I'm using the rock I have on site for a cob house drain trench. It's some kind of sandstone, and I'm worried that it's not strong enough to support the walls, and may get washed away during rains. Is this something to be worried about? If so, does anyone have a good drain rock resource in the San Francisco East Bay?
Yes Conner...it very much can be...If you can get yourself into some "historical structures" in your area and look at there stone foundations you may be able to empirically determine the viability of this stone type. Otherwise I would strongly suggest speaking with a geologist at a local college about your stone and discussion your plans with a "well healed" historical mason for your region and/or in general...I will answer your questions the best I am able.
Photos of the stone, and blueprints/CAD models would also help if you need feedback.