Choosing Drill Rods for Trenchless Tunneling

By Denise Sullivan
Published: December 12, 2017 | Last updated: July 20, 2023
Key Takeaways

For trenchless projects, managers select one of three main steel rod categories, A-series, D-series, or O-series. Each is used for different projects.

Drill rods are one of the most critical components of horizontal directional drilling platforms. It is up to the construction designers and project managers to determine which hardened steel rod works best for their project. Along with the project goal, managers must take into consideration the steel grade, rod length, and rod diameter to choose the right tools.


Steel Grade of Drill Rods

All drill rods are made of hardened steel. However, depending on the working conditions and needs, the steel mixture and the hardening method vary. There are several different series of drill rods. However, for trenchless projects, managers select one of three main categories, A-series, D-series, or O-series.

A-series rods are made of air-hardened material that contains five percent chromium. The addition of the chromium coupled with the air hardening method provides this series with greater water-resistance and less distortion than comparable tools.


D-series drill rods are also air-hardened tools. However, unlike the A-series, this device has a higher concentration of chromium. Due to the higher chromium concentration, this series has a higher wear resistance and seldom changes shape after repeated usage, if it is well maintained.

O-series rods are oil-hardened. Unlike the A and D-series tools, this rod is hardened at a much lower temperature, making it more resistant to deformation over time. As with the D-series, the O-series also contains a high amount of chromium, giving it a wear resistant quality.

Length of Drill Rods

In general, drill rods come in lengths of 3, 4.6 and 9.1-meters. Each section has male and female threading on opposite ends, which allow for the attachment of one to another. The total length is dependent on the project design plans and obstacles in which they may have to drill around. For large jobs, it is not uncommon for some HDD platforms to carry up to 305-meters of the rod.

When calculating overall length, project managers consider the entry and exit angle in degrees, the curve radius in meters for both the entry and exit points, the depth in meters, and the length of the obstacle in meters.

In addition to calculating the total length, the bend radius of the drill rod is also calculated. Essentially, the bend radius is the forward distance a drill string must make to make a 90-degree turn. The exact calculation to determine the bend radius of a rod varies from manufacture to manufacture. However, exceeding the calculated variance damages the rod. While the rod may not fail immediately, it will eventually fail due to this damage causing costly replacement and downtime on future projects.


Diameter of Drill Rods

As with length, the drill rod diameter size chosen by the project manager is decided based on the overall scope of the project. Mini-horizontal directional drilling will take a much smaller diameter size than a standard HDD project. Managers must factor in diameter of the pipe when calculating the needed length of the rod. The width affects the bend radius and is part of the consideration.

Traditional trenchless projects which employ drill rods find them ranging from 2 to 5-inches in diameter. Mini-HDD rigs may use much smaller diameter starting at just 2.3-inches (6cm). Some horizontal directional drilling rigs can handle drill rods larger than 36-inches. These “maxi” rigs are equipped to use rods around 6 5/8-inches.

Additionally, the shape of the rod affects the outer diameter calculation. While not often considered, employing square or rectangular drill rods might benefit the project more significant than a more massive diameter traditional round rod. Planners should carefully weigh the benefits of going with a non-traditional shape.

When it comes to determining the best drill rod for a trenchless project, the planners and managers have several factors they must consider. The grade of steel is only the beginning when choosing the proper equipment. Performing the mathematical calculations to determine the bend radius to select the best length and diameter is tedious as an error can cause problems on future jobs. Additionally, deciding not only how broad of a bore needed to lay the new line is just part of the diameter question. Managers must also determine if they want to go with a traditional round rod or try a square on instead.

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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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