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District Energy System

Extensive Tunnel Repair Projects Completed

4/15/2011

Tunnel RepairThe Metro Nashville DES distribution system consists of approximately 4 miles of trenches and tunnels with 84,000 feet of piping. Of this total, approximately 5,800 trench feet is contained within the system’s main tunnels below Broadway (2,100 feet), 4th Avenue North (2,000 feet) and 7th Avenue North (1,700 feet). These tunnels were constructed during an expansion of the distribution system in the mid 1980's and are quite remarkable. Large boring machines drilled 8 foot diameter holes through solid rock 40 feet below the City of Nashville. Since the tunnel construction, ground water and water from naturally occurring springs in the area has always seeped through the rock and into the tunnels. This water hardly ever accounts for more than an inch or so of water in the floor of the tunnel, however its seepage through the rock walls and ceiling has slowly deteriorated the rock. This has subsequently led to portions of the rock detaching from the tunnel walls and ceiling. To fix this problem, MNDES hired a contractor to stabilize the worst tunnel sections. This stabilization involved the drilling and placement of 6 foot long rock bolts at the 10 o’clock, 12 o’clock and 2 o’clock positions in the tunnels perimeter. Drainage wicks were then installed along the tunnels length at the 12 o’clock position and also circumferentially along the tunnel with varied spacing depending upon the amount of water infiltration but a minimum of every 4 feet. Then, a 4 inch by 4 inch heavy wire mesh fabric was attached to the tunnel walls and ceiling leaving a 2" gap between the wire and the surface. A concrete mixture (shotcrete) was then blown onto the walls and ceiling to create a new interior tunnel surface a minimum of 4” thick. The goal with this design was to strengthen the tunnel walls to prevent rock detachment while also re-directing water infiltration along the sides of the tunnel so that the water could then drain to the tunnel floor. Approximately 700 feet of the Broadway tunnel was repaired, approximately 2,000 feet of the 4th Avenue tunnel and 1,700 feet of the 7th Avenue tunnel. The work took approximately 9 months to complete.