A lot of expanding foams contain ghastly chemicals (like dioxins) which can leach into the water. There was a massive environmental scandal in Sweden about this during the building of a railway tunnel. They were using such a foam to grout the wall before lining it. A key problem was that the tunnel was too cold for the foam to set quickly enough and chemicals leached into the groundwater.....resulting in poisoned milk, damage to various endangered species, furious farmers etc. etc. The question of temperature is obviously of important in UK caves. Check the spec carefully of the foam you want to use, because otherwise you might find that it doesn't set properly in average UK cave temperatures.
I also worry about the long-term perspective of using foams. They will presumably break down over time, again leading to possible environmental problems. From a conservation perspective, I think good old cement is a much better alternative, since it's basic make-up and chemistry is pretty similar to limestone and it's reasonably chemically inert. It's hard to imagine any long-term environmental problem arising from a lump of concrete in a limestone cave.
As an aside, what kind of concrete mix can you get away with? Obviously the more sand and aggregate (sourced from within the cave) you use, the more efficient the process is, because you need correspondingly less cement. But too little cement would make the resulting concrete too weak. Obviously it depends a bit on what is available down the cave (a mixture of different grain sizes is better than, for example, pure fine sand). Any thoughts?
Now all you have to do is work out how to get the concrete mixer down there :-\
Mark
