Sub-Mississippi River water tunnel illustrates feats of modern engineering

The Mighty Mississippi has long been both the source of Minneapolis' drinking water and one of the most significant obstacles to its distribution. Starting this summer, the marvels of modern engineering may solve a problem that has bedeviled city engineers for more than a century.

A high-tech machine will grind through rock deep below the Mississippi, carving a tunnel to keep one of the city's most important drinking-water pipes safe from the elements.

In the early 20th century, laborers battled subzero temperatures, torrential flooding and lumber floating downstream to dig a trench and yank a massive riveted water pipe across the river just south of downtown. In the 1940s, that pipe was replaced with another one hanging from the 10th Avenue Bridge. But years of exposure to the elements and roadway salt have corroded the pipe, and the bridge's upcoming rehabilitation spurred discussions about relocating it.

Enter the microtunnel boring machine, a mechanical mole that will burrow a 5-foot hole several stories beneath the riverbed to make way for a new water main. The $24 million microtunneling project is a first for the city, but the machines are increasingly being used across the country to help dig holes beneath tricky areas like rivers and freeways.

"We think going under the river is a 200-year solution, potentially," said Glen Gerads, the city's director of water treatment and distribution, "vs. putting it back on the bridge where it's exposed to the environment and we're going to be back at this again the next time the bridge is rehabbed or even before."

It's no simple endeavor. Contractors will dig two wide shafts on each side of the river — one of them 130 feet below ground — to launch and retrieve the machine. The toothed cutterhead will carve through about 900 feet of sandstone on the journey to West River Parkway. A jacking system at the starting point will push sections of steel pipe behind the machine to advance the cutterhead and secure the tunnel.

"For engineers [on the tunnel], these are kind of like once-in-a-lifetime projects," Gerads said. "They don't happen every day."

The pipe will be one of the largest constructed beneath the riverbed in this area. It is an important connection in the city's drinking-water system, which is pressurized and flows based on demand — unlike gravity-based wastewater pipes.

"These big [water mains] really have an impact that's pretty large geographically across the city," said Peter Pfister, a city engineer and the project manager. "Especially as they cross the river — because there's so few of them — they're kind of the main suppliers."

The machine could encounter obstacles like harder rock along the way, but soil samples taken from a boat in the river have given the city a glimpse into what lies below. And while the process is more automated than a century ago, with an operator monitoring the machine above ground, workers are still needed at the bottom of the shafts to assemble the pipe sections as they enter the tunnel.

People have been microtunneling in the United States since the 1980s. Similar machines have been used by the Metropolitan Council in recent history to install wastewater pipes beneath major freeways like Interstate 35 near Elko New Market and Interstate 94 near Woodbury.

"It's a technology that's been kind of waiting for us to need it," Pfister said.

Paul Pasko, a civil engineering consultant with SEH Inc. who specializes in "trenchless" technology, said the machines were once so expensive to use that only larger government agencies could afford them.

"Microtunneling [is] slowly getting down to the price point that the city of Minneapolis can afford to use it," Pasko said.

The machines are also distinct from their larger cousins, tunnel-boring machines. One of those supersized machines weighing 500 tons created the light-rail tunnel at Minneapolis-St. Paul International Airport in 2002 after making an oceanic journey from Europe.

There are about 1,400 pipes and lines crossing over or under water bodies in the metro area that carry wastewater, drinking water, natural gas, electric power, telecommunications and cable television, according to Minnesota Department of Natural Resources license data. The pipe north of Lake Street that still carries most of south Minneapolis' wastewater was bolted together underwater in 1936 by divers navigating in "pitch-dark mud" at the bottom of the river — and weighed down by lead shoes and belts.

Representatives for CenterPoint Energy and Xcel Energy said their underwater crossings nowadays are often installed using directional drilling, which is more akin to pushing a large flexible drill bit beneath the water.

Microtunneling, by comparison, is typically used to dig larger holes with more accuracy.

Contractors will begin assembling machinery to drill the shaft next week, Pfister said. The city expects tunneling will begin this fall. The new main should be delivering drinking water by 2020. The cost will be covered by existing water bill revenue.

The water main project precedes rehab work on the 90-year-old 10th Avenue Bridge, which is expected to cost about $43 million and benefited from funding in the 2017 bonding bill. The city hopes to begin that bridge work this fall and wrap it up by fall 2020.