Development of Analytical Model for Thrust Restraint Design: Part 1 - Continuous Pipelines

Thrust restraint design of buried pipelines is dependent on internal pressure, piping configuration, and the pipe-soil interaction at the pipe-soil interface. Pipe-soil interface behavior is dependent on the pipe material, native and backfill soil properties, and installation conditions. It is generally recognized that a buried pipe has to move through the soil to develop friction and adhesion resistance. It also has to move against the soil in order to develop lateral (passive) resistance forces which, in combination with the frictional and adhesive resistance forces, resist the unbalanced thrust. Axial and transverse pipe movements cause additional pipe stresses (axial tension, bending, and shear) on the pipe at or near the unbalanced forces, which are not considered in the current American Water Works Association (AWWA) pipe design manuals adequately, except to a certain extent in the M9 (AWWA 2008) procedure. The ASCE task committee on thrust restraint design of buried pipelines was formed in 2008 with the objective of documenting and reviewing the current practices, investigating the feasibility of a unified thrust restraint design framework for all materials, and preparing a manual of practice to improve current design practice for thrust blocks and thrust restraint systems. The proposed framework consisted of two distinct design solutions - for continuous (welded, fused or flanged) and segmented (those with some degree of joint flexibility and/or extensibility) - to accommodate the different types of joint types used in practice. Advancing the concept further for continuous pipelines, the task committee presented a case study in 2011 that compared restraint length and calculated the additional stresses in buried continuous steel pipelines. The approach used in the case study utilized analyses procedures outlined in AWWA M9 (2008), M11 (2004) and the simplified procedure published by Robertson (1980) for Reinforced Thermosetting Resin (RTR) pipes. In addition, the task committee has also undertaken a series of exercises towards developing the preliminary unified framework outlined in the draft white paper, which include the development of an analytical model and a software tool. As an initial step, the analysis approach presented in AWWA M9 (2008) for continuous pipelines has already been implemented in the software tool. This paper presents a detailed design example for the purpose of verifying and fine tuning this analytical model for the case of steel continuous pipelines.