Recycled Water Expansion Project – Central Service Area Highway 101 Crossing

Project Details

Pilot Tube Guided Auger Boring

  • 281′ of 24″ steel casing for 16″ PVC recycled water
  • Caltrans ROW crossing of Hwy 101

Geotechnical Conditions

  • Surficial medium dense clayey sand fill
  • Soft to medium stiff silt at pipeline horizon


  • Caltrans encroachment permit
  • Mitigation of settlement risk to highway
  • Constrained work areas
  • Minimize impacts to residences and businesses

Project Overview

Bennett Trenchless Engineers provided trenchless design services to North Marin Water District for 281 feet of pilot tube guided auger boring beneath Highway 101 in Novato, CA.  The 24-inch steel casing was installed for 16-inch DR 14 PVC water pipe to provide the sole connection between service areas on opposite sides of the highway.  Challenges included minimizing impacts to the businesses and neighborhoods near the shafts, threading the needle between multiple subsurface structures (soundwall footings, electric vault), and selection of pipe elevation to minimize risks presented by ground conditions, including settlement risk beneath Caltrans ROW.  The pipe alignment was selected within soft to medium stiff silt, above the underlying sandstone, with groundwater at approximately pipe invert. 

BTE made recommendations for the geotechnical investigation, reviewed the geotechnical data, and provided feasibility analysis to determine the most appropriate trenchless construction method.  Based on the selected method, BTE performed a settlement evaluation to identify the appropriate pipeline horizon to mitigate settlement risks to Highway 101.  Final design services included drafting technical specifications, providing drawing edits, providing technical assistance acquiring the Caltrans Encroachment permit, and review of construction submittals.  The project was successfully completed without any observed settlement of Highway 101.