Evaluating & rehabilitating interceptors throughout Fort Worth, Texas
Fort Worth, Texas, is the 15th largest city in the U.S. and provides water and sewer services to more than 1.2 million North Texas residents. Its wastewater collection system exceeds 3,300 miles of pipelines with more than
255 miles of 24-in. and larger diameter pipe. The city is divided into six collection basins, of which the Sycamore Creek interceptor basin is one. The existing Sycamore Creek interceptor is a combination of 48-, 54- and 60-in. diameter unlined reinforced concrete pipe installed in the early 1960s.
In 2010, the city began a multi-year program dubbed the Interceptor Condition Assessment Program (ICAP) with the goal of inspecting its large diameter sanitary sewer lines, evaluating their condition, and prioritizing repairs and response strategies. The program deployed sonar, 3D lasers and high-definition video equipment to inspect pipes from the inside, and at the time was one of the largest known implementations of this technology in the world. As part of a systematic response to the ICAP findings, the city addressed condition deficiencies in the Sycamore Creek basin in 2014.
In 2014, the city selected Lockwood, Andrews & Newnam Inc. (LAN) to evaluate the assessment data and develop a rehabilitation plan for the existing interceptor. During the development of plans for selective rehabilitation of the pipeline, it was determined that the rehabilitated gravity sewer would not be able to carry projected peak flows through the basin. Rehabilitation would provide improved hydraulics, but not enough to carry the projected 2030 flows.
The city, as required by the Texas Commission on Environmental Quality (TCEQ), must ensure that proposed improvements can handle design flows 30 years in the future. The projected flows for the basin were calculated to be in excess of 79 mgd though, while the existing interceptor allowed a flow capacity of 48 mgd–well below the future required capacity. In response, the city directed LAN to design a parallel relief interceptor that would serve two key functions:
- Provide sufficient capacity for projected flows; and
- Provide a low-cost, low-risk method of bypassing wastewater flows from the existing interceptor during future rehabilitation efforts.
Because the new parallel and relief interceptor would be completed in phases to accommodate capital budget allotments, LAN designed strategic interconnections between the existing and relief interceptor.
The existing interceptor crossed community parks, a municipal golf course, hike-and-bike trails, a railroad, as well as other recreational areas. With the project impacting several stakeholders, this quickly became a logistical challenge. Stakeholders included the City Water Department, City Parks and Recreational Department (PARD), Texas Department of Transportation (TxDOT), Union Pacific Railroad (UPRR), Tarrant Regional Water District (TRWD) working in conjunction with the U.S. Army Corps of Engineers, four different gas and petroleum companies, and several private-property owners. Each stakeholder had specific requirements to use their property or easements.
The TRWD required general public protection from construction activities, storm water runoff contamination prevention, specialized grass restoration, and an extended warranty for work in their area. TxDOT required specialized temporary fencing to limit contractor activities with a focus on erosion prevention and public traffic safety.
Gas and petroleum companies wanted differing separation distances from their existing pipelines and guaranteed access to their facilities during construction. UPRR focused on the tunnel liner plate strength and tunneling shield requirements to minimize track settlement. Private landowners and businesses were concerned with access to their properties and continued city services.
PARD was concerned with the impacts large diameter pipeline construction would cause in an active municipal golf course. Affected by proposed improvements on the nine-hole course were two par-five fairways, one tee box, several bunkers and two greens.
Coordination of all stakeholders and the physical conflicts created several design challenges for the new parallel and relief interceptor. Most notable was how to accommodate the vertical conflicts between an existing creek crossing, several high-pressure gas pipelines and a petroleum pipeline that served Dallas Fort Worth (DFW) International Airport.
Within the golf course, it became necessary to go under the drainage tributary of Sycamore Creek with a two-barrel siphon with specially designed erosion protections. This minimized impacts to the golf course features, future game play and course aesthetics as opposed to an aerial crossing.
Just upstream of the creek crossing were existing gas and petroleum pipelines, as well as water distribution and sanitary sewer collection lines. With each of these utilities at different locations, threading the needle with a gravity interceptor required an innovative approach.
Throughout the alignment, the goal was to maintain positive grade wherever possible while minimizing the number of siphons and the length of the proposed siphon. To minimize the limits of the creek crossing siphon and avoid a second siphon, LAN designed a bifurcated wye to pass beneath an active high-pressure steel gas pipeline and provide clearance.
The bifurcated wye allowed the larger 66-in.-diameter interceptor to reduce the profile while maintaining hydraulic capacity with two 51-in.-diameter lines. Further downstream, at a petroleum pipeline, crossing clearance was gained with the design of a smaller diameter pipeline laid at a steeper grade to maintain hydraulic capacity.
Understanding that the flow changes patterns with the use of siphons, bifurcated wyes, and pipelines of different diameters, the city specified polymer concrete junction boxes at transitions and passive odor control installations. Additionally, rather than utilizing physical weirs within the flow control junction boxes, elevations were used to force cleansing velocities and minimize future maintenance by reducing debris collection locations. This was critical to keeping the parks and golf course stakeholders on board with the approach as any future maintenance activity will inherently impact gameplay for golfers.
Engineer’s Design Tool Box
With the conflicting challenges presented by a creek and series of pipeline crossings, the solution ended up being a combination of ideas and design tools. No single solution could address the challenge effectively. Therefore, the lesson to take from this project is to not limit the size of the engineering tool box by relying on traditional design thought processes or methods.
Changing pipe sizes can be an effective way to avoid conflicts and manage separation distances required by other utilities without negatively impacting their operations. This can be particularly important when utilities have a critical service such as DFW International Airport. However, when changing pipe sizes, consideration must be given to the smaller pipeline carrying capacity and the effects of hydraulic changes such as surcharging potential, head loss, or odor development.
The key to successful delivery here was early engagement of all project stakeholders and coordinating with multiple stakeholders simultaneously in some cases. This allowed parties to understand and appreciate the perspectives of others and needs of impacted parties, which enabled early buy-in on innovative design approaches and material selection.
The improvements were broken up into two separate construction phases because of the challenges and timeline with the UPRR permit. Phase 1 is approximately 80% complete with all the tunneling under Interstate 30 finished and much of the carrier pipe installed. The remaining portion of this phase is construction of the interconnection junction box between the existing 60-in.-diameter interceptor and the new relief interceptor that will manage flows through the system. Phase 1—including approximately 1,800 ln ft of 66-in. Fiberglass Reinforced Polymer (FRP) pipe—is expected to be completed in summer 2019.
Phase 2 design is complete and expected to be advertised for construction in summer 2019, with a planned construction schedule of 12 months. Phase 2 provides a second system interconnection between the existing and new relief interceptors while also allowing for the abandonment of an existing 18-in.-diameter sewer line within the golf course. Overall, it will include installation of approximately 3,650 ln ft of 27-, 42-, 48-, 54- and 66-in. FRP pipe with corresponding corrosion resistant tee-base polymer concrete manholes and polymer concrete junction boxes.
As the city looks to future phases to provide hydraulic capacity for the system and subsequently address maintenance of the existing interceptor, there are more parks, highways, private parcels, creeks and pipelines to cross. With a complete toolkit at the ready and experience in addressing conflicting interests and alignment challenges, the city is ready to go under the river and through the pipelines again, if needed.