On November 17, 2008, DWSD issued Notice to Proceed to Vinci/Frontier-Kemper JV for construction of the Modified Detroit River Outfall No. 2 (MOD DRO2) project (DWSD Contract PC-771). The contract duration is 1,826 days.
DRO-2 is a $299 million project and is intended to meet a National Pollutant Discharge Elimination System (NPDES) Permit requirement to construct and place into operation a second Detroit River outfall for discharge of treated municipal wastewater from the Detroit Wastewater Treatment Plant (WWTP).
Construction work for a prior project (DWSD Contract PC-709) was undertaken in November, 1999 and was planned to be operational in May 2005. That project consisted of an approximately 1-mile-long, 21-foot finished diameter rock tunnel approximately 300 feet below ground surface with associated shafts and terminal effluent diffusers. It was designed to transport and discharge during the critical design flood condition 1,000+/- million gallons per day (mgd) of treated wastewater from the existing Rouge River Outlet conduit to the Detroit River. However, construction of this initial project was halted following a flood event during tunneling activities in April 2003. To date, the tunnel excavation remains flooded and the construction contract (PC-709) has since been terminated.
The new proposed MOD DRO-2 project is described as follows: Plug a portion of the as-built flooded tunnel. Backfill the lower portion of the entrance shaft, lower portions of two (2) access shafts and lower portions of the six (6) Detroit River diffuser risers. At the entrance shaft, construct a pile-supported concrete box structure in soft ground to intercept and transfer all flows from the existing dual-box River Rouge Outlet conduit. Construct a new 6,200-feet–long, 24 +/- feet excavated diameter rock tunnel (21-1/2 foot finished) along the original tunnel alignment, but at a higher elevation (approximately 160 feet below ground surface) and connect with remaining portions of access and diffuser riser shafts. Other work includes modification, rehabilitation, and construction of new hydraulic control structures (gates) and associated mechanical, electrical, and system components.
Tunnel construction will be through permeable rock, and below the groundwater table with artesian groundwater pressure conditions (approximately 5 bar) with hydrogen sulfide and possibly other harmful gases. A slurry type pressurized-face tunnel boring machine (slurry TBM) erecting a single-pass, gasketed segmental precast concrete tunnel liner will be required for tunnel construction. Short adit tunnel connections to the two (2) existing access shafts and six (6) existing diffusers risers will be made with conventional mechanized excavators. Ground improvement, such as grouting and ground freezing, will also be required for groundwater control for construction of access shaft and diffuser riser connections.