Opening of the Borehole
In order to open and condition the bore hole to a diameter adequate to the product to be inserted, it should be reamed to a diameter 50% larger in the case of small pipes or 12 inches more in the case of bigger pipes. The borehole should be reconditioned taking care that it does not collapse, and to that effect, openers and reamers of various diameters are pulled through the length of the borehole, in a succession of smaller to larger size.
The type of soil of the crossing establishes the choice of the reamers, these can be:
a. Open fly cutters
b. Crown reamers with inserts or teeth
c. Conical reamers
It is important that a permanent injection of drilling fluids is maintained during all stages, as this will remove the clay, sand or whatever soil content is being drilled.
For effective cleaning of the borehole it is necessary to inject four to five times the volume of fluids to that of the its final diameter. Ensuring positive pressure at all times aids in the containment of the walls.
The gel strength and carrying qualities of the bentonite based fluids allow them to function as a conveyor belt, extracting the borehole cuttings and transporting them towards the exit pits. A low content of solids remaining inside the bore hole ensures for a smoother pull of the product pipe to be installed.
Swabbing and Cleaning of the Borehole
In order to even out the borehole walls and prepare them for a smooth and safe pipe pull a swabbing run is carried out with a cylindrical barrel reamer.