A Case Study in Corestones - Challenging Pipejack Beneath I-8 in San Diego

A Case Study in Corestones - Challenging Pipejack Beneath I-8 in San Diego


Matthew Wallin, Bennett Trenchless Engineers, Folsom, California United States
Mark Niemiec, Padre Dam Municipal Water District
Michael Hindle, Padre Dam Municipal Water District
John Harris, Michael Baker International

Abstract: Padre Dam Municipal Water District (PDMWD) recently completed construction of a new water reservoir, pump station, and pipeline that improves reliability within their service area near San Diego. One of the most high-risk elements of the pipeline construction was a 350-foot trenchless crossing of Interstate 8. As well as crossing a major arterial highway in San Diego County, the tunnel alignment passes beneath several existing utilities including an 18” water line and a natural gas line. The ground conditions at the project site generally consist of weak granodiorite rock. However, the formation is known to include floating, boulder-sized corestone material that can be up to 10 times harder than the surrounding rock. These conditions posed significant challenges to the installation of the 48-inch steel casing.

This paper presents the characteristics of the challenging corestone material found on site, and its effects on the trenchless design and construction. Design efforts focused on the specification of appropriate pipejacking equipment to handle the varied rock conditions, given that construction of a rescue shaft was not feasible within Caltrans ROW. Contractual geotechnical baseline conditions were established for bidding purposes, as well as for evaluation of potential differing site condition claims (DSC). During construction, the contractor’s initial approach to the pipejacking work ran into difficulties when corestones were encountered. The contractor attempted to continue with the original method before ultimately changing tunneling equipment to complete the bore. The paper closes with a discussion of how the baselines were used to resolve the potential DSC claim.

Publication Date: 2019

Presented at:
NASTT’s 2019 No-Dig Show Chicago, Illinois
March 17-21, 2019 | Paper Number: TM1-T4-01

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