This is more of a educational statement, at least thats what its meant to be, and I'm sure its been brought up before. In my days of underground cast iron waste plumbing under homes that have hit the 35 to 40 years level and the pipe that has corroded through and collapsed it always was where limestone gravel touched the pipe and never where mud touched. The pros told me that limestone sets up a very small electrical current and just sets up an electrolysis and causes corrosion like a battery cable and a battery post. If anyone thinks otherwise, let's hear it. Also I might add is that Germany used plastic type sewage pipe in the thirties and tyler cast iron pipe makers in the US was making political donations to the states to keep it out. The Federal government stepped in and said no more FHA money until it is okay to use. Kentucky, and I believe Alabama were the last two states to approve PVC plumbing above and below ground.