Grand Prairie, Texas
Perkins Engineering Consultants, Inc., serving as a subconsultant to another firm, was responsible for identifying hydraulic relief improvements needed to enable this 152-mgd regional plant to better distribute flows from its 12 secondary clarifiers to its 32 low-head effluent filters, some of which are being abandoned while others are receiving significant capacity upgrades via cloth media retrofits. Two relief tunnels were identified and designed. The first involved a relatively short but complex 84-inch relief tunnel to connect the effluent troughs from two isolated banks of clarifiers. This pipeline was partially installed by open cut, but due to electrical duct banks and other large utilities in the vicinity, specific portions had to be installed by tunneling in tight, confined operating plant areas and beneath roadways. This project was completed in 2016. A second cast-in-place relief tunnel is scheduled for construction in 2017.