Earthbags on top of rock

I live near Tulum Mexico and the property I have is most almost all rock. I've read all over that a trench needs to be dug to put a foundation for the earth bags, but this just isn't feasible without the help of a jack hammer. Can I built directly onto the rock or is there some special technique that will protect my walls?

I am from the Kootenays and I now have a place down in Tulum. I am interested in earthbag, etc. I realize some of the challenges there. I'll be interested to hear about how things work out for you. I will be in Tulum Dec. 20 - Mar 1. Here is a link. Not sure if you saw this one before: http://momenttomomentdk.blogspot.ca/2011/08/sustainable-building-in-tulum.html

S

The first thing I though of when reading this is this house. http://permaculturenews.org/2011/02/07/freeform-earthbags-between-boulders/

Which incorporates large rocks into the walls. Not sure how it's held up but certainly looks really cool if you live in a climate that this wouldn't hurt.

As far as directly on rock, the only thing that I'd be concerned about is tying it into place to prevent walls from moving. Depending on material weight by itself might be sufficient. I'd just be worried about the base slipping somehow especially in an area with seismic activity. Heaviely bermed might help there as well.