PROJECTS TO WATCH | REGIONAL INFRASTRUCTURE

Some of the region’s most substantial construction investments will never become part of its skyline.

Approximately 200 feet beneath St. Louis, a planned wastewater storage tunnel will help address a challenge facing many older communities: what happens when heavy rainfall overwhelms a combined sewer system.

MSD Project Clear’s Lower & Middle River Des Peres Storage Tunnel represents a major investment in that solution. MSD, an SLC3 owner member, describes an approximately nine-mile-long tunnel with a 30-foot inside diameterdesigned to capture and temporarily store combined stormwater and wastewater before it can overflow into River Des Peres.

For the architecture, engineering and construction community, the project offers a look at the scale and coordination behind essential underground infrastructure.

Managing Water Before It Reaches the River

In a combined sewer system, stormwater and wastewater travel through the same pipes. During heavy rain, flows can exceed the system’s capacity, resulting in combined sewer overflows into nearby waterways.

The planned tunnel will generally follow River Des Peres, capturing that mixed water before it enters the river. Drop shafts will carry flows from shallower sewers into the deep tunnel, where the water can be stored until it is treated at MSD’s Lemay Wastewater Treatment Plant.

That storage function is central to the project. By providing space to hold wet-weather flows, the tunnel will help the system manage periods when rainfall places the greatest demand on existing infrastructure.

A Substantial Underground Construction Program

The tunnel itself is only part of the planned work. MSD’s public project overview identifies several major components:

  • An approximately nine-mile main tunnel with a 30-foot inside diameter
  • Two connecting tunnels, each approximately half a mile long
  • Large construction shafts providing access to the underground work
  • Thirty-four intake structures
  • A substantial excavation for a future cavern-style pump station

MSD lists Jacobs as the design engineer and Black & Veatch as the construction manager. Its published project information identifies an estimated construction cost of $995 million, with construction anticipated to begin in 2029 and finish in 2038.

Those dates are planning estimates, but they illustrate the long delivery horizon associated with infrastructure of this scale.

Deep Construction, Surface-Level Coordination

Although much of the finished infrastructure will be underground, delivering it requires careful coordination above ground.

From a project-delivery perspective, shafts, intake structures and connections to existing sewers create interfaces with streets, utilities and surrounding neighborhoods. Construction access, material movement and site restoration must be considered alongside the tunneling work.

For owners and project teams, this highlights the importance of planning the entire system. The main tunnel, connecting structures and treatment operations must function together, while the construction approach must account for the communities above them.

Planning for the Long Term

MSD’s public project page includes materials from a June 2026 neighborhood meeting at Alaska Park, giving residents access to information about the planned work.

Community communication is particularly relevant for a project with a lengthy planning and construction period. Residents may experience individual work sites and construction phases without seeing the full underground system those activities support.

Clear explanations of the project’s purpose, local work and anticipated sequencing can help connect those visible activities to the broader infrastructure investment.

A Project Worth Following

The Lower & Middle River Des Peres Storage Tunnel brings several important AEC issues together: deep underground construction, connections to an operating utility system, neighborhood coordination and long-term capital planning.

For SLC3 members, it is a substantial owner-led project to follow as its design and delivery plans advance.

Its lasting contribution will be measured in how well it manages wastewater during heavy rain and helps protect the waterways serving the St. Louis region.

Learn more: Explore MSD Project Clear’s Lower & Middle River Des Peres Storage Tunnel overview.