Breaking down the digging of a massive tunnel

March 23rd, 2017

By Doug LeDuc | Greater Fort Wayne Business Weekly

The tunnel boring machine being used to excavate the Three Rivers Protection and Overflow Reduction Tunnel this year will be 19 feet in diameter.

That’s 7 feet more than a standard lane within the U.S. Interstate Highway System. It’s 350-foot length will be longer than a football field.

Built underground, the machine’s cutter head will bore through a layer of limestone that is between 200 and 250 feet beneath Fort Wayne’s surface while part of the machine behind its head mixes the broken rock with water and deposits it into a conveyor and pumping system, which takes it back through the working shaft it is creating.

“And so, as we get further and further away, ultimately we’ll be pumping that rock five miles backwards to the wastewater treatment plant,” said Matthew Wirtz, deputy director and chief engineer at City Utilities in Fort Wayne.

Five miles is how long the deep rock tunnel will be, which City Utilities will start building this year to improve Fort Wayne’s rivers and bring it into compliance with Environmental Protection Agency regulations.

The machine will be used to erect foot-thick, pre-cast concrete panels and line the tunnel with them, then pump grout onto them to lock them in place, and “we’ll be left with a 16-foot, inside-diameter concrete pipe,” Wirtz said.

The tunnel will start at Fort Wayne’s water pollution control plant on Dwenger Avenue, next to the Maumee River and east of North Anthony Boulevard. It will run parallel to the Maumee, cross Swinney Park and the St. Marys River, then continue south along the St. Mary’s until it ends near Foster Park.

The project only required City Utilities to obtain about 100 underground easements, because most of the tunnel will be bored through city property or public right-of-way areas. Some of the underground easements were obtained from property owners along Lake Avenue and Bluffton Road.

“Sometimes we were just going under their yard, not necessarily their house,” Wirtz said.

Unless the properties are near a drop shaft built to send water collected from the surface into the tunnel, the property owners will not notice any of the related activity “even during construction,” he said.

The tunnel’s depth will make most of its construction unobtrusive. It takes between 50 feet and 70 feet to reach bedrock in most of Fort Wayne.

The project is going well past that depth “to get down in a rock that is good to bore in, where the contractor can be the most productive” Wirtz said.

Project engineers sought a level with rock with consistent hardness and without many fractures that could let through much groundwater, he said.

The rock hauled out of the tunnel at the plant will be trucked to a location where it is screened and processed for later use in city projects.

About 80 percent of the project’s labor will be local, hired through local labor halls or local contractors. And according to the U.S. Department of Labor, every $1 million spent on a large construction project generates between 18 and 32 jobs.