I just got off the phone with somebody who was having their toilet and tub back up and spill a lot of water onto the floor - after which it found it's way through various holes down into their ranch house's basement. There was enough water so that the doorway out into the hall had to be dammed up with towels to keep the water from getting throughout the first floor.
There was a huge amount of rain in that area just before that happened (and still is). The basement usually does get flooded. Also, the area is a mile or so from a river, so the ground water also usually gets filled from the river, besides from the rain.
But here's the thing: how could water back up through the drain pipes for toilet and tub when the rims of those two things are at least four feet above ground level? The back yard was *not* covered in water, like in a flood. How did water get pressured upwards like that?
Could it be because of the basement sink? There's some kind of sink there, that uses an electric pump to boost the water up and out into the septic system. Could it be that water flooded down into that sink, and its pump kicked in to somehow create pressure in all the drain pipes and make the water eventually go up and out the toilet and tub? Should the electric to that sink's pump be cut off? (That's what I suggested.)
The ground water itself couldn't have pressure to do that, could it? Especially when the lawn is not flooded?
Or is there some other mechanism involved? Gas pressure in the septic tank? The whole thing seems strange. Btw, the tub did eventually drain by itself.