Does anybody know where to find information on calculating the allowable pipe loads or capacity of buried pipes? Are there any comprehensive resources on calculating the capacity of pipes of all types (hdpe, pvc, steel, clay, etc.)?
There are many sources, but I don't know of a single source that answers all of your questions. Start by looking through such sources as pipe handbooks, State Highway Department design manuals, and your old textbooks. Many of these are on line so Google them ( Google is now a verb but Giggle is not.)
Download Australian standard AS 2566.1 Buried Flexible Pipeline Design. This gives you clear advice as to how to determine the loads and the deflection,stres, strain, buckling and combined loading criteria.
Also Structural Mechanics of Buried Pipes by Watkins is a good reference. Also Buried Pipe Design by Moser should be a considered text.