Abstract：It is a complex engineering problem to protect foundation beds for large open-caissons against bed-scouring. The
size of the protecting materials should be neither too small to ensure safety against floods nor too big to affect the cut-in of the
open caissons. By physical model testing，the size and gradation of crushed rocks for bed protecting and the stability of the
bed protection for open caissons in the process of maneuvering, flooding control and bed-sitting were studied. The range of bed
protection and the gradation of crushed rocks for bed protection were thus achieved to make the bed protection for large open
caissons not only stable but also more economical during flooding and bed-sitting operations, which can not only satisfy the
construction needs but also make the overall bed protection meet the general requirements