Trenchless Installation of Trunk Interceptor with HOBAS CC-GRP Pipes in Ohio, US
The wastewater system of the city Marysville in Ohio serves its growing population and several major industrial firms including the Nestlé Corporation, Goodyear Tire & Rubber Co., and Honda. To meet future wastewater requirements, reduce maintenance costs, and increase the reliability of the system, the city decided to build a new water reclamation facility in 2008. A new wastewater conveyance system was also planned, with the objective to eliminate pump stations throughout the existing collection system and reduce the number of future pump stations. The city concluded five contracts for this purpose, one of them for a trunk interceptor at a cost of 35 million USD. HOBAS Pipes DN 1535 were used to realize this trunk interceptor sewer: It discharges into a pump station, which conveys the wastewater to the water reclamation facility.
The presence of industrial sites posed challenges to the local engineering company DLZ: Throughout the project area, there was potential for contaminated soil and water. “This was one of the reasons we chose to specify trenchless installation techniques for the major part of the project,” Debarati Bardhan, public works project manager with DLZ explained. The pipe could be jacked below the potentially contaminated soil. Other reasons included the depth of over 12 m, interferences with subsurface utility networks, and issues related to easement acquisition – various open surface and subsurface easements, both permanent and temporary, were necessary for the construction of the new line. By placing the jacking pits outside the possible contamination zones, the construction works were greatly facilitated. Trenchless simply proved to be the most economic and convenient method of installation in every aspect.
Long Jacking Runs
Two pipe materials were specified for the trunk interceptor project: Centrifugally cast, fiberglass reinforced plastics and reinforced concrete pipe with internal PVC lining. The DN 1535 CC-GRP Pipes and GRP Fittings provided by HOBAS Pipe USA convinced the investor. “The initial material cost increase of HOBAS Fittings over a concrete alternative was offset by the decreased labor cost due to the easy connection with the supplied couplings,” stated Jake Keegan, Project Manager with Super Excavators, Inc. The fittings were equipped with appropriate stubs to connect the existing lines to the new DN 1535 line.
Super Excavators, Inc. installed 6.2 km of HOBAS CC-GRP Pipes DN 1535 – 1.9 km by open cut and 4.3 km by microtunneling methods. The jacking pipes supplied by HOBAS Pipe USA featured an allowable thrust force of 5295 kN and were delivered in 3 m lengths in order to reduce the size of the thrust pit. The 4.3 km long jacking pipeline was installed in 18 drives, the longest one amounting to 321 m, with an average jacking force of 980 kN. For the longer drives, intermediate jacking stations were installed in order to distribute the thrustforce along the pipe string by dividing it into independent sections. The hydraulic jacks were installed inside a steel casing fabricated with the same outside diameter as the pipe. The remote-controlled tunnel boring machine managed the difficult ground composition of cobbles, boulders, gravels, and sticky clays without problems.
Optimal Performance Thanks to HOBAS
Once the pipeline was installed, a series of tests was conducted to ensure its proper installation and performance. The construction management checked the entire pipeline for deflection by means of a proof stick with a length equal to 97 percent of the internal pipe diameter. “The tests were conducted in accordance with the latest specifications and all results were found to be acceptable and within the limits specified in the contract,” stated Bardhan. When the job was completed, all parties were pleased with the project’s success.