By Jason Allen

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DALLAS (CBSDFW.COM) – Dallas is going under the river, and through the mud, for a project you rarely get to see up close.

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One-hundred-forty feet under I-35, excavation crews are in the final stages of hollowing out dual 9-foot wide tunnels.

One of the tunnels, three-fifths of a mile in length, is already lined in slick, cast fiberglass pipe. The other is still a skeleton of wood and steel panels, as crews dig out the last bits of shale mud. On a good shift, they can get through about 25 to 30 feet.

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When the entrance to work is a hole in a river bed, it can make getting to work a little difficult some days. The heavy rains in 2015 slowed down the timeline for the $25 million project.

The project to keep Dallas flushing though, is on track to finish early next year. And it’s all to move away wastewater. The more people who move in, the more there is to get rid of.

“It will be very low maintenance,” said Matt Penk, the pipeline project manager for the city. “We won’t have the mechanical systems of the pumps to maintain and operate. It’ll be by gravity flows”

The plan is for the dual pipes to eliminate the 60-year-old system, and keep everything moving for the next century.

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