Applying the hollow "primary-secondary" driller to expand the holes for reducing the peripheral frictional resistance of the internal drive pipe can make the internal drive pipe submerge to the position of the broken pile.
应用空心“子母”钻对桩基进行扩孔,通过减小内护筒周边的摩擦阻力,可使内护筒顺利下沉到断桩位置。