IE. was shoveling clay fines from tailwheel of B-11 belt. IE was working parallel to wall, throwing material in front of EE, so EE was not twisting EE's back, nor was material wet or in large chunks. EE was working at a moderate pace about 15 mins, when EE felt a sharp pain in EE's lower back. Taken to phys., placed on restricted duty, released to full duty 2 days later.