturally from one portal to the other. “The station is a major attraction in itself,” Wilson says.

Artwork in the station includes a “time line” railing with glass-encased rock samples from each period in the hills’ 16-million-year history, along with drawings and descriptions etched into the station’s granite walls. “The art and architecture give it a warm, welcoming feeling,” Wilson says.

Platforms in the Washington Park Station are separated by a pillar of rock, with enough room on either side to handle the entire passenger load for which the station was designed. The pillar incorporates fire doors so that, in an emergency, either platform can serve as an area of refuge.

Two vertical shafts between the tunnel and the surface house the elevators, stairwells, and emergency ventilation and power systems. The stable rock surrounding the tunnel provides excellent natural protection from earthquakes, and special details at the interfaces between shafts and tunnels provide additional distortion tolerance and help the shafts move with the ground.

Four high-speed elevators whisk passengers from the platform to the station’s surface level in less than 15 seconds. The surface level was carefully designed to blend in with the surrounding hills. Theshowcase entry level features an outdoor amphitheater set in a garden-like setting. Up to 2,770 passengers can move through the station every hour on their way to and from the nearby Metro Oregon Zoo, World Forestry Center, Oregon Vietnam Veterans Memorial and Portland Children’s Museum.

The entire Westside line is equipped with low-floor train cars. “We had tried lifts and special platforms in the past to achieve ADA compliance, but found them slow and difficult to use,” Wilson says. “When this project was in design, we searched for a vehicle that could accept curbside boarding without a lift, to simplify boarding for the young, elderly and disabled, and to prevent delays.”

Low-floor technology removes auxiliary equipment from the underside of the transit vehicle so that the body can hang closer to the ground. In Europe, where the technology was developed, the motor is typically reduced in size, the wheels are located at the front, back and center of the cars, and electric brakes are used instead of pneumatic compressed brakes.

For the Portland system, the first in the United States to use the technology, Iselin, N.J.-based Siemens Transportation Systems built the cars with powered wheels at the front and back only. A special section with small-diameter, unpowered wheels is in the center. Two of the four doors on each side of the cars are equipped with an extending bridge plate that can be lowered to provide wheelchair access, eliminating the delays typically associated with wheelchair boarding.

The cars were built with a heavier articulation joint than their European brethren to achieve a design speed of 55 miles per hour. An operator’s cab is located at each end for fully reversible operation. The traction electrification system is conventional overhead catenary.

All elements of the new Westside line are compatible with older Banfield segments, allowing the two lines to operate as a fully integrated system. The extension, which opened last fall, has been even more successful than its planners hoped. “Our goal was to achieve a daily ridership of 20,000 by the end of the first year,” Wilson says. “We met that goal by the end of the first month.”