1.
If you ~ a quantity of things or if they ~, they gradually form a pile.
Bulldozers piled up huge mounds of dirt...
Mail was still piling up at the office.
PHRASAL VERB: V P n (not pron), V P
2.
If you ~ work, problems, or losses or if they ~, you get more and more of them.
Problems were piling up at work...
He piled up huge debts.
PHRASAL VERB: V P, V P n (not pron)