Pleasant Hill Grayson Creek Trunk Sewer

Project Details

Pilot Tube Guided Boring

  • 19 drives totaling over 5,800 feet
  • 18-24-inch VCP gravity sewer

Front Steer Guided Boring

  • 7 drives totaling over 2,000 feet
  • 20-inch HDPE gravity sewer

Geotechnical Conditions

  • Medium stiff to stiff lean clay
  • Groundwater up to 10′ above crown


  • Highly constrained work areas
  • Limited work hours
  • Groundwater infiltration and disposal

Project Overview

As a subconsultant to West Yost and Associates, Bennett Trenchless Engineers provided specialty construction inspection for over 5,800 feet of pilot tube guided boring for the Grayson Creek Trunk Sewer project.  The project included over 9,200 total feet of 18-inch and 24-inch gravity trunk sewer pipelines in Pleasant Hill, California.  The project was  located within residential neighborhoods and along heavily travelled corridors with multiple existing utilities.  Trenchless construction solutions were specified to minimize impacts to residents and traffic for construction of the 20-25-foot deep sewer.  Additional crossing features included a Bureau of Reclamation 96-inch storm drain with minimal clearance, two creeks, and an archaeologically sensitive area.  The trenchless construction included 19 pilot tube guided boring drives and 7 front steer guided boring drives, with a total of 35 shafts. Project challenges included highly constrained work areas with overhead utilities, short construction windows, groundwater infiltration into the shaft , and groundwater disposal.  Automated, real-time vibration monitoring was installed on houses  near launch and reception pits to assess whether  construction activities were creating vibrations that could possibly damage nearby houses.

Bennett Trenchless provided full-time on-site construction inspection services for the pilot tube construction and evaluated the contractor’s VE proposal for use of front steer guided auger boring.