Andrea,
Is this a one time event that the stuff shot out the returns or is it a recurring event? When you say the pump may be too big, how about some numbers like the pump info. Personally I don't know what size the T-60 is. Info on that would be helpful too, simply to not require looking it up. The sand on the floor does indicate an oversized pump as a possibility. Is there anything you did or changed that may be related to the incident?
Best for now.
Al