Approach
A series of geotechnical test borings were completed along the pipeline alignment. The geotechnical investigation also included research into geologic setting, development history along the alignment, coordination with U.S. Army Corps of Engineers work, and construction precedent for past Napa River undercrossings.
Both horizontal directional drilling and microtunneling were considered for construction of the siphon beneath the Napa River. Due to limited surface area on either side of the crossing, microtunneling was selected as the crossing method.
The presence of cobbles and boulders was documented in both the project field investigation and the construction records for a nearby tunnel undercrossing of the Napa River. As a result, it was recommended to change the design of the siphon from twin 30- inch diameter pipelines within separately installed 36-inch diameter steel casings to installation of the twin 30-inch pipelines within a single 72-inch diameter steel casing. The switch to a larger casing (and thereby larger tunneling machine) provided increased ability to ingest cobbles and boulders.
Other project challenges included the design and construction of 55-foot deep ‘watertight’ shafts for tunneling, and a section of “inclined” tunneling at over 15% upslope which required significant modifications to standard tunneling equipment and procedures.
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