The Problem:

Trains could only run on reduced speed for one weekend and steep penalties would have been enforced if regular train operation was impacted during construction.

The Solution:
California contractor Pacific Boring Inc. had a rough task on their hands.  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 undercuts 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. 

Project Overview:

Located in southern San Francisco, the crossing is part of a 23,000-ft long, back-up cross town water main.  The new line is the first of many for the SFPUC’s East/West Transmission Project, which will build in redundancy in several key water lines in case of emergencies such as earthquakes or large scale pipeline failures.  “This region is semi-arid and does not have readily available water supplies – all the supplies are actually groundwater piped in from the Hetch Hetchy reservoir located in the Yosemite National Park 300 miles away.  The East/West project will help secure water supplies by piping in water from the eastern side of the city,” said Stephen Martin, construction inspector for the San Francisco Water Department.  Once the pipeline is finished, the city’s two main reservoirs will be connected to one another. 

Choosing the Motorized SBU:

Pacific Boring Inc., an auger boring and tunneling contractor from Caruthers, California, was subcontracted for the crossing and began site preparation in autumn 2008.  The company had already completed two shorter bores using an ABM and conventional soft ground head for the project.

While Ranger Pipeline was excavating the 12-ft wide by 40-ft long launch pit for the BART tracks crossing, it hit much harder rock than anticipated (about 14,000 psi UCS).  During pit excavation the contractor also hit a mixed face of rock and soft ground at a 20-ft depth, prompting the crew to lower the boring pit another 20 ft into more stable rock.

The Project team changed the initial contracted method, which specified pipe ramming.  After encountering the diameter TBM or other methods would not be successful.  We wanted a more versatile and accessible cutting head, and Pacific Boring suggested the SBU.

Crossing Excavation:

The small construction site (40 ft wide by 80 ft long) on a residential street in the city was about 15 ft from a row of houses and prompted the contractor to move residence to a nearby hotel during the weekend launch of the SBU-M excavation began on a 24-hour basis on January 31st.  Boring was given until Monday morning on February 2nd to make it past the 70-ft mark and the BART tracks.

The machine utilizing 11.5-in disc cutters began excavating the Franciscan formation consisting of greywacke rock.  “After we got through the initial section of hard rock, we ran into a muddy fault zone for about 18 ft until we got back into the fractured rock we were expecting,” said Iles.

The fault zone, consisting of fractured rock and hard pan, allowed the machine to advance at a rate of about 1 ft every 30 minutes.  Once back into hard rock, the machine averaged about 3 ft per hour.  Every 20 ft, crews lowered a new section of 60-in Permalok steel casing for installation.  Muck was removed using a crane with hydraulic clam. 

By Monday morning, the SBU-M had advanced about 80 ft meeting the project goal.  After the tight initial schedule, crews resumed normal 10-hour boring shifts.  Pacific Boring expects to have the crossing completed in the first quarter of 2009.

As taken from:  Trenchless Technology Magazine, March 2009