Upper Northwest Interceptor, Section 3 and 4

Project Details

Microtunneling/Open Shield Pipe Jacking

  • 4,930′ of 84″ RCP gravity interceptor EPB Two-Pass Tunneling
  • 5,930′ of 84″ RCP gravity interceptor
  • 8,010′ of 66″ RCP gravity interceptor

Geotechnical Conditions

  • Rapid raveling, poorly graded sand
  • High groundwater


  • Limited work areas
  • UPRR crossing
  • Drainage canal crossings

Project Overview

As the trenchless subconsultant to CH2M Hill, Bennett Trenchless Engineers provided preliminary and final design services for the trenchless installation of 19,000 feet of 60 to 84-inch gravity interceptor pipe through a congested urban area. In addition to the limited work area, other challenges encountered in the design of the interceptor included high groundwater levels, utility conflicts,  crossing beneath the Union Pacific Railroad and two drainage canals.   The railroad crossings and environmentally sensitive features presented significant permitting challenges.

Bennett Trenchless Engineers led the evaluation of alignment, shaft locations, staging areas and alternative trenchless construction methods as part of the Basis of Design Report. Bennett Trenchless Engineers also reviewed geotechnical information, prepared cost estimates, pipe material recommendations, settlement evaluations, specifications, construction schedules, reviewed design drawings and provided permit assistance.  Bennett Trenchless developed the settlement instrumentation and monitoring plan, including allowable and calculated settlements, and threshold values.  Bennett Trenchless co-authored the Geotechnical Baseline Report.  The construction methods selected for the final design were microtunneling, open-shield pipejacking and two-pass tunneling. Bennett Trenchless also provided on-call inspection services and Engineering Services during Construction, including submittal review and response to RFIs.  The project was successfully completed with no construction claims.