structural requirements for earth shelter houses

I am in the process of designing my own earth sheltered/underground house and I have been considering a variety of methods. The wofati design detailed by Paul Wheaton http://www.richsoil.com/wofati.jsp has helped greatly in refining my design. However, I am curious as to how design and evaluate the structural needs of the roof. I would like to do all the work myself with minimal modern materials, sticking with on site lumber and dirt as much as possible. So if anyone has any expertise on structural geometry, or knows of any good publications, i would appreciate the input. Thanks

All of that work is already done for you.

Mike Oehler paid a structural engineer to design the modular tables in the back of his book, "The $50 and Up Underground House".

There is no other book that I know of that will give you that information and how to do it safely for low cost.  Try it any other way and be prepared to pay into the mid hundreds of thousands. 

Mike's engineer designed the roof to hold 2 feet of earth and a foot of water, however Mike recommends reducing loading to 18". 

His info is at www.undergroundhousing.com.