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RFID doesn't seem like the right technology for such a project to me but if you want to do it that way
for some reason you could do worse than look around the examples pages of companies such as
microchip or mikroelectronica for the fundamental information and examples and build from there.
If the other threads for toll gate rfid questions that elicited no response here are anything to go by
I'd suggest the above is your only answer.
Long range rfid relies on careful choice of frequency and very large recievers - which I would expect
would be very prone to noise issues and unreliability in such an environment.
(I'm assuming you dont have the resources to place your devices underground when I say that - if
you do you already know more than me)