Sink Trap SLIPS - please help
We have a garage sink and the washer dumps its water into that sink and i believe the pressure of the water being dumped into the sink and down the drain slowly makes the sink trap slip and floods the garage. This has been on-going for a couple of years. My husband couldn't figure it out so he wedged a board between the floor and the trap so the weight of the water could not make the trap slip. Then recently I decided to hire a plumber to permanently fix it, and he used all new metal pipes/trap and it still slips! The plumber came back and re-tightened everything and said it would 'not' happen again - and it has! What else can be done? Has anybody encountered this before? Please advise of a permanent solution, as I never want to flood my garage again. Thank you.