Thrust Induced Movement of Pipe Bends for Differing Pipe Materials

Theory and practice of thrust restraint design of restrained-joint buried pipelines are dependent on the pipe-soil interaction at the pipe-soil interface, which can be quite complex. It is generally recognized that a buried pipe has to move against soil in order to develop aggregate friction, adhesion and passive resistance forces. In combination, these forces resist the unbalanced thrust forces in a bend. Design equations currently in use in the water/wastewater industry are often based on simplifying assumptions that do not necessarily address the interfacial behavior clearly. These pipe movements potentially cause additional pipe stresses (bending and shear) on the pipe at or near the unbalanced forces. However, these additional stresses are typically not considered in the designs of water/wastewater pipelines using most of the current AWWA pipe design manuals. Pipe movements in a pipe-soil interaction system should be considered in the design and selection of pipe and its associated joints. There exist numerous joint configurations, some with substantial axial and rotational slack (deflection flexibility) and others that are instead rigidly harnessed or welded. While most manufacturers have information on joint configuration that could be used to estimate available axial and rotational slack at the joints, the impact of the slack and/or pipe bend movements on the design of the pipe itself is less understood. The ASCE task committee plans to gather information to document the current understanding of allowable pipe bend movements in the design of pipelines at the unbalanced thrust sources. Having understanding of these differing movements, and being able to quantify basic levels of movements available for each pipe material and joint configuration will form a key component of efforts towards future practice improvements on the subject. This paper presents information gathered to date on the subject in terms of available data on thrust-induced movement of commonly used pipe materials (e.g., concrete, ductile iron, fiberglass, polyinyl chloride (PVC), polyethylene (PE), and steel) and joint configurations to form the basis of the respective thrust calculations for these materials.