Opportunity
The Stanford Linear Accelerator Center was created in 1962, on 426 acres (172 hectares) of land just west of Stanford University. Two miles (3.2 kilometers) long, the particle accelerator on the site remains the longest in the world.
In 2009, the renamed SLAC National Accelerator Laboratory unveiled the world’s most powerful X-ray laser, known as the Linac Coherent Light Source or LCLS.
As explained with an animation by the Department of Energy, a drive laser generates a precise pulse of ultraviolet light, which strikes a copper plate inside the injector gun. The plate produces a burst of electrons that are guided into the accelerator. The electrons pass through two “chicanes” or bunch compressors, which organize electrons of different energies.
The electron pulse leaves the accelerator at nearly the speed of light and enters the Beam Transport Hall, where it passes through diagnostic monitors and focusing magnets. The pulse then passes through the Undulator Hall, where special magnets force the electrons to give off X-rays. The electrons are drawn off while the X-rays continue in a straight line to the target. The entire process occurs up to 120 times per second.
To create the LCLS it was necessary to add almost half a mile of new tunnel to the existing tunnel housing the original accelerator. A high degree of accuracy was required for this project.
Solution
In 2006, Mott MacDonald was retained by the construction manager/general contractor to provide construction management and tunnel inspection for the tunnel segment of the LCLS facility, which includes 2,400 feet (731 meters) of horseshoe-shaped tunnels with an internal radius of 10 feet six inches (3.2 meters). The tunnels terminate at the new Far Experimental Hall, which is 55 feet (17 meters) wide, 35 feet (11 meters) high, and 230 feet (70 meters) long.
Before construction, we provided constructability reviews, value engineering, cost estimating, and schedule support, and led the procurement effort and helped prepare the rate schedule and established procedures for a Dispute Review Board. Under a separate contract, we also prepared the Geotechnical Baseline Report for the project.
During construction, Mott MacDonald managed the tunnel subcontractor, processed submittals and RFIs, reviewed schedules and pay applications, and responded to claims. We provided full-time staff for inspection work inside the tunnels.
The tunnels were excavated through weak Ladera sandstone using two roadheaders and the Sequential Excavation Method (SEM). The tunnels were built with a shotcrete lining 12 to 18 inches (0.3 to 0.5 meters) thick, containing fiber and mesh reinforcement. Lattice girders were installed at 4-foot (1.2-meter) intervals.
Mott MacDonald was closely involved with safety management, providing tunnel safety expertise to the client’s own construction safety staff. A tunnel safety specification authored by us was incorporated into the documents for the tunnel construction contract.
Outcome
Mott MacDonald's proactive approach to procurement yielded three competitive tunnel bids, two of them below the Engineer’s estimate. Quality of construction was high, with no remedial measures needed. Settlement was minimal, never reaching a level requiring action.
The tunneling was completed on budget and two months ahead of schedule. We were able to keep the project on track even when the parent company of the tunneling contractor announced that it was withdrawing from the tunneling business.
In December 2009, Science magazine selected the LCLS as one its top-ten breakthrough innovations.
“The LCLS is a tool,” the magazine wrote, “but it deserves the appellation ‘breakthrough’ because it takes a qualitative stride far beyond its predecessors. For decades, scientists have used x-rays to probe the atomic-scale structure of materials. Shining a billion times brighter than any previous source, the LCLS produces pulses of x-rays as brief as 2 millionths of a nanosecond, short enough to snap stop-action images of chemical reactions in progress.
“Simply put, the LCLS is the first device to combine atomic-scale spatial and temporal resolution.”