Procurement of Missouri's Longest Tunnel Project and the Largest Clean Water SRF Project in State History

Phase II of the Lower Meramec River System Improvements Project, also known as the Lower Meramec Tunnel (LMT), consists of a 6.8-mile-long, 14.5-foot excavated diameter, 78 to 286-foot-deep sanitary sewer tunnel. The project is located in south St. Louis County and traverses' portions of unincorporated St. Louis County, City of Sunset Hills and the City of Fenton. The LMT is part of Metropolitan St. Louis Sewer District's (MSD) Project Clear; a program planned to span 23 years to improve water quality throughout MSD's service area. The tunnel's main objective is to intercept flows and to take offline the interim Fenton Wastewater Treatment Facility (WWTF). This project is currently under construction and our presentation will discuss project details, including planning, detailed design, hydrogeologic challenges, use of hydraulically driven Milwaukee H4 vortex drop shafts, lessons learned from Phase I (Baumgartner Tunnel), procurement and the current status of construction. The project was originally envisioned in 1979 as part of regional treatment plan for pollution abatement within the Meramec River Basin. Previous investigations concluded that a regional concept involving a major interceptor sewer conveying flow to a single treatment facility that would discharge its effluent to the Mississippi River, was the most cost-effective scheme for the area. The Meramec River, one of the longest free-flowing waterways in Missouri, is widely used for recreational boating, fishing, canoeing and floating and is the central natural element for a series of parks located along the river's banks. Between 2005 and 2012, alternatives for expanding the Lower Meramec WWTF were evaluated through MSD's program management contract. This work included an alignment study for Phases II and III of the LMT project. In 2014, MSD awarded a design services contract to HDR Engineering, Inc. and, as a sub-consultant, WSP USA, Inc. who is serving as the lead design firm. The project received bids for construction on July 22, 2020. The tunnel will be constructed entirely in rock with an invert elevation of 255.1 feet NAVD88 where it connects with the Baumgartner Tunnel and at elevation 291.1 feet where it terminates near the Fenton WWTF. The proposed tunnel 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 feet per second. to 4.9 feet per second, which provides sufficient velocity between the minimum (2 feet per second) and maximum (10 feet per second) flow velocities as identified in the Recommended Standards for Wastewater Facilities for grit and solids suspension. Drop structures have been spaced along the alignment to accommodate as much gravity flow from MSD's collection system to the tunnel as practical and economical. Selection of the drop structure type has considered technical and functional criteria that influence the capital and life-cycle costs associated with constructing and operating the drop structures for the duration of their intended 100-year service life. Four types of drop structures were considered for the project, including: tangential vortex, vortex with helical ramps, plunge inlet and baffle type drop structures. Both plunge and tangential vortex drop structures were considered suitable. For locations where the design flow rate was expected to exceed 10 cubic feet per second, tangential vortex drop structures were recommended. Smaller plunge style drop structures were utilized elsewhere. During construction of the Baumgartner Tunnel (Phase I), which concluded in 2007 forethought was considered and the receiving shaft, the Baumgartner Shaft, was built to facilitate construction of LMT. The existing Baumgartner Shaft is located on MSD property at a formerly decommissioned sludge lagoon facility. The shaft is approximately 190 feet deep with the upper 130 feet constructed as a slurry wall which was keyed four feet into bedrock where supplemental grouting was performed. The lower 60 feet of the shaft was constructed through limestone bedrock. The shaft is currently unlined and is serving as LMT's tunnel boring machine (TBM) launch shaft. Up to 27 acres have been made available to the Contractor at the Baumgartner Shaft site to utilize as the contractor's primary work/staging area. Additionally, a unique reuse of a decommissioned lagoon located on the site will be used for muck disposal which greatly reduces the project's carbon footprint by eliminating haul traffic and disposal requirements. Three shafts will be provided near the Fenton WWTF, with the following key functions: - Accommodate tunnel construction for TBM retrieval and the ability to launch a TBM for the Grand Glaize Tunnel (Phase III), if required; - Intercept the Fenton WWTF flows via drop structure, deaeration chamber and connection adit to the LMT; and - Provide adequate venting at the upstream terminus of the LMT to mitigate potential surge and transient flow conditions. The Fenton Construction Shaft is located on property owned by MSD near the Fenton WWTF and the site is approximately eight acres. The shaft is anticipated to be approximately 136 feet deep with a depth to bedrock of approximately 63 feet. A slurry or secant pile wall will be constructed for initial support in overburden with supplemental grouting at the bedrock-soil interface. The site is located within the 100-year floodplain and significant flooding has been experienced, including recent historic flood events in December 2015 and May 2017. A portion of the site will be raised for flood protection and resiliency purposes above the recently revised 500-year flood elevation. Given that LMT is an extension of the Baumgartner Tunnel, design has taken an approach to focus on lessons learned from previous tunnel construction in similar geologic conditions. From the onset of the design, risk management has been implemented to mitigate known challenges as previously encountered, including risk mitigation as it relates to water-bearing zones within the rock formations; the presence of hydrogen sulfide and methane gas; and abrasive chert. The project has been delivered utilizing a traditional design-bid-build approach with contractor prequalification a part of the selection process. A comprehensive Geotechnical Data Report (GDR) and a Geotechnical Baseline Report (GBR) were prepared for the project to mitigate risk on behalf of the Owner.