We installed the new septic system without too many glitches. The next step is to dig the trench for the foundation, line it with rebar, and then have it inspected before pouring the concrete.
George had given us the plans (approved by the county health department) for the septic system he put in 30+ years ago when he moved the house here. The plans showed that the 750-gallon tank was located exactly where the addition is to be built. A few weeks ago, Scott went out with a long metal-rod probe and hit what he thought was the tank right about where the plans said it was. We assumed that was its location.
Well, this week, Scott started digging exploratory holes to see exactly where the tank is in relation to the trenches we need to dig for the foundation. And guess what? The tank is no where to be found. He found the exit pipe and followed it for quite a ways past the house, past the end of where our additon is going, and way past where it shows on the plans. We still don't know where it is. The probe must've hit a rock that Scott mistook for the tank.
This is good news and bad news. Good news: it will be easier for us to dig the new foundation knowing we don't have the old tank to either remove (heaven forbid) or demolish in place after having it pumped out. Bad news: we probably could've revised our plans so we didn't have to spend the extra money to install a new septic system.
Lesson learned: don't trust the plans given you for earlier work you can't see. In our case, what was installed is way different from what shows on the plans. I wish we'd gone looking for the old septic tank before we contacted the engineer to design a whole new system for us. Maybe by posting this, you won't make the same mistake we did.