Thinking Outside the Box, Inside of the Box - Planning, Designing, and Constructing Reliability Improvements into City's Forcemain System by Inserting 36-inch Valves with a Thrust Box Methodology

With Florida's flat topography, transmission of wastewater to the treatment facility almost always requires pumping via forcemains. As Florida grew in population, forcemains were constructed to centralized treatment facilities, with these mains often running for several miles as 'one-way' pipes. These mains were typically constructed with little to no valves given their single direction design with no looping and minimal connections. However, over time as utilities grew, additional pump stations and forcemains were constructed and tied into the original forcemain. In some cases, parallel forcemains were constructed to address growth. Now utilities are often faced with the original forcemains which are exceeding 50 years in age, yet they cannot be isolated for maintenance or repair due to a lack of valves to isolate portions of the forcemain. This was the case for the City of Boynton Beach. The City's primary forcemain included a 36-inch main down Congress Avenue, constructed nearly 50 years ago. The main runs for approximately 4 miles to the South Central Regional Wastewater Treatment Plant (SCWWTP) with numerous pump stations manifolded along the way. As growth continued, the City later constructed a parallel 24-inch main towards the east. These mains were effectively looped, with both mains ending at the SCRWWTP. Although the City had parallel forcemains, they did not have the ability to isolate the original 36-inch main if required given no in-line valves existed. This left the entire City collection system at risk should maintenance or repair be required in the future. Knowing the risk which existed in the collection system, the City requested Carollo to develop and execute a plan to build reliability in the system. This plan included several steps. The initial planning phase utilized the previously developed wastewater model to determine flow patterns and to conduct emergency scenarios. This included stress testing of the system with theoretical 'break points' in the main, allowing the City to visualize flow patterns and limitations during potential scenarios. Based on this analysis, 7 potential locations were identified where addition of a valve would benefit the system. The next step included preliminary design, where the identified locations were evaluated in the field for potential valve addition. Given the size of the valve and construction area along a major roadway, survey and utility locates were conducted at each location to determine the feasibility for valve installation. The 3 final selected locations were based on providing the most reliability/use, minimized service interruption, and having the least construction impact. Final design followed, requiring some creative engineering concepts. The 36-inch forcemain was alongside a 6 lane major roadway, with two of the selected locations being next to a park and school. The City's intent was to minimize impact to traffic and the community. Therefore, a quick construction process with minimal construction area was established as a goal. However, the City was faced with a forcemain that could not be shut down along with unrestrained bell joints. Valve insertion typically requires thrust restraint via thrust collars/blocks and additional joint restraints, which was undesirable given the extended impact length along the pipeline. As a solution address the challenge, Carollo developed a 'Thrust box' concept. Rather than constructing collars, a pre-cast box structure was installed over the active forcemain, similar to a doghouse manhole. The pipe was then restrained to the box, which became a structural member to restrain the forces of the pipe. Flow-thru line stops with a bypass were utilized to maintain service while the section of pipe was cut for valve installation inside of the thrust box. In addition to providing thrust restraint, the box also provided safe access for cutting of the existing main and installation of the new valve. With the insertion of three 36-inch valves in the forcemain system, the City has built a significant amount of reliability in the system and greatly reduced risk for the future. The presentation for this abstract includes design concepts along with detailed photos of the construction process of the valve insertion at each site.