It is quite possible that the culprit used some sort of adhesive under the particle board, which is why you may be having such difficulty. If this is the case then a hammer, chisel and alot of $g##$@%&* will eventually remove it all. The next phase involves a question on my part. Can you tell what type of flooring is underneath this particleboard? Also what room is directly underneath the dishwasher? Till I hear from you. You may EMail me for a faster response if you choose.