The problem was, when the washing machine would drain the tub for the
spin cycle, the water would backup and shoot out of the drain pipe.
I dug up the distribution box and found the dist. box full of mud and
sludge... mostly mud.
The idiots who installed the septic system 30 years ago busted the 4"
PVC connecting the tank to the dist. box. So, sand, dirt and what
little sludge made it to the dist. box filled it up to the top.
I replaced the busted PVC pipe, shovled out the mud/sludge, sprayed
out the drain field entries to clear the mud, threw some copper oxide
(or, whatever that stuff was) and covered it all back up.
Draining like a champ now, and I suspect is will, for many, many years