EE was observing the installation of roof bolts with Fletcher CB on the 4WTG section. A rock approx 4'x6'x12' fell striking ATRS then ee on the lt lower leg resulting in a comp fracture. On 10/8/17 the injury resulted in amputation of the leg at the knee. Cause of injury was awareness. Compliance with rules/regs,PPE,mining equip or Miner proficiency not a factor at this time