As a subconsultant to GHD, Inc., Bennett Trenchless Engineers provided trenchless design services to relocate a water main crossing of the Mad River near Blue Lake, CA. The existing water line was attached to a local railroad bridge that was no longer maintained by the rail company and was falling into disrepair. The feasibility analysis evaluated replacement with a new pipe bridge or installation by trenchless construction methods. HDD was the preferred construction method due to lower estimated construction cost and reduced permitting requirements. The ground conditions included extensive near-surface gravel, cobble, and boulder deposits within the river channel; claystone bedrock for most of the bore length; and a 200-foot wide fault gauge comprised of highly plastic clay and sheared bedrock.
The primary challenge on the project was understanding the fault gouge and its effects on hydrofracture risk. Other challenges included minimizing impacts to the steep, densely vegetated work area at exit while accommodating the pipe layout area which was at a severe angle to the bore. A nearby railroad ROW that roughly paralleled the river was used for pipe fabrication and layout. The bore geometry was designed at an oblique angle across the river and incorporated a slight horizontal curve near exit to reduce the angle between the pipe layout and the bore. BTE evaluated trenchless construction alternatives, evaluated alignments, evaluated pipe materials, conducted design calculations for hydrofracture risk and pipe stress and pull load, provided technical specifications and drawing edits, evaluated submittals, and provided on-site specialty construction inspection services.