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.