Horizontal directional drilling (HDD) is one of the most common methods used for installing piping, conduits or cables underground without digging open trenches. Trenchless methods of construction like HDD have become popular due to the low impact it imposes on buildings and commuters, as well as the environment, during construction.

The History of HDD

The idea behind horizontal directional drilling was birthed when Martin Cherrington observed the installation of a gas line using a hand held air drill in the 1960’s. Martin developed the concept of guided drilling eventually building a custom designed drilling rig and launching his own company.

HDD’s major breakthrough came in 1971 when Martin used his drilling rig to install a 4” gas line underneath the Pajaro River for Pacific Gas & Electric Company (PG&E). It was a 500-foot crossing, which took about a month to complete, but the success of this project opened the eyes of the construction industry to the potential of HDD.

Since the early 1980s, the technology for HDD has developed extensively. In Martin’s early years of development, they would have to dig “potholes” along the bore path to verify the depth and direction of the drill. Since then, sophisticated methods of measuring and controlling the HDD trajectory have made the technology more robust and efficient.

Basic Description of HDD

Horizontal directional drilling is a method of construction where a tunnel is drilled through the ground to enable the installation of piping, conduits or cables without digging trenches.

The first step in an HDD installation is to set up the launch site. A drilling rig is established on site, which supplies the rotation and thrust to the drill. Drilling rods connect the drilling rig to the drill bit, and additional lengths of drilling rod are added as the drill progresses through the bore. The drill bit is chosen based on the type of soil to give the most efficient progress possible as it cuts through the ground. (Learn more in "The Right Drill Bit for Soft, Medium & Hard Ground Conditions.")

Once the drill passes all the way through to the destination point, the first phase is complete and the next step is to enlarge the bore size by pulling a reamer back through the tunnel. The drill bit is removed from the end of the drill string and is replaced with a reaming tool. A number of reaming passes may be required to open the diameter of the bore to approximately 1.5 times the diameter of the product pipe. Once this is achieved, the product pipe can be installed.

The product pipe is pulled back through the bore in the same way as the reaming tool was used. A pulling head and swivel is now attached to the end of the drill string. The pipe is attached to the pulling head and slowly pulled back through the bore to complete the installation.

Key Components of HDD

Directional control of the HDD drill path is limited. An operator can adjust the direction of the bit by stopping the rotational movement and then thrusting the bit forward. The slanted face of the drill bit causes it to adjust its angle into the ground. Some HDD rigs also allow for a transmitter to be installed just behind the drill bit. A person holding a receiver above the ground over the drill bit’s location will be able to determine the position and direction of the drill. This information can be used by the operator to make minor adjustments to compensate for any deviation. (Read "A Complete Guide to the Usage and History of Drill Bits and Tooling.")

Drilling mud is pumped down the drill string and into the drilling bit, where it exits through holes in the bit. The purpose of the drilling mud is to keep the transmitter cool, stabilize the walls of the tunnel and remove the spoil. As it washes back up the hole to the surface, it removes the spoil from drilling along with it. Drilling mud is recycled by using screens and hydro cyclones to remove the spoil. It is processed back into a tank on the surface before being pumped into the drill string again. This is called reclaiming or recycling the mud.

Advantages & Disadvantages of HDD

HDD has clear advantages over traditional trenched methods of piping installation. It is possible to navigate underneath obstacles like roads, rivers and sensitive ecological areas without any disruption to the environment or users. Trajectories can also be more flexible with curves instead of being limited to straight lines.

The use of drilling fluid does bring some risks to the HDD process. It is possible for a blowout to occur, where the drilling fluid pushes through the tunnel walls and leaks into the surrounding soil and may lead to contamination of groundwater. Drilling fluid must also be disposed of in an environmentally acceptable manner once the project is complete.

A major advantage of HDD over other trenchless methods is that the operation is carried out from the surface without the need for entry and exit pits. The overall efficiency of this method makes it one of the quickest methods to install underground piping.

The HDD method cannot be used in gravel or loose soils. The particle size of the spoil in these cases is not conducive to being suspended in the drilling fluid, which is a critical requirement for HDD to be successful.

What Projects Are Suitable for HDD?

HDD is most suitable for installation of pipes underneath rivers, roads or sensitive ecological areas. The angle of the drill into the ground can be set by the drilling rig to create an arc that neatly navigates the desired path. (Learn more in "Why the Oil and Gas Pipeline Industries are Eyeing HDD.")

Improvements in the directional control of HDD rigs are continuously being developed. As the technology improves, this method will remain a popular choice for a growing number of applications. (Read on in "Horizontal Directional Drilling: 7 Key Facts to Know.")