Tualatin Valley Water District North Transmission Line
Cornforth Consultants (CCI) performed geotechnical investigations for 65,000 linear feet of 72-inch water supply pipeline and 19,000 linear feet of 48-inch water supply intertie pipeline. The investigations included reconnaissance and 30 exploratory borings. The route crossed five railroads, six rivers and streams, and about 20 highway/street crossings. Geotechnical recommendations included pipeline support, bedding, boring/jacking under railroads, groundwater conditions, dewatering, shorings, pipeline loading, backfill materials, and compaction.
CCI also assisted the City of Hillsboro during construction and microtunneling under the Tualatin River. Buried logs presented a high hazard for the microtunnel boring machine and CCI helped evaluate alternative options to complete the tunnel.
High groundwater levels also presented a challenge to the contractor in terms of trench shoring and subgrade stability beneath the pipeline. CCI evaluated options to shore and dewater the pipeline excavation including wells, sheetpiles and deep soil mixing.