TWO INDIVIDUALS WERE IN THE PROCESS OF INSTALLING THE WALLS OF A TANK AND WHILE ATTEMPTING TO CORRECT A BOW IN THE FLOOR OF THE TANK. A 1"" SHIM HAD BEEN PLACED UNDER THE TANK IN ORDER TO ASSIS T IN CORRECTING THE BOW. AS THE EE'S MOVED ABOUT THE TANK, THE 3 STEEL PLATES LEANING AGAINST THE EXISTING WALL FELL, STRIKING ONE OF THE EE'S & A PIECE OF DUNAGE. WAS ABLE TO EXTICATE HIMSEL