For parallel large diameter pipelines, concern should be given to pipe-trench interaction and trench requirements.
When buried pipes are installed in parallel, principles of analysis for single pipes still apply, however soil cover must be greater than minimum. In other word, the design of parallel buried pipes requires an additional analysis for heavy surface loads (given that a common trench is used). In fact for buried parallel pipes soil slip between the pipes becomes the bottleneck for the distance calculation. Talking about 1 m or more separation then the concern becomes faded but it all depends on your pipelines diameters and soil density.
If the pipelines are in separate trenches, then embedment stability becomes a concern when a trench is excavated parallel to an existing buried flexible pipe. You should address what would happen to a buried flexible pipe when some or all the side support is removed in a parallel excavation. Ans also the existing trench stability itself is an issue to concern. At less than minimum side cover, X, side support is lost and the soil on the pipe must be supported by the pipe. If ring stiffness is inadequate, the pipe collapses.
For a detailed discussion, please refer to "Buried Pipe Design" by A. P. Moser, Steven Folkman chapter 3.
