...system (ENR 7/17/06 p. 10). In August 2006, Turner received notice to proceed under a firm-fixed-price $88-million contract. Congressional budget limits forced the project to drop a 74,000-sq-ft laboratory and office complex. SLAC will rehabilitate other facilities instead.

Shortly after groundbreaking ceremonies in October 2006 at the site of the Near Experimental Hall, crews began site work for the Beam Transport Hall, which traverses the Research Yard. “The biggest thing was for existing utilities to be rerouted out of the way of the BTH,” says Owens. After more than 40 years of operation, many utilities didn’t show up on the Research Yard’s as-built drawings.

The first concrete was poured on April 5 for the Beam Transport Hall’s 3.5-ft-thick slab-on-grade. This area is the owner’s first priority because it connects directly to the Linac building. Completion will allow installation of LCLS line equipment to begin, says Owens. The Near Experimental Hall’s sub-basement slab was completed April 2, and work will continue with construction of the NEH’s second level, which lies on the beam line. “The goal is to work on all areas at once, and every area is now in progress,” Owens says.

Since March 12, Affholder has advanced 50 ft into the Far Experimental Hall’s access tunnel with a road header, says Patrick Doig, project manager for Hatch Mott MacDonald, which is managing the tunnel work. A second road header arrived the first week of April and is beginning the tunnel from the Research Yard for the Undulator Hall.

The ground throughout the LCLS site is dry, soft sandstone, lying well above the water table. “The sandstone behaves more like soil than rock,” says Doig. “It has a good stand-up time.” He doesn’t anticipate any surprises in the tunneling. “The ground is fairly well known in this area” because SLAC has occupied the site for so long. After the machines cut 4 ft, crews apply 3 in. of gunite to the walls, erect lattice girders and apply 3 in. more gunite before continuing. A final 6 in. of gunite and a concrete floor slab later will finish the tunnel. Progress is 4 to 8 ft per day, says Doig.

The current single shift works 10 hours daily, five days per week, plus nine hours on Saturday, says Owens. Later this month he will start a night shift on the access tunnel, then at the Undulator Hall.

The FEH excavation will begin about mid-May. “The FEH mined cavern is the biggest challenge on the job,” says Doig. The cavern, measuring 212 x 46 x 29 ft high, will be mined in nine cuts on three levels. “It isn’t that often you get to build something this large in soil,” says Doig.  FEH mining will take about four months, with additional time required to apply 12 in. of gunite to the walls and roof.

The Undulator Hall mining that just started will take eight to 10 weeks, while the other machine mines the FEH. The Undulator Hall tunnel, 19 ft wide by 14.5 ft high, emerges at the beam dump, one of three cast-in-place concrete structures that includes the Near Experimental Hall. The road header then will move to the Far Experimental Hall. One machine will continue excavating the FEH while the other heads back toward the NEH, mining out the X-Ray Transport and Diagnostics Tunnel. All tunneling is scheduled to be completed by December.

Standford Linac Center Excavation of access tunnel just began.

Precise

The electron beam must remain precisely targeted, maintaining a tolerance of 2 microns over 130 meters. “This is a laser and focusing beam on a target more than 2.5 miles away from the electron gun that starts the beam,” says Richard Bambam, project architect in Jacobs’ Cypress, Calif., office. The alignment must remain absolutely flat and straight, actually deflecting 1.5 in. toward the Earth’s center to compensate for the Earth’s curvature before angling back up. Forces that might distort the beam include temperature, vibration and barometric pressure, while some parts of the LCLS require concrete thicknesses of up to 72 in. to shield radiation from within.

Still, Galayda says the job has “pretty standard engineering tolerances.” Structures must be aligned within a couple of millimeters but final alignment of line equipment will be performed by machinery. The concrete floors have no expansion joints. “If it cracks, it cracks,” Galayda says. The equipment can compensate better for shrinkage alone, he says.

Sitting just a mile from the San Andreas Fault, SLAC has site-specific seismic criteria beyond the building code, but seismic events are not a major threat. “Generally, underground structures tend to be pretty safe in an earthquake,” Galayda says. Buildings fall because of differential motion between base and top, he notes. SLAC is designed for a magnitude 7 quake centered a few miles away.

For the equipment, “we need 0.1°C temperature stability,” Galayda says. “HVAC is pretty important to us, particularly the regulation” to maintain a narrow range of equipment temperature. Ambient temperature can fluctuate because the equipment’s mass holds the temperature stable, he notes. Temperature control presented “interesting challenges in the geometry of it,” says Dennis Hickman, manager of engineering at the Portland, Ore., office of Jacobs, which performed the mechanical, electrical and plumbing design. Jacobs designed a system of zone control, in effect separating the tunnels into “segments of the Tootsie Roll,” he says.

Turner is to complete civil construction by June 2008. SLAC then will complete installation of the line equipment. When LCLS operation begins in 2009, the facility is expected to beat its nearest rival, in Hamburg, Germany, by no more than a few months. It will cast the future of science in a whole new light.