Comparison of ASCE’s Unified Approach and Current Practice for Thrust Restraint Design of Segmented Pipelines Using Ductile Iron Pipe as Example

The American Society of Civil Engineers (ASCE) Task Committee on Thrust Restraint Design of Buried Pipelines is currently developing a Manual of Practice (MOP) on the subject under the Pipeline Division of the Utility Engineering and Surveying Institute (UESI). This MOP, currently in its final stages of development, proposes a unified method that can be applied to all common pipe materials used in the water and wastewater industry. This proposed unified approach utilizes advanced soil-structure interaction concepts and consistent soil and pipe-soil interface parameters to better model the conditions of a buried pipeline subjected to thrust forces. The proposed framework consisted of two distinct design solutions: (1) continuous (welded, fused, or flanged) pipelines and (2) segmented pipelines (those with some degree of joint flexibility to allow angular deflection and axial slack) to accommodate the different types of joint types used in practice. Analytical models based on beam on elastic foundation (BOEF) theory were developed to evaluate restrained joint design for each pipeline type. The segmented model incorporates individual beam segments connected by nodes that represent the joints. Each node has six equations in six unknowns; therefore, advanced numerical computational methods are used to obtain the solution. On the other hand, the continuous model is a special case of the segmented model and results in a quadratic equation that can be solved without advanced numerical computational methods. Based on the work completed to-date, the task committee concluded that it would be appropriate and reasonable to solve the segmented pipeline problem as a continuous pipeline to obtain conservative, yet economical restrained lengths for buried pressure pipelines. In order to validate this conclusion, several different problems were analyzed using the numerical models developed for segmented pipelines using both segmented and continuous models, and the results were compared. Due to space limitations, only limited number of thrust restraint analyses and results pertaining to ductile iron pipe, which is a segmented pipe, are presented in this paper. In addition, this paper compares the results obtained from the segmented model with the results obtained using the current thrust restraint design approach contained in the Ductile Iron Pipe Research Association (DIPRA) design guidelines and AWWA design manual M41 for ductile iron pipe.