Fifteen years ago, new to his role as a sewer engineer for the Department of Public Works, Joe Klejwa entered a tunnel 70 feet below downtown Minneapolis.
He was on a quest to assess the city’s subterranean tunnel systems, which were built to help drain stormwater from the surface into the Mississippi River. Some of the tunnels were more than a century old; others were built in the 1930s. In the time since, the wooden framing surrounding the concrete tunnels had rotted away, leaving gaps between the concrete and the surrounding sandstone. The city had been built out over the same time, with impervious pavement forcing more water into the tunnels. To top it off, storms — including two intense rainstorms in 2010 — had put so much water pressure in the tunnels that they had begun to bulge out into the soft sandstone and crack…