Not So Quiet On The Western Trunk--HRSD Sets The Standard For Connecting To Concrete Pressure Pipe

The purpose of this abstract is to provide engineers and utility owners with valuable design and construction considerations for connecting to and working with existing, large-diameter (LD), pressurized concrete pipe – which in the context of this presentation includes asbestos cement (AC) and prestressed concrete cylinder pipe (PCCP). Risk exposure increases exponentially when working with LD pipe and working with older, susceptible, cementitious pipe materials can exasperate construction challenges. The Hampton Roads Sanitation District (HRSD) recently completed a project with an unusual amount of work with and connections to AC and LD PCCP force main (FM). Located along the eastern region of Virginia, HRSD owns and maintains more than 500 miles of pipelines, most of which is LD, pressure FM. Although HRSD has developed detailed design and construction standards for working with these pipe materials, this project included additional, innovative design and risk-mitigation elements to achieve success. The Western Trunk FM Replacement project, located in Virginia Beach, VA, was an US Environmental Protection Agency Consent Order (EPA CO) Rehabilitation Action Plan required project with associated deadline of May 5, 2021. The EPA CO required abandonment of a 6,000 linear foot (LF) section of 20-inch AC FM and transfer of flows to a parallel section of 1970's-era, 48-inch PCCP FM. The project scope also included (non-EPA CO requirement) installation of two 'valve complexes' along the 48-inch PCCP FM. This project carried substantial risks to regional sewer operations, property damage, and public safety if a failure were to occur. Therefore, this presentation will focus on identifying risks during planning, mitigation through design and results and lessons learned during construction. Topics include: - Proper design and execution for approximately 17 planned connections to LD FM - Structural considerations and mitigation to reduce failure of existing pipelines - Corrosion mitigation - Contractor/Subcontractor prequalifications Connections: To meet the project scope, the temporary and permanent work included: - Over 700 LF of new 8-, 12-, 16-, 42- and 48-inch FM - Three FM reconnections from the 20' AC FM to the 48-inch PCCP FM - Installation of two 'valve complexes', each consisting of an inline, emergency 48-inch isolation plug valve and branch 42-inch isolation plug valve (for future connection) and associated LD appurtenances - Two 48-inch by 36-inch dual line-stop and bypass systems To execute the above tasks, the project included two connections to the existing AC FM (including one impromptu connection due to a faulty valve) and 15 connections to the existing 48-inch PCCP FM. For each connection, the design had to evaluate a multitude of failure mechanisms and proper mitigation efforts. Another important factor to consider was the requirement to maintain system flow during construction. The presentation will detail the planning, design and construction of these connections and elaborate on the critical components, including: - Risks of pipe failure, property damage, public safety, - Use of HRSD standards and what additional provisions, design elements were included, - Development of a very complex and ultimately successful sequence of construction, - Results during construction, how design standards and construction checklists added value, - and improvements to design/HRSD standards based on lessons learned. Structural Considerations: Every connection required special attention to newly applied hydraulic, weight loading and stress to the existing pipelines, regardless of size. To minimize risks of settlement and corresponding pipe failures through joints or other means, the design needed to include specific measures, materials and structural designs to ensure the proposed improvements did not cause failure to the existing, active FMs. The massive weight of each 'valve complex' would place stress on the existing 48-inch PCCP joints if not supported properly. The design had to consider provisions to minimize potential for settlement and reduce risks to the existing 48-inch PCCP including detailed bedding design, interior and exterior welds, concrete support pads and testing during construction. The presentation will detail the design and construction of the structural elements of this project and elaborate on challenges and lessons learned during construction. Corrosion Protection: The piping design had to consider exterior corrosion from corrosive soils, internal corrosion from sulfuric acid buildup and galvanic corrosion due to material electrode potential differences. Local soil corrosivity was a major concern with soil resistivity readings below 5,000-ohm/cm and potential for aggressive exterior corrosion to DIP. Although HDPE was considered, certain constraints ultimately led the engineer to select DIP. To protect the new pipe, a complex system of protection was designed including special interior and exterior coatings, cathodic protection system and simple, yet innovative approaches to isolate galvanic corrosion between differing metals. The presentation will cover the design elements included to protect the pipe and the specialized work, including: - system design and innovative approaches to limiting corrosion migration - specialized construction and how systems were inspected - challenges during construction, lessons learned and any improvements to design standards Contractor Prequalification: Due to the high-risk, critical nature of LD FM work and specialized experience for tapping and line-stop work, rigorous qualifications were required of the general contractor and subcontractors to ensure success and timely execution of the work. HRSD's frequent need for pipe tapping and line-stop activities for work on active LD FMs has led them to develop very detailed subcontractor qualifications for both specialties, which were utilized for this project. Ultimately, an experienced contractor, teamed up with a well-known reputable subcontractor to perform both specialties. This presentation will touch on the value of the prequalification requirements, how these impacted bidding and award of the project and contractor/subcontractor performance during construction. Conclusions: On paper, these connections can often appear very simple. However, without proper consideration of historical installation techniques, material properties, construction methods and site specific and locational challenges, reduced service life and/or catastrophic failures can occur. At the time of this abstract submittal, the project has achieved substantial completion and met the Consent Order deadline. Given the location of this project within a busy, multi-lane corridor, any damage to existing FMs could have been catastrophic. However, based on a robust design and an experienced contractor adhering to HRSD's PCCP and AC pipe handling and tapping protocols, zero pipe failures occurred. Engineers and utilities that work with critical, LD pressurized AC and PCCP pipelines, will find value from this presentation with new insights and valuable tools to help during their next project. The presentation will explore the success of HRSD's standards and construction checklists used, additional design provisions for risk mitigation, lessons learned during construction and what improvements could be made in the future. Attendees will enjoy a summary of unexpected issues incurred during construction and how they were resolved without missing contract or Consent Order deadlines.