What is Directional Boring?


Directional boring, commonly called horizontal directional drilling or HDD, utilizes a surface launched drilling rig and electronic locating devices to steer a drilling tool along a prescribed path consisting of a shallow arc. It is a trenchless method for installing infrastructure such as telecommunications, power, water, sewer, gas, oil, product pipelines, and environmental remediation casings. HDD is suitable for a variety of soil conditions and because it eliminates the need to dig a trench, it falls under the broad industry headings of “trenchless technology” and “no-dig” underground construction.

The video below would give you a basic understanding of how it works.
Press the Play button to view it.

HDD is preferred when installing conduits or casing infrastructure under waterways, roadways, tarmacs, near shore-lines, congested areas, environmentally sensitive areas, and areas where other methods are cost prohibitive. Installation lengths up to 6,500’ (2,000 m) have been completed, and diameters up to 56" (142 cm) have been installed in shorter runs. Pipes pulled through the drilled hole can be made of materials such as PVC, polyethylene, Ductile iron, and steel.


The HDD method is comprised of a two or three stage process wherein the first stage consists of drilling a pilot hole on the designated path approximately 3-9” (7.6-22.9 cm) in diameter. The second stage enlarges the pilot hole by pulling a cutting tool, also known as a hole-opener or reamer from the exit end of the tunnel, back to the machine. This stage is also known as pre-reaming.

The third stage places the product or casing pipe into the tunnel. If information about the product’s pull force and annular mud pressure are required, a tension monitoring device is installed between the reamer and the product.

Drilling Process

The HDD machine and drilling tool have directional control capabilities that assist the rig operator to make necessary changes in the directions of the drilling tool.

As the tool is pushed and rotated through the ground, pieces of hollow steel pipe are threaded together resulting in what is called a drill string.

Drilling Fluid

The horizontal directional drilling procedure is done with the help of a viscous fluid known as drilling fluid or mud.

The mud recipe varies with soil conditions. It is usually a mixture of water and bentonite or polymers that are continuously pumped to the tool through the drill string. It facilitates the removal of cuttings, stabilizes the bore hole, cools the tool, and lubricates the passage of the pipe. It is used on both the pilot hole bore and during the reaming and product pull-back.


Location and guidance of the drilling tool is a very important part of the HDD operation, as the tool is under the ground and not visible.

A typical HDD locating system consists of three things a transmitter (sonde, beacon, or probe), a receiver (hand held locator), and a remote display for the drill operator. The transmitter is positioned in a housing behind the tool; it registers angle, rotation, direction, and temperature data. This information is encoded onto an electro-magnetic signal that is detected and decoded by the hand held locator at the surface.

The receiver sends the information from the transmitter to the machine operator’s remote display.

A non-typical locating/guidance system is called a wire-line system. All of the transmitter’s data is sent to the drill operator through a wire that is fitted within the drill string. Both systems have their own merits. A particular system is chosen depending upon the site conditions.

DCI designs and builds locating/guidance systems of both types as well as the TensiTrak tension monitoring device, call us for more information.