EE WAS BACKING 100 TON TRUCK TO A BERM ON LEVEL #1 IN ORDER TO DUMP A LOAD OF RIP-RAP INTO THE BASEMENT. UPON CONTACT WITH REAR TIRES TO BERM, LEVEL BEGAN FALLING AWAY, STARTIG WITH RIGHT REAR SIDE. THEN ENTIRE BERM FAILED CAUSING TRUCK TO FALL BACKWARDS OVER HIGHWALL.