Horizontal Directional Drilling (HDD)

Definition - What does Horizontal Directional Drilling (HDD) mean?

Horizontal directional drilling (HDD) is a basic trenchless technology that involves drilling into the earth to create a horizontal bore under the surface along a planned pathway through which pipes and conduits may pass. Once the HDD creates a bore of suitable size - which may require one or multiple passes by the drilling apparatus - the conduit or pipe is pulled into the bore and connections are made to the appropriate utilities.

Trenchlesspedia explains Horizontal Directional Drilling (HDD)

When the installation of underground utilities by trenching cannot be accommodated, the other option is to bore or microtunnel beneath the surface. Horizontal directional drilling is ideal for situations in which trenching isn’t practical. Perhaps the utilities must be installed under a roadway where traffic interruptions are impossible, such as an interstate highway. HDD may be the most cost-effective way to construct utility installations that cross beneath lakes and rivers. It may be done with a drill or an auger, depending on the type of material the bore must pass through. It can also provide the most effective means of installing a pipe or conduit that requires support from bedrock.


HDD may require not only a drill, but mud pumps, mud reclaimers, mixers and other support equipment. Or, it may involve a small crew and minimal equipment, depending on the extent of drilling activities.


HDD is a three-step process. After a receiving hole and entrance pit are dug to allow for collection and reclamation of drilling mud, the first step is the drilling of a pilot bore along a planned pathway.


The second step is known as reaming. The drill, equipped with a back reamer, passes through the pilot hole again, enlarging the pilot hole. The back reamers come in a variety of sizes and the reamer is chosen based on the size of the pipe or conduit that will be pulled through the bore. The driller can further increase the diameter of the reamer to create a custom fit in the bore for the diameter of the pipe/conduit. This allows the driller to operate in the most productive and cost-effective manner.

In the third step of HDD, the crew inserts the pipe or conduit into the bore, using the drill stem. The pipe/conduit is then pulled into the bore by the reamer, which centers the pipe in the bore.

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