Rene, At 07:36 AM 8/19/04, you wrote:
However, we're worried about the sound/vibrations that could reach us through the ground. When we listened with our ears to the ground we could hear a low rumbling and a high whining sound.
This type of foundation might help a little compared to a monolithic concrete or brick foundation. I am sure though that you will still be able to hear the low rumbling. An alternative not as green is to build on a thick expanded styrofoam slab this will give a better contact sound isolation. You could combine both techniques ie tires with thick foam strips on top and then the framing for the walls and a wooden subfloor. For good sound isolation the wood subfloor will requier extra mass in the form of a thick layer of sand witha floating finish floor ontop of the sand. The design and execution of this structure will have to be done carefully to avoid local sagging of the building structure causing plaster cracks.
What you should seriously consider, although I am sure you have, are the amount of times and time during the day when these trains pass. If they are in the night time I personally don't think the site is a very good one as you will experience sleep disturbance. SB walls are indeed a good sound insulator but then again a normal SB wall will only give you about 55dBA noise insulation. This is an average figure for the whole sound spectrum lower frequencies will have a much lower performance of around 35dB and much higher performance for higher frequencies.
Rene