Thrust Restraint Design and Analysis for Continuous Pipelines: Case Study of Steel Pipe Using M9, M11, and Fiberglass Analyses

Thrust restraint design of buried pipelines is dependent on the pipe-soil interaction at the pipe-soil interface, pipe bend configuration, pipe and soil properties, and internal and external loading. It is generally recognized that a buried pipe has to move against the soil, in order to develop maximum aggregate friction, adhesion and lateral (passive) resistance forces, which in combination resist the unbalanced thrust forces. Pipe movements (axial and transverse) cause additional pipe stresses (axial, bending and shear) on the pipe at or near the unbalanced forces, and these additional effects are currently not considered in American Water Works Association (AWWA) pipe design manuals, except to a certain extent in the recently revised M9 procedure which uses significantly improved assumptions. In 2010, the ASCE task committee on thrust restraint design of buried pipelines presented a draft white paper on the subject and proposed a preliminary framework for the design of thrust blocks and thrust restraint systems to improve current practice. Recognizing the different joint types used in practice, the proposed framework consisted two distinct design solutions for: continuous (welded, fused or flanged) pipelines; and segmented (discrete) pipelines. Continuous pipelines consist of welded steel (WSP) and concrete pressure (CPP) pipelines with welded joints and polyethylene (PE) and polyvinyl chloride (PVC) pipelines with fused joints. Since the publication of the white paper, this task committee has undertaken a series of exercises to evaluate and compare current practice in detail and to evaluate alternate approaches in order to advance the development of a potentially unified approach. As part of this effort, the committee undertook a case study for the comparison of restraint length and resulting stresses in buried continuous steel pipelines utilizing the analyses procedures outlined in AWWA M9, M11 and the simplified procedure published by Robertson (1980) for Reinforced Thermosetting Resin (RTR) pipes. This paper presents the results of this case study containing bend displacements and additional bending stresses calculated using two WSP examples.