Thank you for your question regarding multi-layered soil, it is our pleasure to help.
Rarely does one ever find soil conditions that could be considered one-layer (uniform) or even 2-layer. Most commonly you will find soil conditions that are best modeled using 3, 4 or 5 layer conditions. These layers impact the way electrical energy propagates through the earth dramatically, and should be accounted for, particularly when dealing with human safety. Here is a link with more information on this subject for you:
If the above link does not answer all your questions, please feel free to contact us again in the future.
The Engineering Team at E&S Grounding Solutions
Photo credit: http://www.flickr.com/photos/soilscience/5094932244/sizes/o/in/photostream/