Why Reamers Are Important to Trenchless Boring, Plus Available Types of Reamers

By Denise Sullivan
Published: January 28, 2021 | Last updated: July 5, 2023
Key Takeaways

Many trenchless drilling projects utilize reamers to help enlarge the pilot borehole. With proper reamer and fluid use, this method can help to stabilize the newly created hole.

Boring a hole is only the first step in a trenchless project. After the initial dig is complete, the hole must be enlarged to meet the project's demands. At this stage, reamer tools come into use. By understanding the differences and when to use reaming, project managers can ensure success.

What is the Difference between Boring and Reaming?

Boring and reaming may sound like the same thing. However, the words are not interchangeable. While some projects require both methods for completion, they are different tasks entirely.


When boring in the earth, workers create a hole, well, or tunnel. The process dates back to the Han dynasty in China. Workers used the boreholes to complete mining projects that were as deep as 600 m. Since then, workers use boreholes for mineral exploration and geological surveys, hydrology, and civil engineering.

While many fields use boring to make holes in surfaces, in earth-boring, the purpose is to better understand the soil composure before the project starts. However, it is sometimes used to create large openings for wells and to lay pipelines.


Reaming, on the other hand, occurs after workers bore the hole. It is the process of enlarging an existing hole to accommodate the new pipe to run through it. As the new opening needs to be approximately 1.5 times the new line's outside diameter, reaming is a regular part of any underground drilling project.

Workers can ream a hole either in a forward or backward direction. Back reaming will create a larger hole and may be used if the project enlarges the boring hole significantly. The placement of the reamer is dependent on which direction the enlargement goes.

The reaming tool attaches to the drill rod on the entry side and moves forward for forward reaming (also called push reaming.) With back reaming, the device is connected on the exit side and pulled back through the hole.

When Is It Necessary to Ream?

Some jobs occur in soil which may become unstable if the hole is a wide bore. With the danger of the hole collapsing, workers must remove the dirt gradually. They can then stabilize the hole as it is reamed to keep it from falling in on itself. Workers use a reamer in these situations. (Read also: Understanding the Importance of Wellbore Stability.)

Likewise, in pipe replacement projects, reaming may be a necessity. The existing pipe is broken up by attaching a pipe reamer to a horizontal directional drilling (HDD) machine, allowing a new line to easily slide into place behind the reamer. Workers remove the broken pieces of the previous pipe at the exit point along with drilling fluid.

Oilwell drillers frequently use reamers when tapping a new oil source. However, unlike other applications, they don't drill the pilot hole and then return with a reamer.

Instead, these drillers still use a pilot hole drill bit to better direct the path. Behind the drill bit is a special reamer known as an underreamer. The tool allows drilling workers to dig and ream the well in one pass, saving time and money.

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.

What Do You Need to Consider When Choosing the Right Reamer?

Before beginning a trenchless project, project managers need to determine which reamers are appropriate for the job. To do so, they must consider the ground condition, cutting action, rig size, and drilling fluid volume.

Ground Conditions

Before beginning the project, managers should sample the ground to determine what the composition is. Different types of soil require different types of reamers. Clay and other sticky soil will require enough fluid pressure to keep from leaving large chunks behind. Softer ground, such as sand, needs a packing reamer that mixes the cuttings and drill fluid to create the borehole wall's natural support. (Read also: The Right Drill Bit for Ground Conditions.)

Cutting Action

Cutting action is also dependent on ground conditions. More rigid substrates, such as rocky conditions, need a reamer with a cutting action that pulverizes as much as cuts. Cobble substrates need a tapered reamer that will push the smaller rocks out of the way, not necessarily crush them up. Limestone requires a reamer that fractures and grinds. Sand and clay, on the other hand, both need multiple reamer types to be successful.

Rig Size

The rig size considerations are not just the cutting diameter and shaft size, although that is important. Workers and planners should be cautious not to put a reaming tool on a rig with too much power for the instrument. For example, a 20,000-pound reaming device should not be placed on a rig with a 40,000-pound pull back. To do so would damage the rig and the reamer.

Planners should obtain reaming bits that are strong enough for the rig employed at the site. If it is necessary to mix sizes, it is better to put a larger bit on a smaller rig than vice versa. However, it is strongly encouraged that those in charge use the appropriate size tool for the rig.

Drilling Fluid Volume

Appropriate drilling fluid volume is important to creating a successful hole. If there is not enough drilling fluid, the cuttings will not mix with the fluid for adequate removal. Should workers try to pull the reamer out of the hole without enough fluid, it could cause the cuttings to clump and form a blockage. (Read also: An In-Depth Look at the Role of Drilling Fluid in Trenchless Construction.)

Inappropriate rig and reamer size can cause an issue with drill fluid volume. If workers use a large with a reamer meant for a small one, the fluid holes would not be large enough to allow the appropriate amount of fluid out. Likewise, if the reamer tool is meant for a larger rig and is used on a smaller rig, large holes on the reamer will allow the fluid to come out at the wrong pressure. This would cause cuttings to ball up and potentially jam the rig.

Final Thoughts

Reamers are an essential part of almost all trenchless boring projects. They help to enlarge the boring hole to the appropriate size in increments to keep it from collapsing. However, project managers should be careful about their type and size choices to ensure that they use the right tools.

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Written by Denise Sullivan | Technical Writer @ Trenchlesspedia

Denise Sullivan

Denise Sullivan is an accomplished freelance writer from Louisiana, with a Associate's Degree in Journalism from Eastern Oklahoma State College. She also graduated from East Central University with a Bachelor's in Biology. Denise began her writing career writing operations and maintenance manuals and software utility manuals for flight simulators. Since, she has expanded her writing to a broad spectrum of topics.

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