D8R Bulldozer was dressing floor of overburden pile. D8R was north and 30 offset to the west of 374F, loading material. 773D dumptruck lined up and responded on signal from excavator by backing up towards loading position. D8R backed into path of dumptruck. 773D struck ROPS of D8R. Collision caused D8R operator to strike self in the mouth with fist, fracturing teeth.