Improved Thrust Restraint Design Considering Displacement of Pipe Bend and Joint Separation

The stability of pressure pipe bends is evaluated by the equilibrium between the thrust force and resistance forces in the current design, and the behavior of pipe bends is not considered. Force–displacement (F–D) relationships, which are used for predicting the displacement of pipe bends, include the performance of pipe joints in the design of pipe bends and shift the design method to a performance-based approach. However, F–D relationships have been proposed only under plane-strain conditions. The behavior of pipe bends cannot be represented in two dimensions. Therefore, in this study, the prediction method of the F–D relationship for buried structures is extended to a three-dimensional condition to improve the design method of pipe bends with thrust restraint. Lateral loading experiments on thrust restraint, using rigid thrust blocks and flexible thrust restraints with geogrids and gravel, were conducted in dry sand to investigate the lateral behavior of rigid and flexible thrust restraints and to obtain F–D curves with different dimensions of thrust restraint. The experimental results revealed that the deformation of the flexible thrust restraint had little effect on the lateral resistance force if proper dimensions of the thrust restraints were determined. Using the experimental results, the F–D relationship was formulated based on a hyperbolic curve. The proposed equations were able to predict the resistance force relatively well at small lateral displacements. In addition to the formulation of the F–D relationship, a new design procedure considering the pipe displacement and performance of pipe joints was developed by combining the proposed F–D prediction method and the joint separation model proposed in previous studies.