Fundamentals of a New Manual of Practice for the Design of Flexible Liners for Pipes Operating in Gravity Flow Applications

Over the past 20 years there has been much research and verification as to how buried pipes interact with the surrounding soil(s) (soil-structure interaction theory) and also how this relationship is evolving over time. Concurrent with this body of study, other engineering academicians have also developed an understanding of how the addition of a flexible liner into this existing soil-structure interaction system will perform after the liner's installation. This paper introduces to the practicing engineer the work currently underway by a task group of the PINS committee charged with producing an ASCE Manual of Practice (MOP) for flexible liners in host pipes that are designed for gravity applications such as culverts, storm sewers, and wastewater collection and transportation systems. This task group, in conjunction with others working around the world, is seeking to take this vast body of engineering knowledge and consolidate it into a practical and usable form for the practicing engineer. In keeping with the professional engineer's need to properly document the design process he or she is employing, the new MOP is currently envisioned to cover not only the design process itself, but also the engineering assessment of the existing pipe-soil conditions that must precede the design process so that assumptions made will reflect the true needs of the renewal process. This knowledge of the state of the stresses in the existing soil-structure interaction system is a prerequisite to qualifying the liner for the typically desired service life of 50 years or more. This proposed design methodology considers the impact of both the global and the local imperfections present in the existing pipe and/or liner system. The new methodology can handle noncircular standard and nonstandard pipe shapes in addition to just the circular-shaped pipes addressed by the nonmandatory design appendix of ASTM F1216, which has, by lack of a more formal design standard, become the default design methodology for most practicing engineers in North America. Written in 1985 essentially for the pipe size range 8 in. to 12 in., its extension into the larger size pipes has led to inefficient wall thicknesses in these larger sizes that are hard to install well and lack the compliance with the shape of the host pipe structure desired by engineers and owners.