While being loaded with muck from the 3944-119, haul truck 101 was parked in the 3950-215 cab first. As loader 94 was placing a bucket of material into the truck's bed, a fall of ground occurred 15.5x16x11 releasing material onto the bed/articulation of the haul truck. The operator could exit the cab, evaluate the condition of the back and find a safe path out of the 215 drift.