Understand that the fire shelters are only to protect in an emergency from fire that are traveling fast and pass over the firefighters quickly.

For the idea of digging a hole in the ground I am not sure how deep or how much dirt has to be piled on top of them.
I do understand that the entry to it needs to have a door capable of resisting the heat or a bend to prevent directly radiated heat from cooking you inside of it like an oven.

Most of the information on fire bunkers for protection from bush fires (wild fires) is coming from Australia.
They have been doing a lot of work on this because they have had really terrible fires over the past few years.

Here is a sample of an article you might find introducing the idea.
http://fireshelterfirebunker.com.au/
A deeper search would get you more in depth article or you could introduce yourself to the Australian Survivalist
http://www.aussurvivalist.com/
and ask them about it too.
They had a really good set of discussions about this last year.

Some of them were even posting on discussions we had right here on this forum about it.

http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=155573&page=1

http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=164777&page=1