Butterfield/Meadowcroft – Arroyo Gravity Sewer Improvements Project

Project Details

Pilot Tube Guided Auger Boring

  • 500′ of 18″ VCP gravity sewer
  • 2 drives in densely travelled corridor

Geotechnical Conditions

  • Very stiff to hard sandy fat clay
  • Moderately hard sandstone


  • Densely developed urban neighborhoods
  • Constrained construction window and hours
  • Tight line and grade tolerances
  • PTGAB in rock required drilling modifications

Project Overview

Bennett Trenchless Engineers provided design review services to Ross Valley Sanitation District for their trenchless installation of 500 feet of 18-inch VCP gravity sewer by pilot tube guided auger boring (PTGAB).  BTE identified several areas where the design could be improved to increase constructability, decrease traffic impacts, and to ensure that qualified trenchless subcontractors bid on the project.  The project had many logistical challenges including tight utility corridors; a very short construction window to minimize traffic impacts to the nearby elementary school; constrained work areas at all shaft locations; and minimizing impacts to the neighborhood. 

The largest challenge on the project was the anticipated geotechnical conditions which included very stiff to hard sandy fat clay and moderately hard sandstone.  Typically PTGAB construction is used in displaceable soil conditions.  BTE recommended additional geophysical investigations to characterize the interface between the soil and underlying bedrock; identified advancements in PTGAB methods for non-displaceable soils; researched qualified, experienced contractors; evaluated the constraints of the method; and proposed design changes to optimize the project.   BTE provided trenchless contractor pre-qualification requirements, and evaluated pre-qualification packages.  BTE also provided engineering services during construction as a subconsultant to the construction management consultant, Psomas, including submittal review,  responding to RFIs, and full-time specialty construction inspection during PTGAB work.