Partnership and Innovation Ensure Success of Bypass Project
Collaboration and communication — these were at the heart of the Cadiz lift station y-inlet rehabilitation project in Dallas during the summer of 2019. This complex sewage bypass project was made even more challenging by extreme temperatures, up to 130 F on the asphalt, and busy downtown traffic.
Partnership was critical to the success of the project, which aimed to repair a deteriorating y-inlet built in the 1950s. The y-inlet was used as a diversion structure to send sewage flow around the Cadiz lift station to a gravity interceptor. Three lines entered the lift station through the y-inlet. There was a 90-in. horseshoe-shaped pipe (130 million gallons per day or MGD) and 51- and 54-in. circular pipes with a combined flow of 20 MGD.
For the repairs to take place, an engineered pumping and bypass solution was required.
Sunbelt Rentals offered bypass design services during the pre-project planning design phase, partnering with the City of Dallas Water Utilities; Chris Story, a project engineer from JQ Structure; and Erin Flanagan of Freese and Nichols Inc., who was the bypass engineer.
Sunbelt Rentals began consulting when approximately 30 percent of the design was completed and worked with the engineering team to bring the plan to fruition. In total, it took one year to establish a plan, which included an initial job walk by Sunbelt Rentals to assess the space. The partnership was key to establishing a solid understanding of the bypass portion of the project and mitigating risk.
After the design plan was complete, it was let for bid and BAR Constructors, Inc. was awarded the project, with Michael Ayers serving as the point person. They chose Sunbelt Rentals as the bypass subcontractor.
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The Challenges along the Way
The route for the bypass was determined by Flanagan and the City of Dallas to cross the I-30 Tom Landry Freeway using the Hotel Street Bridge — closing a lane of traffic and finally reaching the discharge siphon, which acted as a bypass for the lift station.
There were several challenges to overcome to complete the project.
Challenge 1: Time and Monitoring
The teams discovered that the discharge pipe route along the Hotel Street Bridge was very narrow. The project required seven 24-in. and three 18-in. runs of HDPE discharge pipes to fuse, while still allowing a lane of traffic to remain open. They also had to bury the pipe at the main entrance of the Cadiz lift station to allow access the site.
The original specifications didn’t allow enough time to manage these challenges, so communication became key to determining a solution. By working with the City of Dallas, the engineers, and BAR Construction, the teams were able to come to an agreement to extend the timeline for this portion of the job. The extra two weeks they received allowed for discussion, brainstorming, and ultimately proper installation.
As part of the spec for this portion of the job, Sunbelt Rentals was also required to develop, maintain, and deploy an internet-based monitoring interface/portal to monitor water surface levels of suction pits, total available pumps, pumps running and water levels of the siphon inlet structure.
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Sunbelt Rentals had an internal website that offered these monitoring insights, which allowed them to provide personal logins to all stakeholders to view the project in real time. This live monitoring also allowed for an ongoing dialogue of questions and answers.
Challenge 2: Containment
Another challenge was the need for secondary and spill containment under the discharge piping spanning the Hotel Street Bridge to safeguard against leaks that could fall to the six-lane major highway below.
Sunbelt Rentals laid 10 mil poly plastic sheeting underneath the discharge piping, securing it with adhesive. They also placed inflatable hydrobarriers along the side of the plastic that could direct any leakage to designated reservoirs created at the bottom of the pipe. This system would allow for pumping any leakage back to a redundant line on each bypass in the event of a failure and prevent any leakage from touching the ground.
Challenge 3: Blocking Flow
Figuring out how to plug the 90-in. horseshoe line to block flow during the bypass also posed a challenge, one that BAR Constructors, Inc. solved. Using the measurements of the pipe while the sewage was still flowing, they fabricated a metal plate with a rubber seal to fasten to the inside of the pipe as the plate was placed against the open face. It also had a 36-in. valve that could be used as a flow through port. In case of an emergency, it would send the flow to the Cadiz lift station y-inlet.
Challenge 4: Line depth
The 30-ft depth of the sewer line posed the biggest obstacle for the team to address, since pipes that deep make overcoming suction lift obstacles very difficult. Sunbelt Rentals determined the project required suction pit locations to be excavated and placed at optimal elevations to overcome the net positive section head (NPSH) issue needed to perform the job.
Suction pits were dug approximately 100 ft by 100 ft and 8-ft deep for a 90-in. interceptor and approximately 40 ft by 30 ft and 15-ft deep for 51- and 54-in. interceptors. A complete bypass of the y-inlet was performed with a bypass designed for 150 MGD.
With the elevations and slopes of the 51- and 54-in. sewer lines, it was determined the teams could utilize a plug at the upstream manhole (MH) where they combined flow, which eliminated a suction point, ultimately saving money on the project.
The 90-in. primary and redundant system was designed using fourteen 18-in. Pioneer HV QF diesel pumps and over 14,000 lf of 24-in. HDPE fused discharge pipe. The 51- and 54-in. primary and redundant system used four 12-in. Pioneer HV QF diesel pumps with over 3,500 lf of 18-in. HDPE fused discharge pipe.
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Bringing it All Together
Taking on such a complex project is one thing, but completing it successfully in a relatively short timeframe is another. Mobilization and the project build took place from May 12 to July 25, 2019. The run time lasted from July 25 to Aug. 26, 2019, with breakdown and demobilization spanning Aug. 27 to Sept. 26, 2019.
The partnership between the City of Dallas, JQ Structure, Freese and Nichols, BAR Construction and Sunbelt Rentals was essential to overcoming the unforeseen obstacles encountered on the job. Also, BAR Construction’s role in understanding and helping with the pumping portion of the bypass project was considered vital.
Communication governed every step — that and a bit of innovation.
Jason Chambers is regional strategic account manager at Sunbelt Rentals.