A septic tank normally is full to the overflow/outlet port. So, put some waste in, some (tries) to flow out the other side. THe inlet and outlets are separated, so solids fall to the bottom (which is the part that needs to be pumped out periodically). All other liquids eventually go to the leach field(s). If everything is working right, the solids have time to fall down enough that the overflow does not contain any, and thus eventually clog the fields up with fine particles. If the soil is saturated, there isn't much you can do - waste in, waste out, instead of percolating down into the soil, it will try to go somewhere. Depending on the elevation changes, the permeability of the soil, and probably other things, you may not get an overflow into the house, but if the line feeding the septic tank was also full back into the house, adding more WILL cause it to overflow in the lowest point(s) in the house. It could get quite expensive, but there may be ways to provide for temporary leach field saturation. A new tank with a pump and various controls could be used as a buffer. Normally, it would only be a pass-through, using the pump to move it to the existing septic tank. But, if the leach fields became saturated, it would allow it to fill up. Depending on the size of the tank, this might give you a couple of days (or maybe more) buffer. Not sure how feasible this would be, but to get it to work automatically, could get quite expensive.