A、<span style="FONT-FAMILY: 宋体; FONT-SIZE: 10.5pt;">保护孔壁，防止坍塌</span>
B、<span style="FONT-FAMILY: 宋体; FONT-SIZE: 10.5pt;">有利于钻头钻进</span>
C、<span style="FONT-FAMILY: 宋体; FONT-SIZE: 10.5pt;">便于下钢筋笼</span>
D、<span style="FONT-FAMILY: 宋体; FONT-SIZE: 10.5pt;">其它目的</span>