Microtunneling Key to Outfall Water Line in California
The City of Tracy, California, executed an alteration agreement with the California Department of Fish and Wildlife and San Joaquin County Public Works to install a new 18,000-lf, 42-in. diameter outfall pipeline. The pipeline was needed for the dramatic expansion taking place in the City of Tracy. The overall project consisted of a new diffuser in Old River, water body crossings including Sugar Cut, Channel 7, Tom Paine Slough and Paradise Cut. The new diffuser would be installed approximately 800 ft west of the existing diffuser. The water crossing of the Paradise Cut would include a 680-lf microtunnel, exiting out of CMS tunnel shaft, with portal stabilizations.
Nada Pacific was hired by Mozingo Construction to install the Paradis Cut Microtunnel. The project scope also included the installation of the welded steel AWWA 42-in. carrier pipe after completion of the tunnel. Nada Pacific also performed the soft eye (Acrylate grout) portal stabilizations at both the entrance and exit shafts.
The project was designed by CH2M for the City of Tracy and construction tunneling oversite was provided by Jacobs. Microtunneling was selected due to the sensitive nature of the Old River/San Joaquin River waterway. The site geological conditions included sands, clayey sands, lean clay (CL), and (CH) residuum. Due to the sensitive nature of the waterway, a minimal amount of settlement was allowed. The depths of the tunnel ranged from 70 ft to 50 ft deep, with groundwater located just below the surface. Not only was settlement monitoring required, but also a complex system of spoils management. Nada Pacific provided a mass accounting system to monitor, weigh and test the spoils.
Due to the ground conditions Nada Pacific used an Akkerman SL52 microtunneling system, with a skin of 55.5 in. and a mixed ground cutterhead. The cutterhead selected was also beneficial since unexpected obstructions were encountered in the many layers of the flood plain, including a significant amount of wood. Permalok steel casing pipe was selected as the Tunnel Pipe, with a Derrick Separation plant custom built by Nada Pacific.
The work area located in the flood plain, had a tight schedule constraint. Nada Pacific met this tight schedule challenge and worked diligently to complete the tunnel in a timely manner.
Mobilization began mid-July 2020, with the completion of the tunnel in mid-September, and completion of the carrier pipe install by mid-October 2020. The tunnel was successfully completed on time, on-line and on-grade. The sand conditions and high groundwater pressure posed to be difficult as expected. However, after some time and efforts modifying the slurry spoils with different polymers, this condition did not pose to be an issue. The mass accounting system that Nada Pacific provided proved to be successful, with no major discrepancies from theoretical volume. The complex settlement monitoring system also showed no major variations to trigger any stop protocols.
Success of the tunnel was a team effort between the owner City of Tracy, Mozingo Construction, CH2M, Jacobs and Nada Pacific.
