No One Wants It but Everyone Needs It - Building an 8.5 mgd Wastewater System for the 5-Billion Dollar Mile

摘要

The Dallas-Fort Worth area has been growing at a rapid pace and the City of Frisco, located approximately 25 miles north of Dallas, is leading the growth. A 1-mile stretch along the Dallas North Tollway includes four mixed-use developments totaling $5 Billion in capital investments. This area has been coined 'The 5 Billion Dollar Mile.' These developments required a rapid response by the City to formulate a smart plan and to quickly build the needed wastewater infrastructure. This resulted in planning, designing, and building $13.5-million of wastewater infrastructure in 34-months. The 5 Billion Dollar Mile is all within an undeveloped area in the same wastewater basin. The density associated with the high-rise buildings exceeded the wastewater flows projected by the City. When the developers began to approach the City with conceptual plans, analysis quickly began to determine the existing wastewater systems limitations. The first step was to update the City's Info Works model with these new flows which revealed these flows far surpassed the system's capacity. This analysis revealed an interceptor currently under construction would be surcharged with the proposed flows from the denser developments. The first solution was to delineate the development into sub-basins to better convey the flow and maximize use of the existing system. This aided but an additional study showed that further improvements were required to adequately convey the anticipated flows of the four developments. As the development plans gained clarity, the need for additional conveyance became apparent. An 02.additional 11-mgd needed to be conveyed from the 5 Billion Dollar Mile to the City's Stewart Creek West Waste Water Treatment Plant (SCW WWTP). Several options were considered to upsize the existing system to accommodate the expected flows through the gravity system. The residential impact was deemed too great to parallel an existing interceptor through an established development, not to mention the capital and construction costs. Instead, two lift stations were proposed along opposite sides of the basin. The City agreed to provide the lift station and gravity improvements necessary to convey 8.5-mgd of the proposed flow while the second lift station would serve the remaining 2.5-mgd produced by a single development and would be the responsibility of that developer. The initial location of the City's 8.5-mgd lift station was north of the proposed developments at a convergence point in the existing gravity system. The City proposed to purchase a section of the property at the first site location with intentions to add a trailhead associated with the City's Master Parks and Trails System in addition to the wastewater facility. When negotiations began to stall, alternative sites had to be considered. The lift station location ultimately settled within the northeast boundary of one of the proposed developments. This was a good solution for the overall project but lead to some new engineering challenges. The location was approximately 25-feet higher in ground elevation than the original site, requiring adjustments to the hydraulic design. Because the lift station was now located on the development's site, the size and aesthetics of the facility became a high priority. The visual impact was minimized by including a building with facade matching the developer's high-end buildings. A single force main was proposed to convey flow from both lift stations to the SCW WWTP. The majority of the 16,000-feet of force main (18-inch diameter) is located within the median of a major 4-lane thoroughfare. The pipe installation utilizes open cut and trenchless methods. This alignment also crosses one of the busiest intersections in the City, with an average traffic count of 30,000 vehicles per day. The team had to develop a solution to cross the intersection with minimal disturbance. A 2,500-foot horizontal directional drill was chosen for the crossing. The drill was designed to avoid compounding horizontal and vertical drill radii, avoid existing utilities within the median, and to minimize pavement repair. The drill parallels an existing 12-inch reused pipeline that was previously constructed by horizontal directional drilling. Entry, exit, offset, and crossing of the previous drill were all considered with the design. This project was bid as two separate construction contracts totaling $13.5-million and is currently under construction. As developments continue to seek growing areas, municipalities are faced with the struggle of maximizing existing systems and incorporating capital improvements, often in an expedited schedule, to accommodate the additional wastewater capacity associated with the developments. Flexibility and communication between all stakeholders has never been more critical. The use of Info Works modeling software proved invaluable with the dynamic nature of the developments and their flow estimates. This paper will discuss the unique challenges of designing and constructing an 8.5-mgd lift station and force main in the heart of a 5-Billion Dollar development.