EMPLOYEE WAS OPERATING WATER TANK TRUCK; SHE BACKED IT UP NEXT TO A SHALLOW EXCAVATION WITH THE REAR RIGHT WHEEL VERY CLOSE TO THE EDGE OF THE EXCAVATION. WHEN SHE PUSHED IN CLUTCH TO PUT IN GEAR TO PULL TRUCK FORWARD, THE TRUCK ROLLED FURTHER BACKWARDS INTO EXCAVATION. THIS CAUSED THE TRUCK TO ROLL OVER ONTO THE PASSENGER SIDE OF THE TRUCK. OPERATOR WEARING SEATBELTS/SHOULDER H