North Davis Sewer District East Outfall Replacement Tunnel
The North Davis Sewer District initiated the East Outfall Replacement, Phase 2A project to both increase sewer capacity in the Kaysville area and replace or rehabilitate aging infrastructure. The project included rehabilitation of existing sewer lines, installation of new system components, and replacement via new installation of a sanitary sewer tunnel beneath Union Pacific railroad tracks, Utah Transit Authority tracks, and Interstate 15.
Brown and Caldwell served as the lead engineer. The project design team included Gerhart-Cole as the geotechnical engineer and Lithos Engineering as the trenchless engineer. Lithos coordinated geotechnical investigation aspects related to the trenchless crossing with Gerhart-Cole to ensure adequate data collection relevant to tunneling.
The tunnel was 375 feet long. Initial support was provided by a jacked 48-IN DIA steel casing pipe and the carrier pipe was 30-IN FRPM. Cover ranged from 7 feet beneath the railroad tracks to 12 feet beneath the interstate and soil conditions were characterized by saturated clay, silt, and fine sand lacustrine deposits. Design documents prepared by Lithos included a feasibility review, Geotechnical Baseline Report, trenchless-related specifications, drawing details, and other engineering design elements. Before and during construction, Lithos provided bid support, performed submittal reviews, responded to RFIs, and provided a full-time field engineer to oversee trenchless construction and identify construction risks.
Project challenges included saturated and extremely soft fine grained materials combined with unknown ground conditions beneath the highway and railroad. Allowable trenchless construction methods included microtunneling, guided pipe ramming, and guided auger boring contingent on ground conditions and water inflow observed during test pit excavation.
Whitaker Construction was selected as the general contractor and BTrenchless was the selected trenchless subcontractor. BTrenchless successfully installed the sewer line using guided auger boring from February to March 2018.