In the Trenches: Parallel Pipe Design

Trench design for a flexible pipe is second nature to most engineers. Trench width is typically based on a balance of safety considerations, soil strength, and soil stability to provide reasonable side support and space for compaction. One topic that is generally considered outside the confines of standard pipe design is calculating the minimum separation between parallel pipelines. Providing the correct pipe spacing whether installed in a common trench or next to an existing line is critical to prevent catastrophic failure. This failure can occur from pipe collapse or undermining of the soil support of an adjacent pipe. The authors present a method for calculating pipe spacing using measured soil parameters: soil slip angle and cohesion. The paper will provide an in-depth analysis of the full approach and include sensitivity tables that show the interrelationship of each parameter and how it influences the trench width and depth. This method was used to design the spacing of a parallel 36-in. and 60-in. pipeline and the challenges encountered during soil testing.