Storm Water Detention Structure Built Under Busy Thoroughfare

One of the busiest streets in Chattanooga is MLK Boulevard. This four-lane artery carries approximately 13,000 vehicles per day. Below the street is a combined storm/sanitary sewer. Following heavy rainfalls, the combined sewer facilities surcharged and the street became an open channel for excess flows. The City of Chattanooga hired CTI to perform an analysis of alternatives to eliminate surcharging in a seven-block area. The optimum solution was found to be construction of a storm water detention facility beneath MLK Boulevard.

But beneath the roadway there was a tangled mass of traffic of another sort. Gas and electric lines, sewer junction boxes, BellSouth’s main fiber-optic duct bank, as well as the 110-year-old combined sewers, filled the “nether region” of the busy boulevard, and some of these components were not recorded on any drawings of the project area.

Detention Culvert

The first part of the surcharging solution included the installation of 378 linear feet of 12-foot by 8-foot precast concrete box culvert, 1,240 linear feet of pipe, 10 catch basins, and 6 manholes. These facilities provide nearly 300,000 gallons of underground storm water storage.

During design, it was discovered that gas and underground electrical services to former businesses in the area were still active. Working with the Chattanooga Gas Company and the Electric Power Board, CTI coordinated the abandonment of gas and underground electrical services. The culvert location was also adjusted during design to avoid BellSouth’s fiber-optic duct bank.


Another challenge involved the schedule, which was compressed due to the upcoming conversion of the street from one-way traffic to two-way traffic. The street was closed completely for only 9 days to install the box culvert. At least two lanes of traffic were kept open at all other times.

Chattanooga’s annual Riverbend Festival “Bessie Smith Strut,” which is a parade down MLK Boulevard, also took place in mid-construction; however, the parade was not adversely impacted.

Additional Improvements

Downstream of the box culvert, 900 linear feet of aging brick and clay sewers were replaced. The largest obstacle related to the sewer replacement occurred when two buried sanitary sewer junction boxes that were not shown in the historical records were unearthed during construction operations. Both boxes were incorporated into the new sewer.

In addition to the storm water detention and sewer upgrade, CTI’s scope of services included design of streetscape improvements over the top of the box culvert. Total construction cost for all elements was approximately $1.1 million.


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