Thrust Restraint Assessment and Rehab for the Town of Gilbert’s Zone 1 Transmission Main

Zone 1 transmission main for the town of Gilbert consists of approximately 3.5 mi of 24-, 36-, and 48-in.-diameter bar-wrapped concrete cylinder pipelines installed under three contracts or segments between 1990 and 1993. A failure at a 36-in. diameter 45° bend, which occurred on June 29, 2018, prompted the town of Gilbert to initiate the assessment of the existing thrust restraint designs of Zone 1 transmission main. This thrust restraint assessment, which is the main focus of this paper, is part of a comprehensive assessment and rehab program undertaken by the town of Gilbert to ensure continued Zone 1 water supply. The adequacy of existing restraint length and cylinder thicknesses were checked using both the pre-2008 and 2008 AWWA M9 thrust restraint design procedures. A pre-design report was compiled, which includes the required restraint length and cylinder thicknesses for dead end thrust and horizontal and vertical bends; the report also includes discussion on the probable causes of the failure based on review of available photos and reports. The comprehensive assessment program includes field leakage tests, electromagnetic tests to detect corrosion, tests to detect stray current electrolysis, CCTV inspection to detect joint movement, and valve replacement. The tests performed are described, and results are presented in this paper along with an overall summary and recommendations.