After a rain event, haul truck left the stockpile, began to slide, doing a 180, and came to a stop. 2nd truck came off the dump, attempted to stop, slid sideways into 1st truck. 2nd driver was seen at ER on 10/12 and fully released. Oct. 15 employee complained of back spasms, went to PCP, and taken off work. CURRENTLY ONGOING.