Ever wonder where rainwater goes when it falls in downtown Minneapolis? Landform Surveyors had a chance to find out! A tunnel system built from the late 1800s to around 1930 was designed to carry stormwater and sewage. The systems were separated by a series of construction projects throughout the 20th century, leaving the 80-100 foot deep tunnel system today to carry only stormwater.
The large amount of impervious surface in downtown creates a lot of stormwater when it rains. The rush to get to the Mississippi makes for a lot of pressure in the tunnel system. The concrete liner in the tunnels hewn from sandstone (by hand!) have started to crack under the pressure. A plan is underway to build a parallel tunnel underground to relieve some of that pressure. Landform's task was to map the existing tunnel so designers can figure out where to build the new one.
Surveying 100-feet underground requires a bit more planning than your day-to-day survey. Our field staff had to get confined space training, learn how to operate multiple gas meters for monitoring the air underground and get lowered to the job site each day in a crane basket (along with a couple of extra trips to get equipment down below). When it was all said and done, Landform surveyors mapped just shy of 4 miles of tunnel. We set 61 permanent control markers in the ceiling of the tunnel for future use. We prepared an alignment of the tunnel invert for use in hydraulically analyzing the system. Our crews spent 9 long days underground - A very interesting survey!