Containing Thrust Forces in Municipal Pipelines

Bell-and-spigot gasket-joint thermoplastic pipes such as Polyvinyl Chloride (PVC) are widely used in pressure applications for potable water distribution and transmission, and sewer force mains. The thrust forces which result at joints due to changes in the direction of flow or changes in the diameter size of a pipeline must be counterbalanced to prevent joint separation. This is accomplished using either concrete thrust blocks or joint restraint devices, or a combination of the two. In the past fifteen years, use of joint restraints in PVC pipes has increased significantly while the use of concrete thrust blocks has lowered. In many parts of the US, both are used in tandem. Traditional joint restraints, “lug-type restraints,” are external to a piping system and must be fitted on the outside of the joint, exposing the device to in-situ soils, which makes it susceptible to corrosion. Assembly is also time consuming and prone to human error. When using joint restraints only, without thrust blocks, it is usually necessary to also restrain one or more pipe-topipe joints on either side of the fitting. The number of pipe joints or the length of pipe to be restrained on either side of the appurtenance can be calculated based on several parameters. The next generation of joint restraints for PVC pipe-to pipe and ductile iron fitting-to- PVC pipe connections are internal to the system. The BullDog™ joint is integral to a PVC pipe bell and is intended for pipe-to-pipe joints, while the fitting-topipe restraint is a self-restraining gasket, called the MJ Filed Lok®. These devices are easier to assemble than traditional alternatives and much less susceptible to corrosion. Basic design of joint restraints, outlined in the American Water Works Association (AWWA) Manual M23, PVC Pipe - Design and Installation are discussed and the importance of ASTM F1674, Standard Test Method for Joint Restraint Products for Use with PVC Pipe, in the selection of joint restraint products, is explained. Finally, a case history of a recent installation of the BullDog™ and the MJ Field Lok-PV® at the City of Richardson, Texas is presented.