Do you have any recommendations for modeling a power house cavern excavation?

With regards to cavern excavation, the following discussion is courtesy of Evert Hoek:

I have attached a note on cavern design using Phase2 which may be of assistance to you. Generally I recommend ignoring the shotcrete lining for a large cavern design and relying on rockbolts or cables to provide the principal support. The shotcrete is applied to protect the rock surface and to bridge the gap between the rockbolt of cable heads but, unless the blasting is perfect, it is very difficult to form a load bearing shell with a thin shotcrete lining.

 

You will also find a chapter on large cavern design at http://www.rocscience.com/education/hoeks_corner.