Reamers are cutting tools attached to the cutting head. The dimension of the reamer must be such that it accommodates the final pipeline installed in the borehole. Many times, during borehole construction reaming is a better choice than boring the hole to size. How is this process best used in trenchless construction and rehabilitation?

What Is Reaming?

Reaming is the process of enlarging a hole. For trenchless projects, reaming occurs after workers bore a pilot hole using an auger and appropriate cutting head. As a rule, the borehole size should be 1.5 times the outside diameter of the installed pipe. Reaming the hole to the proper size is often faster and more consistent than attempting to drill the tunnel to the proper size. (Learn more in The Process of Borehole Expansion.)

Workers can ream a hole either through forward or backward motion. For forward reaming, the reamer attaches to the drill rod on the entry side of the borehole. It then rotates through, going in a forward motion to the endpoint. In back reaming, the reamer is attached on the exit side and pulled back through the borehole to the entry point.

Despite the direction, reaming a hole to a much larger size is completed incrementally. Workers may have to make several passes to enlarge the hole to the appropriate size, using slightly larger reamers with each pass. However, back reaming will make a larger hole than forward reaming can achieve.

What Are the Different Types of Reamers?

There are several different types of reamers available. The type of reamer utilized is dependent on project needs and soil condition.

Compaction Reamers

Sandy soil is problematic for boring projects. The soft soil is likely to collapse in on itself and typically needs a support mechanism of some sort. To ream boreholes made in this type of ground, compaction reamers are the optimal choice. This reamer design relies on a bentonite filter cake to force drilling fluid into the surrounding soil. The injection of slurry helps to provide much-needed support to the hole and prevent collapse.

These cutting heads are generally a cone shape and increase gradually in size. The tapered design forces the slurry mixture into the surrounding soil.

Hole Openers

Hole openers are, as the name implies, used to enlarge pilot holes for large applications. These cutters work with ground formations that cannot be penetrated by standard reamers. They use rolling cutters for reaming rock formations.

These reamers often require several passes to open the hole safely. Each pass enlarges the hole slightly more. To accomplish this without damaging the reamer, it is necessary for workers to stabilize each consecutive size.

Mixing Reamers

When reaming a borehole in clay or harder ground conditions, it is best to use a mixing reamer. These reamer types include rippers, fly cutters and beaver tails. They help break up soil particles and mix them in with drilling fluids to move the cuttings out of the hole. Due to the tendency of clay and water to clog the bore, it is essential that workers use a particular type of drilling fluid with these reamers.

While all three mixing reamer types work well in harder soil conditions, the fly cutter is best used in situations that are more compacted that others. A fly cutter’s open design allows larger rocks to pass through the reamer as it moves through the hole. For this reason, the fly cutter is often used for sandstone and siltstone over other options.

When Is Reaming Used?

Workers use reaming in any boring project where the hole needs to be larger than the bore. Generally, these projects are completed in soil where boring a large hole to begin with may cause it to become unstable before completion. By reaming out the hole gradually, workers can help maintain the integrity of the hole while adjusting it to the proper size.

Projects which use horizontal directional drilling often use reaming to expand the hole after the initial pilot hole is complete. These projects use a pre-reaming process to enlarge the hole before installing the pipe. The reamer in these instances is attached at the exit point and is pulled back through expanding the tunnel. On the other end of the reamer, workers connect the drill pipe so that as the reamer enlarges the hole, the pipeline pulls into place, helping to maintain the integrity of the line.

Drillers working on oil wells use reaming as part of their process. In these situations, some may still drill a pilot hole to offer directional control during drilling. However, these projects often employ reaming while drilling, helping keep the overall cost of the job down. These units use special reamers known as underreamers which run in conjunction with the drill bit.

Workers can expand or contract the cutters to enlarge sections of the borehole before casing installation. (For more details on equipment, see A Complete Guide to the Usage and History of Drill Bits and Tooling.)

In some cases, workers use reamers as a method to help replace existing pipelines with new ones. In pipe reaming a reamer is attached to an HDD machine and is pushed through the old pipe. As it moves through, the old line is broken apart and removed at the exit point with the drilling fluid. The machine pulls a new line into a place where the old once was.

There are many reasons project managers choose to use reaming in their borehole construction. The time it takes to enlarge the hole is dramatically reduced through reaming as compared to drilling.

It also offers a better consistency and can help to stabilize the hole with the use of the correct drilling fluids.