Propane is heavier than air, so it naturally goes to the lowest point in the tunnels. In doing so, it displaces air and will not necessarilly mix with it. Without injecting air or O2 along with the gas, the odds of getting the exact combustible mix along the length and depth of a tunnel system would be pure luck. And if those conditions existed, they would not exist for long as the heavier gas would separate and concentrate into it's own lower area with the air setting on top. Even the ones with O2 injected along with the gas need to be fired in a reasonably short time before the gasses separate. Injecting gas alone, the only area where combustion would be possible is the narrow layer where air and gas meet, and that is likley to be a ways down the tunnel. A very slow trickle of gas down the tunnel might leave a combustible air/fuel mix along the tunnel untill the gas pooled at the bottom and forced all the air up and out.
I have convinced some to move on to greener pastures(or perhaps even killed them) with gas alone. I find one end up a tunnel system, preferably on the downhill side. Using an old arrow shaft with a pointed knock end with a few holes in the tube near the point, I probe the ground and I can feel when it penetrates a tunnel or void. Connected to the other end of the hollow pointed arrow tube is a hose to a propane bottle. At the top of the tunnel system, I find a tunnel and open it to the air. I turn on the gas and go wait till it starts pushing air and gas out the top hole. No flame involved, as you do not know what other holes may be open, and if ignited, you may find yourself standing in an inch or two of burning gas that has spread out over the ground and has a large surface area that has a combustable air/fuel mix... CO2 gas would do the same thing, only better. It has no odor, and could present a lethal situation before the critters could run. It also triggers, at least in humans, a panting reflex which causes the lungs to purge whatever O2 they have left.
Cut sections of road flare, lit and sealed in a tunnel seem to also drive them off for a while. I get about 3 good sized 5 minute pieces out of a 15 minute road flare, and they cut real clean and easy with a sawsall and metal blade... Auger hole with cordless drill auger, lght flare section with bic lighter, drop in hole, and set a board or something over the opening. Real nasty smoke...