Since its not the drain field, just the run to the tank, and sandstone, using heavy wall pipe - not the crap from the dollar store- the pipe should bed in the sand mud fine. I would leave it open for as long as possible, and consider some sort of swing joint or slip connector to allow for settling later. You might want to pour a thrust block near the bottom of the run also to help with future movement. And spend a little money on access chambers at critical joints and 'D' boxes and cleanouts. Nothing worse than hidden septics.