Hard Rock Tackled in San Francisco Boring Project
March 1, 2009California contractor Pacific Boring Inc. had a tough task on their hands. In less than 72 hours, the company needed to excavate the first 70 ft of a 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 ft 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.
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.
“Most of this $4.56 billion project is allocated to pipelines. We have some smaller lines utilizing open-cut with jack and bores, but this is the most important pipeline we have in the city,” said SFPUC project manager Saed Toloui. At a cost of $23 million, the East/West pipeline will utilize one of San Francisco’s largest pumping stations and will undergo initial testing in May 2009.
SFPUC conducted geological testing including bore holes along the entire pipeline, as well as both conventional and horizontal bore holes for the Highway 280 crossing. Construction began on the pipeline in February 2007, requiring open-cutting of city streets by general contractor Ranger Pipelines Inc. As of February 2009, the entire pipeline had been open-cut and installed, minus the 300-ft crossing.
Choosing the Motorized SBU
Pacific Boring Inc., an auger boring and tunneling contractor from Caruthers, Calif., 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 Pipelines 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 greywacke, which is very hard rock, we determined a larger diameter TBM or other methods would not be successful. We wanted a more versatile and accessible cutting head, and Pacific Boring suggested the SBU,” said Toloui.
“We had known about SBUs for a while and heard about them from other contractors that had used them, such as Midwest Mole. We were just waiting for the right project with the right geology,” said John Iles, vice president-operations of Pacific Boring.
The contractor also selected an SBU-M over other technologies, such as conventional auger boring. “We didn’t feel comfortable going 300 ft with augers taking all of the cutting torque,” said Iles.
The Robbins SBU-M is a manned entry, hard rock boring machine for use on long utility installations or for crossings with specific line and grade requirements. The SBU-M is utilized with a standard auger boring machine or pipe jacking unit and is mounted with disc cutters to excavate rock from 4,000 to more than 25,000 psi UCS. At machine launch, the SBU-M is welded to the lead casing and a small invert auger inside the casing provides spoil removal. An in-shield motor and drive train provides torque to the cutterhead, while forward thrust is provided by the ABM through the casing.
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 residents to a nearby hotel during the weekend launch of the SBU-M excavation began on a 24-hour basis on Jan. 31. Pacific Boring was given until Monday morning on Feb. 2 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-hr boring shifts. Pacific Boring expects to have the crossing completed in the first quarter of 2009.
Potential in Urban Spaces
The project highlights the urban use of SBU technology, which only requires the space of a standard auger boring pit and can be used close to residential structures without compromising foundations. “SBU technology definitely has potential for urban projects in California in the right type of rock,” said Iles.
“The pit size can be as small as 36 ft long by 10 ft wide for a 24-in. diameter SBU. At minimum, the contractor will need space for lifting equipment and spoil removal equipment, as well as support equipment such as welders and small tools,” said Chris Sivesind, SBU division sales engineer-West & Central U.S. Overall, the SBU-M/ABM or SBU-A/ABM setups need a larger shaft or pit than microtunneling or standard soft ground pipe jacking. However, the surface area required to support a microtunneling system is larger than either an SBU-M or SBU-A/ABM setup.
Distance to structures, such as houses or commercial buildings, depends on the ground conditions. In hard rock, the method of pit excavation is typically by drill-and-blast or rock hammer, which requires some distance to sink a shaft near a structure. The softer the ground, the less critical it becomes to maintain distance from structures due to common methods of ground stabilization such as secant piles. The amount of cover necessary also depends on the geology and pipe diameter, though most hard rock rail crossings require a minimum of 3 to 4 ft of cover.
Desiree Willis is a technical writer for The Robbins Co., based in Kent, Wash.By Desiree Willis