In less than 72 hours, the company needed to excavate the first 70 ft of 300-ft long trenchless crossing underneath high-volume rail tracks. The trains – for the Bay Area Rapid Transit (BART) system – could only run on reduced speed for one weekend and steep penalties could be enforced if regular train operation was impacted.
The crossing undercut not only the rail line but also a 230-ft long section of highway 280. Line and grade underneath the structures also needed to be accurate within 1 foot in predicted hard rock, requiring a boring machine rather than conventional auger boring machine (ABM) cutting heads. San Francisco Public Utilities Commission (SFPUC), in conjunction with Jacobs Associates, opted for a 60-in. diameter Robbins Motorized Small boring Unit (SBU-M) for the job, which is a type of hard rock trenchless boring attachment utilizing disc cutters.