hollow-rod drilling
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螺旋钻头底部阀门防止钻进过程中钻屑进入到空心的混凝土管中，当钻杆提出同时混凝土浆注入时，钻杆底部的阀门将被打开。
The valve at the bottom of auger can prevent drilling bits entering hollow pipe during operation. When drilling pipe is taken out and concrete grout is injected, the valve will be opened.
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