Horner & Shifrin provided design and construction engineering services for the design of a major project for the Metropolitan St. Louis Sewer District. This project (part of MSD’s Lower Meramec River Wastewater System Improvements Program) involves the construction of a deep (approximately 200 feet below grade), large-diameter (96 inch) interceptor sewer, tunneled in rock. The interceptor sewer will deliver wastewater from a substantial portion of southwest St. Louis County to a very large lift station; that will then convey wastewater into a new wastewater treatment plant, located at the confluence of the Meramec and Mississippi Rivers.

The 96-inch diameter interceptor, to be installed within a roughly 12-foot diameter hole tunneled in limestone will extend from the lift station approximately 20,300 feet upstream to allow MSD to eliminate the outdated, under-capacity Baumgartner treatment lagoon, as well as intercepting flows from the Mattese Creek and other watersheds along the way. Ultimately, the tunnel interceptor will be extended roughly another 25,000 feet further upstream, in order to allow MSD to eliminate the existing Fenton and Grand Glaize wastewater treatment plants.

A particular challenge in design of the lift station was determining the most effective shape and method of construction for this relatively deep, 100 foot diameter structure. The design team and MSD ultimately chose a circular configuration, to be constructed with a slurry-wall excavation support method that would also serve as the permanent structural wall of the lift station, as the cost cost-effective approach.

The lift station design includes housing four centrifugal pumps to convey the expected peak wastewater flow to the nearby treatment plant and allows for the future installation of two additional pumps to bring to nearly double the lift station capacity. The lift station also includes a natural biofilter odor control system to treat odorous / corrosive air flows, in order to protect the workers that may have to enter these environments for maintenance purposes and to prevent offensive odors from escaping to nearby residential areas.

As an integral part of Horner & Shifrin’s design services, H & S personnel have worked closely with MSD staff and construction management consultant personnel (a joint venture of Shannon & Wilson / Black & Veatch, retained separately by MSD) to develop final designs for both the tunnel and lift station that were technically sound, cost-effective, and matched the compliance schedule faced by MSD for placing these facilities into operation. During the design phase, constructability/biddability reviews were conducted at the 30% and 60% completion stages of the Contract Documents. Review sessions (rather than an “arms-length” exchange of written comments and responses) were held that involved the design team, construction managers, and MSD. These “brainstorming sessions” proved to be quite effective in reducing construction cost, enhancing schedule coordination between multiple adjacent projects, minimizing potential contractor claims, and improving the technical quality of the Contract Documents; while maintaining the design integrity and functionality of the facilities.

This cooperation between design team, construction manager, and owner was certainly a major part of the reason why the low bids received for these two construction contracts were within 1 % of the engineers’ estimate.