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This paper presents an overview of the design -build of the New Mississippi Tunnel Outfall for the Transportation Expansion Proj ect (T-REX). The Contractor was Southeast Corridor Constructors (SECC), a Kiewit-Parsons, Joint Venture. Parsons acted as JV partner as well as designer and provided a unique perspective in the design build setting. The Transportation Expansion Project’s New Mississippi Outfall is a $30 million constructed value project. This is a small but important part of the 1.2 billion dollar highway widening and Light Rail Transit (LRT) expansion project of Denver’s Interstate Highway I-25. The Outfall conveys highway and other urban drainage to the South Platte River as well as provides long needed relief from flooding problems in the southeastern portion of the Interstate 25 highway corridor. The Outfall tunnel conveys storm water under I -25 then under Mississippi Avenue to the river. The Outfall consists of a number of distinct elements with water flowing and connecting from east to west as follows: an Energy Dissipation Structure, 91 m of 213 cm diameter jacked pipe crossing under I -25, Junction Interceptor Structure, 554 m of 4 m ID tunnel excavated by earth pressure balance machine technology, a Transition Structure (connecting the circular section to a rectangular section and serving as the main tunnel driving shaft), 266 m of 5.5 m wide by 2.4 m high cut and cover box structure, and Outfall Structure at the South Platte River. The Outfall crosses both the existing I -25 as well as the future improvements including under the Light Rail Transit, a new bridge abutment, widened lanes, and new retaining walls. The Outfall also crosses adjacent to several buildings and five bridges, under major traffic and utility intersections, and through difficult geology. The permitting, ROW easements, geotechnical investigation, design and construction was part of the design-build contract package. Close and continuous coordination with City, County, State, and Federal regulatory permitting agencies, railroads and other private businesses was required. |
