With more than 40% of the Far Experimental Hall (FEH) – the most difficult excavation challenge in the new tunnel complex for the Stanford Linear Accelerator Center (SLAC), California – completed by the end of last month, tunnel construction manager Hatch Mott MacDonald is pleased with progress.

Patrick Doig, senior associate for Hatch, which is managing tunnel construction for general contractor Turner Construction, said NATM excavation was proceeding smoothly and without issue, and he expects the FEH to be completed by early December. The FEH is approximately 76.2m long x 15.2m wide x 10.6m high, and there is an associated 78.9m long access tunnel.

Doig said excavation was progressing at approximately 460 m3/week. He added that the FEH was being opened in a staged, multi-drift approach. One of the challenges was getting to the crown of the 10.6m high cavern when crews were coming in at the bottom with the access tunnel. The solution was to gradually increase the height of the access tunnel as it approached the FEH, he said.

The San Jose office of Turner Construction was awarded the US$100M Construction Manager/General Contractor contract in August 2006. Affholder is executing the tunnelling works, which are valued at some US$17M, and ILF is subconsultant for NATM design. The facility, including the tunnels, was designed by Jacobs Engineering for SLAC.

SLAC is constructing the tunnels for its Linac Coherent Light Source (LCLS) complex. Billed as the world’s first x-ray free-electron laser, LCLS will provide scientists a window to the ultra small, ultra fast building blocks of matter, such as x-ray pulses. Neil Calder, a spokesperson for SLAC, said the pulses would allow scientists to make ‘real-time movies of atoms and molecules in motion’.

Extending from the existing Linear Accelerator, LCLS includes 430m of tunnels connecting two underground experimental halls – FEH and the Undulator Hall Tunnel, which extends approximately 183m from the Beam Transport Hall to the Front End Enclosure. There is also the 198m long X-ray Tunnel, which links the cut-and-cover Near Experimental Hall (NEH) with FEH cavern.

The two-part excavation of the 180m long x 5.8m wide, horse-shoe shaped Undulator Hall Tunnel began in April 2007 and was complete when T&TI visited the job site early August. ‘Since then the working invert and the final shotcrete lining of the tunnel have been cast,’ said Doig. The RC invert is to be placed this month.

Ground conditions in the area are fairly well known, consisting of mostly ladera sandstone and siltstone that is weakly to moderately well consolidated. Tunnel support comprises lattice girders on 1.2m centres with 250mm thick shotcrete on walls followed by a cast concrete invert. Work has been underway with two 10hr shifts/day, five days per week, with anticipated advance rates peaking at about 6m/day in the top heading and some 13.4m/day in the tunnel bench.

In an effort to shorten the tunnel contract by a matter of months, Affholder elected to construct an additional access adit to the X-ray Tunnel, thereby allowing excavation of the tunnel and FEH to progress in parallel. The adit is complete, and the top heading of the X-ray Tunnel was about 10% excavated by late September.

The standard support system for the Undulator and Access Tunnels, which are approximately the same size and shape, includes 150mm of synthetic fibre reinforced shotcrete applied in two equal layers, and lattice girders installed between the first and second layers. Contingency support measures, including rock dowels and self-drilled and grouted pipe spiles are available but have never been needed. In addition to the standard support elements, cement-grouted galvanised dowels are being used for permanent support in the FEH.

‘Ground conditions have been ideal throughout,’ said Doig. ‘We have had no holdups and the two Alpine AM50 roadheaders have performed well.’

In March, Affholder’s parent, Insituform, announced it would sell the subsidiary but assured SLAC the LCLS contract will be honoured. The dig is due to wrap this winter for LCLS to open in 2009.

Tunnelling is underway for X-ray lab in Stanford, California The Undulator Hall Roadheader excavation of the Experimental Hall