Gray Lodge Wildlife Area Water Supply Project

Project Details


  • 72″ ID steel raw water
  • Twin drives, 90′ long
  • UPRR and Kinder Morgan crossing

Geotechnical Conditions

  • Medium dense silty sand
  • Stiff sandy silt
  • High groundwater


  • Watertight shafts required
  • Strict environmental constraints
  • Difficult site access
  • UPRR encroachment permit with waivers
  • Tight construction window

Project Overview

As a trenchless subconsultant to Provost and Pritchard, Bennett Trenchless Engineers provided feasibility analysis, full design assistance, and permitting assistance for twin 72-inch trenchless crossings of UPRR to increase water deliveries to the Gray Lodge Wildlife Area.  The US Bureau of Reclamation and California Natural Resources Agency both funded portions of the project.  Trenchless methods were necessary to install large diameter pipelines beneath a high-priority section of UPRR track.  To mitigate the risk of settlement to the railroad, microtunneling and watertight construction methods were specified.

Challenges on the project included high groundwater with a nearby source of recharge, environmental restrictions on allowable work area and construction season, difficult site access, and coordinating with UPRR to secure an encroachment permit.  The UPRR encroachment application requested a casing requirement waiver for the inverted siphon crossing due to the large diameter of the carrier pipe and the increased risk of settlement.  Through meetings with the client and UPRR engineering staff, Bennett Trenchless was able to demonstrate that UPRR’s concerns had been adequately addressed by the design documents.  Mitigation measures included a robust settlement monitoring program, use of Permalok casing to increase speed of installation, and use of contact grouting to stabilize the soils.