EE WAS INSTRUCTED TO DO PM SERVICE ON 1178 992D CAT LOADER. UPON INSPECTING WORK AREA HE NOTICED A LARGE AMOUNT OF MATERIAL (MUD) ON RT REAR CONTERWEIGHT AREA WHICH CONTAINS BATTERIES & ACCESS TO ENGINE COMPARTMENT. HE DROVE LOADER TO REAR OF OIL HOUSE & STARTED SHOVELING MATERIAL FROM COUNTERWEIGHT. HIS FOOT SLIPPED OFF REAR EDGE & HE FELL APPROX 5 FT TO GROUND BELOW.