A massive wastewater tunnel is rounding into shape, more than 100 feet below a busy swath of St. Louis County.
The Metropolitan St. Louis Sewer District says the lining has been completed in the past several weeks on its $150 million Deer Creek Sanitary Tunnel project.
“The tunnel, itself, is basically done,” said Sean Stone, a spokesman for the utility.
Plenty more work is needed before the 4.3-mile tunnel begins operation. Still on track for its targeted completion in late 2022, the project features a tunnel 19 feet in diameter that stretches from Clayton to Interstate 44. Crews finished boring the tunnel early last year, churning through rock 150 to 250 feet below ground with a tunnel boring machine nicknamed “Mrs. Nancy.”
The cavernous tube will collect and temporarily hold wastewater during heavy rains “until treatment facilities can handle the volume,” according to MSD. The project aims to eliminate sewer overflows and basement backups that have historically occurred along the system, and will serve the communities of Clayton, Richmond Heights, Brentwood, Maplewood, Webster Groves and Shrewsbury.
Stormwater runoff can be more problematic — and more prone to overwhelming the sewer system — in such a heavily developed and densely populated area. The region’s vulnerability also isn’t helped by climate trends, with the warming atmosphere gaining the ability to hold more moisture, and increasing the frequency and intensity of major downpours in the Midwest, according to the National Climate Assessment.
The Deer Creek Tunnel is one of the biggest undertakings within MSD’s Project Clear initiative to “improve water quality and lessen wastewater concerns” in the region over multiple decades. The tunnel marks Project Clear’s largest expenditure to date, although it will only hold that title temporarily.
“Some of the other stuff coming will be pricier,” said Stone.
The $6 billion overall effort stems from a decade-old legal settlement that MSD reached with parties including the U.S. Environmental Protection Agency, after years of discharges sent hundreds of millions of gallons of raw sewage into local waterways.
Major tunnels dug into the limestone rock underlying MSD’s service territory are a central part of the utility’s work to boost water quality. Others include the Jefferson Barracks Tunnel and the Maline Creek Storage Facility.
The Jefferson Barracks project runs along the Mississippi River, from south of Interstate 255 to MSD’s Lemay Wastewater Treatment Plant, near the confluence with the River Des Peres. Compared to the Deer Creek project, that tunnel is shorter in length, at about 3.4 miles, and smaller in diameter, at 7 feet. Given the numerous caves and sinkholes in the karst topography along the Mississippi, MSD says the $63 million tunnel had to thread a needle, staying “carefully aligned to avoid the difficulty and expense of building in such challenging conditions.”
Meanwhile, the half-mile, $82 million Maline Creek tunnel is 28 feet wide, dug 175 feet below Riverview Drive, along north St. Louis’ riverfront. After the tunnel fills during a period of extreme precipitation, a pump station moves the stored contents to the Bissell Point Wastewater Treatment Plant before its eventual discharge into the Mississippi.