Canopy tube pre-support installed over the top heading marks a start to excavation of the single-tube double-track NATM tunnel for the Mission Valley East LRT Extension of San Diego’s Trolley system at San Diego State University. From a section of cut-and-cover, the 330m (1,000ft) long x 11.25m (37ft) wide x 8.85m (29ft) high NATM tunnel portals into a steep slope of dense conglomerate beneath one of eight temporary bridges required to maintain normal campus routine during construction.
A secant pile diaphragm wall supports excavation of the slope back to a vertical wall into which the tunnel portal strikes at an angle of approximately 45o to follow its curved alignment into the underground cut-and-cover station at the center of the university grounds.
Clark Construction Group, owner of Guy F Atkinson, was awarded the campus civil works and underground construction contract in early 2001 after submitting a low bid of $82.7 million. Lead designer of the underground section for owner, the Metropolitan Transit Development Board, is BRW (now part of URS) and Hatch Mott MacDonald is specialist NATM tunnel sub-consultant. Construction management for the full extension project is awarded to Washington Infrastructure Services with sub-contractor Underground Construction Managers. Two separate teams led by Balfour Beatty and Modern Continental respectively have been awarded construction of the at grade and elevated sections either side of the tunnel and underground works on the total 5.8 mile (9.25km) long extension.
“Clark Construction started on site in June last year,” said Dave Ragland, MTDB’s Senior Engineer for the underground section of the line, “and we are generally pleased with progress to date. The start of some activities was delayed during negotiations with the SDSU staff regarding the impact of our work on the campus environment, but Clark has made good progress on the open cut station excavation as well as the cut-and cover tunnel sections and we are pleased now to see start of the NATM tunnel section.”
During a visit to the project in early April 2002, T&TNA toured the works and watched crews installing the canopy of 180mm (7in) diameter x 18m (56ft) long pre-support tubes. Cover at the portal is about half the tunnel diameter and the canopy tubes are installed on about 450mm (18in) centers into the gravel, cobbles and occasional boulders of the sandy clay matrix conglomerate.
Using a Liebherr tunnel excavator, Clark plans to advance the full top heading of the tunnel length to the underground station first and work the bench and invert back to the portal. Primary support will comprise lattice girders and wet mix shotcrete and progress will advance in rounds to suit ground conditions.
“Side drifts are also an option depending on how the conglomerate material behaves and how the contractor approaches the mining works,” explained Lee Warnock, Southwest Area Manager for Hatch Mott MacDonald. “A range of different NATM sequences likely to be required on the project were included in the contract documents as bid.”
The single-tube, double-track NATM tunnel replaces an earlier option for a deeper twin-tube tunnel using shielded-TBM excavation and a precast concrete segmental lining. The deeper alignment was below the water table and required a deeper underground station. The selected NATM tunnel at a higher elevation raised the alignment above the water table and allowed for a signature open cut station that daylights to the campus common and requires less tunnel excavation.
“There are also natural voids in the unconsolidated fill material used to level some of the gullies in the hilly topography of the campus as well as lenses of high permeability sands in the conglomerate,” said Ragland. “The higher NATM alignment avoided the need to undertake potentially extensive surface consolidation grouting along the alignment and reduced the risks associated with the longer tunnel and deeper station in water bearing material. The trade off is more extensive open cut tunnel work. This has required diversion of more than 5 miles (8km) of utilities and services, completed under separate contract, and the need to provide temporary bridges and walkways to retain road and pedestrian access on the campus.”
The selected option has also included steeper alignment gradients. “From the elevated alignment that follows the I-8 freeway, gradients of 4.3% are required to connect with the 20m (65ft) deep campus station at the top of the hill,” said Siegfried Fassman, a Vice President for URS and Chief Engineer on the project for BRW. “These are maximum gradients for the downhill breaking system of the Trolley LRT. Threading the elevated and open cut sections between the adjacent buildings has also been a challenge.”
The chief milestone for the underground contract within the total $431 million Trolley extension project is November 2003 when the civil works are to be turned over to the program wide track and systems contractor. Passenger services on the extension are scheduled to start in December 2004.
