EE and two others were hanging 4"" water discharge line. The line was wet and covered with rockdust, it slipped and struck him on the head/back of neck. Other end anchored w/hoist to prevent falling. He refused medical attention. 2 others helped with hanging. Went to Dr. and referred to orthopedic. Diagnosis, back strain on 8/17/10. He is seeing a chiropractor.