Peak Temperature Inside Concrete-Lined Tunnel

Are you looking for a procedure to conduct experiments on your own, or, experiments by others to use as a reference?

I am looking for suggestions on how to estimate the temperature three inches deep in the concrete based on a fire event that lasts 60 minutes. Assuming a peak fire temperature of 2000 degrees F.

This is a thermodynamics problem. You should know the concrete specific heat capacity, resistance and mass.
Also you should determine the volume of the concrete which stays between the fire and the cables.
Than you can calculate the temperature(versus time) of the concrete next to the cables, according to the temperature of the fire.

You can use FEM (Finite Elements Model) software for this calculation.

There is an interesting report published by NFPA in its July/August 2008 edition NFPA journal. It shows results of tunnel fire in an experiment tunnel and an actual tunnel. In addition, it also shows calculation results. I hope those information will be helpful to you.

put the communication cables for life safety at the lowest point in the tunnel. ... also use a redundant system on the outside of the tunnel for end to end communication.

..Inside the concrete? I'd guess the fire will burn it self out long before a properly rated cable will fail.

...whatch out for margarine trucks.. the silent killer

...in short, I have no answer