Definition - What does Back Reamer mean?
Back reamers are used in Horizontal Directional Drilling (HDD) operations primarily for enlarging the pilot bore to accommodate the carrier pipe. Once the pilot bore is drilled, the drill string is attached with a reamer, and the reamer is pulled back through the hole. The purpose of the reamer is to enlarge the hole. It make take multiple reaming passes before the desired hole diameter is achieved.
The reamer makes a hole at least 50% larger than the outer diameter of the product pipe to accommodate pulling grips, facilitate spoil removal and to prevent excessive pulling force.
Trenchlesspedia explains Back Reamer
Back reaming is used in most HDD projects except for installation of very small diameter pipes. For up to 4-inch nominal diameter, it is possible to carry out the reaming and pullback of pipe simultaneously. A larger tool than the drill head is attached to the drill string after taking off the drill head. To reduce torque and thrust loads on the HDD, larger pipes are reamed in multiple passes. It also compacts the walls of the borehole, reduces the chances of voids, settlement and drilling fluid frac out at the surface. Back reaming prepares the borehole for installing the product pipe.
Before pull-back, a swab pass, i.e. a final reaming pass, is made using the reamer size that will be used for pullback. The swab pass helps in cleaning the borehole of fine gravel and clay, while compacting the walls - this process is known as "swab the hole."