Nonconnah Sewer Repair and South Cypress Creek Stabilization

Memphis, Tennessee

In the 1970’s, design and construction was underway for the T. E. Maxson Wastewater Treatment Facility located in southwest Memphis along with a large diameter interceptor sewer in the Nonconnah Creek drainage basin. Nonconnah Creek provides drainage for the southern part of Memphis and Shelby County and it drains into McKellar Lake. McKellar Lake was constructed in a U. S. Army Corps of Engineers project in the late 1940’s to connect President’s Island, then the largest island on the Mississippi River, to the mainland thus creating the deep-water harbor (later named McKellar Lake) and the President’s Island industrial center that exists today.

To avoid McKellar Lake, it was necessary to route the Nonconnah Interceptor Sewer parallel to the lake shoreline to the treatment facility. The route crossed South Cypress Creek, a tributary to McKellar Lake with a drainage area of more than 15 square miles extending into Mississippi. The crossing, when constructed, was located approximately one quarter mile from the lake shoreline. Erosion of the creek channel resulted in the lake and Mississippi River shorelines reaching the interceptor sewer in 2015. When the Mississippi River water level was above 16 feet (MSL= 200 feet) on the Memphis River Gauge, the entire 96-inch interceptor sewer was submerged.

On March 30, 2016, following a four–inch, two-day rainfall with the Mississippi River level at approximately 10 on the Memphis gauge, high storm water flow in South Cypress Creek eroded the soil supporting the interceptor sewer across the creek channel causing the sewer to collapse. The collapse resulted in the discharge of untreated wastewater into South Cypress Creek and McKellar Lake and eventually into the Mississippi River.

The initial task was to install a bypass pumping system to pump the wastewater around the collapsed sewer. This task required the construction of a road in T. O. Fuller State Park to provide access to the 13-bypass pump setup and a route for the 13 discharge pipelines. It was also necessary to install a temporary retaining wall on adjacent Mitsubishi Electric Power Products, Inc. property for access to an existing sewer structure to be used for pump suction piping.

City of Memphis Public Works departments, local general contractors and a bypass pumping contractor, with an all-hands-on-deck approach, completed the task and had the pumps operating in 6 calendar days after the collapse.

Immediately after the collapse, Allen & Hoshall was selected to provide field surveying, concept development of the repair, engineering design drawings, specifications and contract documents for bidding and constructing a repair of the interceptor. Allen & Hoshall also provided assistance during the bidding period, construction management services and on-site construction inspection.

Allen & Hoshall provided a field survey of the site, concept ideas for repair of the sewer, engineering design drawings, specifications and contract documents for bidding and constructing a repair of the interceptor. Allen & Hoshall also provided assistance during the bidding period. After a construction contract was executed and a notice-to-proceed issued, Allen & Hoshall provided construction management services which included numerous coordination meetings with the general contractor, subcontractors, the Owner and Mitsubishi. As construction difficulties were encountered, Allen & Hoshall evaluated ideas and suggestions from all stakeholders and revised design drawings to incorporate approved changes. Construction management also included review of material submittals, checking pay quantities and recommending periodic payments to the contractor and providing on-site construction inspection services. With the ongoing expense of bypass pumping, the challenges created by flash flooding, the potential of another release of untreated wastewater and site flooding from the Mississippi River, Allen & Hoshall worked tirelessly to disseminate information and manage relationships.

The repair included approximately 225 feet of 96-inch fiberglass sewer pipe, however, during and after the collapse, the creek channel eroded to an elevation approximately 10 feet below the sewer. The major challenge of the design was to construct a support structure across the creek channel to support the sewer and to stabilize the channel to prevent future erosion. The final design/construction included over 40,000 square feet of heavy sheet piling with some sheets being 60 feet in length, more than 1,300 cubic yards of reinforced concrete for support and encasement and 17,000 tons of large stone rip-rap with some grouted.

A known and anticipated hazard that turned out to be a serious obstacle was the existing 96-inch reinforced concrete sewer pipe that fell into the creek channel and sunk to a depth of 20 feet or more. In addition to the 28,000-pound, 8-foot sections of sewer pipe, some of the pipe had a poured concrete arch cap which added 38,000 pounds of additional weight per 8-foot section. With the unstable (“muck”) creek bottom, the numerous flash floods that occurred and the depth of the pieces of concrete sewer pipe and arch cap, removal of the concrete pipe “debris” was finally judged to be impracticable if not impossible. For this reason, the design of the sheet pile “coffer dam” was revised to enclose the underground “debris”, all of which was left in place. The debris was covered with rip-rap encapsulated in geotechnical fabric and a reinforced concrete slab, anchored to the sheet piling, was placed above the rip-rap to support the new sewer pipe.