Make Way for Progress—The Challenges of Relocating Large Diameter Water Mains for Light Rail System Expansion

Abstract

The Valley Transit Authority is working with the Bay Area Rapid Transit (BART) to ultimately expand BART's existing light rail system south to San Jose. Several new stations are proposed as part of the Light Rail Track extension. However, two of the new stations will impact two existing large diameter water pipelines, owned by the Santa Clara Valley Water District. These existing lines have to be relocated away from the new stations. This paper will discuss the technical design aspects of installing new pipe and fittings into an existing 66" concrete pressure pipe raw water main and into an existing 42" steel pipe treated water main in order to properly relocate both lines. A key consideration in this modification to the existing lines is the thrust forces generated from the realignment. The thrust restraint of the old 66" line posed a number of challenges since the original pipe in this area did not have longitudinal thrust forces. Additionally the owner, the Santa Clara Valley Water District, has their own specialized thrust restraint procedure that is based on a 1960s paper. This approach is different than what is recommended by the AWWA in the pertinent AWWA design manuals. Other challenges encountered during the relocation design included a number of pipe design considerations such as internal and external load, buckling, and fittings design. Further complicating the relocation effort is the need to account for a special "rattle box" casing to protect existing gas lines under a new "floating slab" for the Light Rail Track at one location. Included in the discussion will be recommendations on a few "lessons learned" from the relocation design efforts. BACKGROUND