The capability of the pipe to carry the load will depend on the size of the pipe. Generally larger pipes can have less cover because the load is spread out over a larger area. PipePac is a program distributed by the American Concrete Pipe association that will determine the required class of...
I have always used 33% as the void ratio unless I have better information. This may be conservative but usually I am designing a dry well for infiltrating stormwater and clogging can be a factor so a conservative estimate is practical.
