As the trenchless subconsultant to Carollo Engineers, Bennett Trenchless provided full design services for the trenchless components of the separation of the combined sewer from Miller Park to the Pershing Detention Pond in Omaha, NE. Trenchless construction methods were chosen due to the very deep gravity stormwater pipeline alignment and to minimize disruption to the surrounding neighborhood. Microtunneling was chosen to minimize settlement risk and because the groundwater level was above the crown of the pipe. The microtunnel drive was approximately 1,300 feet of 60-inch ID unlined RCP, with the downstream end exiting through a very steep bluff into the Pershing Detention pond.
Challenges on the project included protection of existing utilities and limited work area at both ends of the microtunnel drive, resulting in locating the jacking shaft within the detention pond where the soils were very weak. The lack of sufficient thrust capacity from the soil resulted in the contractor constructing a very robust thrust block using ten 80-foot tiebacks and ten 40-foot H-beams. The project was completed within budget and schedule with no claims.