Lower Meramec Tunnel Project

The 6.8-mile-long tunnel project — the largest ever for Metropolitan St. Louis Sewer District (MSD) — will improve water quality within the Meramec River watershed and help ensure regulatory compliance.

Location

Missouri, USA

Project Status

Construction Start, January 2021

In 2011, MSD began the 23-year implementation of a complex capital improvement program (CIP), called Project Clear, to protect public health and safety while ensuring compliance with the spectrum of regulatory issues and requirements included in its Combined Sewer Overflow and the Sanitary Sewer Overflow Control Master Plan. MSD serves approximately 1.3 million residents and is the 4th largest wastewater collection system in the nation.

Tunnel depth (m)

87.2

Tunnel diameter (m)

3.7

Tunnel length (km)

6.8

A key element of the CIP, the Lower Meramec Tunnel (LMT) consists of a 35,932-foot extension of the Baumgartner Tunnel, designed to intercept flows and to take offline the interim Fenton Wastewater Treatment Facility (WWTF). The tunnel ranges in depth from 78 to 286 feet and is approximately 12 feet in diameter with an 8-foot internal diameter final lining. It will be constructed entirely in rock using a rock tunnel boring machine, traversing through portions of unincorporated St. Louis County, City of Sunset Hills and the City of Fenton.

The LMT is anticipated to operate under both open channel and submerged flow conditions — dry weather flow velocities of the Phase I and Phase II portions of the tunnel are anticipated to range from 2.5 to 4.9 feet per second. Six drop structures are included within the design — plunge inlet and tangential vortex — to accommodate as much gravity flow to the tunnel as practical and economical. The project also involves design of construction shafts, connection to the existing Baumgartner Tunnel, demolition of existing pump stations, and near surface sewer connections to drop structures.

WSP serves a major subconsultant for the project, providing overall project management and lead design responsibilities related to the deep tunnel alignment, Tunnel Boring Machine (TBM) operations, drop structure design, geotechnical engineering and design, risk identification and design services during construction. Key elements of this project include groundwater inflow analysis, assessment of hazardous gases, karst geologic conditions, connection to the existing Phase I tunnel and consolidation of connecting sewers.