Tunnel Project to Help Nashville Conquer CSO Problems

The Lewis Street Combined Sewer Overflow (CSO) Tunnel project is one of several projects designed to bring Nashville, Tenn., into compliance with state and federal regulations governing sewer overflows.

The Lewis Street Combined Sewer Overflow (CSO) Tunnel project is one of several projects designed to bring Nashville, Tenn., into compliance with state and federal regulations governing sewer overflows.

The project was designed to include 5,010 feet of hard rock tunnel, 460 feet of soft ground tunnel, one 32-foot diameter construction shaft and three diversion structures. By design, the dry weather sanitary sewer flow into the interceptor represents a very small percentage of the flow during storm events. Engineers allowed as much slope as possible in the tunnel for low flows while maintaining as much storage volume as possible for storm events. The dry day average daily flow is 0.09 mgd, while storm flow is 204 mgd.

Once the tunnel was bored, 5,470 feet of 102-inch diameter reinforced concrete sewer pipe was installed. The pipe incorporated steel end-ring gasketed joints conforming to ASTM C-443 and an Ameron T-Lock PVC lining, mechanically cast 360 degrees around the interior of the pipe. The liner was specified by the program manager, Consoer, Townsend Envirodyne Engineers (CTEE), to provide a protective barrier against hydrogen sulfide.

The successful bidder on the Lewis Street CSO Tunnel project, with a bid of $13,053,400, was a joint venture between W.L. Hailey and Co. of Nashville, and Frontier-Kemper of Evansville, Ind. Work started on the project in mid-March 1995. Mining with a Jarva Mk-12 tunnel-boring machine began in July 1995. The 5,010 feet of 12-foot diameter hard rock tunnel was completed Dec. 20, 1995.

Three diversion structures were constructed by Superintendent Edwin Proctor and his crews. The last diversion structure along the tunnel alignment was the Lewis Street Diversion Structure and was built on a fast-paced construction schedule in order to meet the August 1996 completion schedule. The diversion structures are heavy in electrical and mechanical components. Craftsmen from W.L. Hailey and more than 25 separate subcontractors and 22 major suppliers helped install the major components of the structures.

The 102-inch reinforced concrete pipe used in the tunnel was designed and manufactured by Sherman-Dixie Concrete Industries of Nashville. The size and weight of the pipe (42,000 lbs. per 12-foot joint) created some serious engineering challenges. Also, the distance form the main construction shaft where the pipe entered the tunnel to its placement site was almost a mile.

To accelerate pipe movement in the tunnel, Keith Miller, W.L. Hailey’s senior project engineer, and John Branson of Frontier-Kemper, designed a special pipe carrier to run on rail tracks. The carrier was propelled by an on-board eight-ton Plymouth diesel locomotive motor and had a self-contained hydraulic package. Plates and beams were designed with steel rolled to the same internal diameter of the pipe and attached to the top of the main needle beam. The invert of the pipe was covered with neoprene to protect the T-lock lining. To allow precise placement of the pipe, the carrier was equipped with hydraulic cylinders which allowed the carrier to move the pipe into any position.

Scheduling the delivery of the pipe from the Sherman-Dixie manufacturing plant to the main construction shaft presented logistical challenges. The working area around the construction shaft provided little storage area for the pipe.

Once full-scale pipe installation started in mid-February, two shifts worked more than 20 hours a day on the installation. The average daily production rate was 15 pieces of pipe (180 feet) per day.

To negotiate several tight radius curves and to allow for minor alignment variations in the soft ground tunnel, W.L. Hailey designed and Sherman-Dixie fabricated several short sections of beveled pipe. The beveled sections were designed using the same watertight steel end-ring gasketed joints as the full-length pipe. The beveled pipe worked as planned allowing the installation of pipe through three curves with a radius of less than 150 feet.

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