Navigating a crowded underground utility space through a dense Silicon Valley neighborhood, a design-build team is using a number of alternative technical concepts on a $2.3-billion Berryessa VTA/Bay Area Rapid Transit (BART) extension.

A Skanska-Shimmick-Herzog joint venture won the $772-million design- build contract in 2012 to build 10 miles of electrified, double-track, grade-separated alignments and two stations. Bernice Alaniz, spokeswoman for the Santa Clara Valley Transportation Authority, the project owner, says it is the largest capital project in South Bay history.

The team has suggested a number of changes to the initial request for proposals, to minimize construction impacts on neighbors both during work and once trains are running, zipping by at 70 mph. At peak times, one could arrive every eight minutes.

To minimize settling in an area with a narrow right-of-way, the contractor suggested lightweight cellular concrete (LCC) for the riding surface. Designers considered stone columns and driven piles but settled on LCC for mechanically stabilized earth approaches in a residential area with as little as 4 in. of clearance to the property line. Crews replaced existing soil with LCC. "We tricked the soil," says Brad Nystrom, Skanska deputy project director.

Crews installed 12,500 linear ft of sound walls and noise-insulating windows in 246 homes. Crews also installed 4,200 tons of aggregate, made from recycled tire rubber, as sub-ballast under floating tracks in four sections to reduce vibration from trains to a frequency of 8 Hz. Sound-absorbing paint further limited reverberation.

The new Milpitas Transit Center houses a trenched track, and bus-terminal designs reflect the industrial history of the area. Skylights and six industrial fans will ventilate the station in case of fire and bring in light. Water-balancing pipes will control stormwater. An alignment with a gradual grade into the station should improve ease of maintenance and operations.

Thirteen utilities were included in an internal permitting process that required sign-offs daily before excavation. The 10-mile stretch was divided into 31 design units. "It requires planning in smaller, bite-size pieces, so everyone knows what 500-ft section will be worked on each day," Nystrom observes.

One alternative technical design entailed lowering the track alignment into a cut-and-cover section at an intersection, instead of using an at-grade guideway trench, which would have required expensive utility removal and sewer siphons. "This was a much cleaner solution," says Timothy Schmidt, senior associate director for lead design firm Lockwood, Andrews & Newnam Inc. Sewer pipes were held in place with straps while the train box was built below.

"The biggest challenge is scale," Nystrom says. More than 700,000 cu yd of earth had to be excavated and 280,000 cu yd of concrete poured.

At the Milpitas station, crews worked night shifts, hauling excavation material without backup beepers because that would have disturbed neighbors. Extra workers acted as spotters to guide trucks safely.

In a section where the BART track parallels Union Pacific Railroad tracks, railroad intersection detection lasers alert BART operations control if anything penetrates the right-of-way so the light rail can be remotely shut down. Contractors use drones for inventory purposes.

The Berryessa extension is on schedule to open in fall 2017, with no major injuries reported to date. The project will be followed by a $4.7-billion, 6-mile extension to Santa Clara that is planned to open to passengers in 2025.