One Project, Four Trenchless Methods

The Alameda County Water District recently completed the construction of over 2,750 meters (9,100 feet) of 400- and 560-millimeter (16- and 22-inch) diameter water pipeline through residential neighborhoods and commercial/industrial areas of the cities of Fremont and Newark, California. Along its alignment, the project pipeline crossed under the Hetch-Hetchy Aqueduct (a major water supply transmission pipeline for the City of San Francisco and surrounding communities), several major arterial roadways, two flood control channels and Interstate 880. The majority of the project pipeline was constructed by open-cut trenching; however, site specific geotechnical and operational concerns (e.g., existing utilities, space constraints, etc.) at critical project undercrossings warranted the evaluation and eventual selection and use of four different trenchless construction methods. Trenchless construction methods that were evaluated and utilized include microtunneling, horizontal directional drilling, auger bore and jack tunneling and pilot tube guided boring. The paper will present case histories for four of the project undercrossings, discussing: why specific trenchless construction methods were selected for the individual crossings; the design measures that were implemented to account for specific subsurface conditions (e.g., high groundwater table, flowing ground, contamination, etc.); issues that arose during construction; and how construction- related issues were resolved.