As a subconsultant to WRECO, Bennett Trenchless provided design services for the trenchless replacement of an existing 48-inch culvert beneath US Route 1 along the Northern California coast for Caltrans. The original CMP culvert was installed on a steep grade under a hairpin turn in the road along a steep section of the coast, resulting in high velocity flows. The invert of the culvert was completely deteriorated and the culvert diameter was too small to pass large debris during storm events. Although the culvert was only about 90 feet long, open-cut methods could not be used due to the lack of a convenient detour around the remote project location. In addition, Caltrans required that the pipe be upsized to 108-inch to reduce the frequency of debris removal at the upstream end. Increasing the culvert diameter significantly reduced the depth of cover beneath the roadway, increasing the risk of settlement. The existing culvert was installed at the same time as the road was constructed. No geotechnical borings were drilled, but the ground conditions at the crossing site were anticipated to be debris from the road construction, increasing the likelihood of cobbles and boulders.
Bennett Trenchless conducted a feasibility evaluation of the trenchless crossing. Due to severe work area restrictions and a two-month construction window, pipe ramming was selected as the recommended construction method. Bennett Trenchless developed a pipe ramming (non-standard) specification for Caltrans as this project was the first time Caltrans had used pipe ramming. Bennett Trenchless also conducted settlement analysis, provided drawing mark-ups, and provided engineering services during construction.