Open-Cut Installation Innovations

There have been many updated recommendations for the installation of cut-and-cover pipelines in the past few years. Uncompacted bedding, uncompacted padding, trench width, soil migration, native soil flowable fill, revised terminology, new test method for the maximum density of cohesionless soils, and soil classes are some of the many changes appearing in new manuals and standards. The PPI Handbook of Polyethylene Pipe is being revised, and Chapter 7 on Installation will feature all of these changes in the new version. These updates reflect the recent revisions to the Second Edition of AWWA Manual M55 PE Pipe—Design and Installation as well as other revised standards and manuals. The handbook encourages the use of basic installation and engineered installation. The basic installation is for a pipe stiff enough not to need special bedding and embedment, for shallow burial with no live load, and for stable trench wall support. In this case, the pipe can be laid on the trench bottom and backfilled with compacted soil from the excavation. This covers most pressure pipe installations. For other conditions, the engineered installation means selecting a pipe and corresponding installation details to meet deflection and buckling requirements. Other changes in installation recommendations incorporate new design approaches such as revised E prime (Modulus of Soil Reaction) values for the embedment soil and for the trench walls, the use of a composite E prime, new material compounds, flowable fill uses, and time-lag considerations. These are significant updates to design and installation recommendations that designers and installers should be aware of.