What is Horizontal Directional Drilling?

By Phil Kendon
Published: March 16, 2021 | Last updated: July 5, 2023
Key Takeaways

Horizontal directional drilling is a process of boring a horizontal hole for the installation of new pipes without the use of conventional trenching.

What is Horizontal Directional Drilling?

Horizontal directional drilling (HDD) has come a long way since it first came onto the trenchless scene in the 1970s. It offers a curved trajectory instead of using point to point solutions in a straight line. As such, HDD opened up the possibility for laying pipes in utilities under roads, railways, rivers and congested areas.


The technology around HDD continues to develop, with advances in steering and mud systems being instrumental in broadening the use of this trenchless method.

How Does HDD Work?

HDD lays underground pipes or utilities by drilling a bore along the planned trajectory. There usually are three stages involved – drilling the pilot hole, reaming and laying the casing, and finally pulling the pipe into position. There are some variations on this sequence depending on the project requirements, but most HDD installations follow this approach.


Required Equipment for HDD

The drilling rig is the main element of an HDD setup. Drilling rods are added one segment at a time as the drilling head progresses through the ground. A drilling head attached to the front of the drill string cuts through the soil and drops the cuttings into the tunnel. There are various drilling heads designed for different ground conditions.

Most HDD projects rely on a drilling mud system. This bentonite mixture seals the tunnel walls and prevents subsidence (sinking or settling). It also carries the cuttings out of the tunnel and provides lubrication and cooling to the drilling head.

Step 1

Step 1 in an HDD project is the drilling of a pilot bore. The purpose of the pilot bore is to ensure that the pipe follows the design trajectory. The pilot bore is relatively small and easier to drill, allowing contractors to discover any issues with the ground condition so that they are well-prepared to pull the pipe into position.

Step 2

Step 2 is to ream the pilot bore and increase the bore size to fit the product pipe. Reaming may take several passes, each one bigger than the last until the final size is reached. A reaming tool fits onto the end of the drill string in place of the drilling head. Drilling mud plays a significant role in removing the cuttings from the tunnel and maintaining the stability of the walls. (Read also: Why Reamers Are Important to Trenchless Boring.)

Step 3

Step 3 of HDD is the laying of the pipe. A pulling head and swivel replace the drilling head at the front of the drill string. Piping is often pulled back into the bore from the destination site, which takes a great deal of force and places significant tensions on the pipe. One of the problems that can occur here is a stuck pipe, where the pipe refuses to move any further down the tunnel.


The History and Development of HDD

Martin Cherrington is widely acknowledged to be the father of HDD and the pioneer of this trenchless construction method. The first seeds of the idea were planted when Martin observed a gas line installation using a handheld air drill in the 1960s. With his general mechanical background, Martin began to work on something much bigger.

Having started with road boring for utility companies, Cherrington was approached to attempt a river crossing in 1971. He was successful and laid 500 feet of a 4-inch gas line underneath the Pajaro River in California. News of this spread through the industrial construction world, and HDD became an established and sought after method.

One of the challenges of the early days of HDD was steering the drill. Crews would dig potholes along the route to check the depth and direction of the drilling head. This technique was cumbersome and invasive, which led to the development of more sophisticated guidance systems. (Read also: The Evolution of Horizontal Directional Drilling.)

These days the most commonly used guidance system for HDD uses walkover electronic tracking devices. A transmitter on the drilling head sends information to the surface. Operators standing directly above the drilling head receive the information on a handheld receiver. This realtime feedback allows the operator to make adjustments to the drilling head to keep the bore on track.

Other steering options for HDD include wireline steering and gyro steering. Wirelines involve a direct connection with the drilling head by a cable that runs back to the operator. Gyro steering tools are also mounted at the drilling head and are not susceptible to magnetic interference.

Recent developments in HDD technology have improved the installation of intakes for desalination plants. Subsea intakes have lower environmental impacts and deliver a better quality of water to the plants. HDD allows engineers to plan the inlet pipe trajectory, lay a sleeve into the pipe, then insert the intake pipe and remove the casing.

Drilling Mud and HDD

Drilling mud is a vital part of most HDD installations. It is pumped into the bore via the drill string and carries cuttings back out of the tunnel. There are some important factors to bear in mind when using drilling mud:

  • What is the best mud composition for the ground conditions?
  • What volume and pressure drilling mud do you need?
  • Is there an environmentally safe disposal point for used drilling mud nearby?

Besides the drilling mud composition, there is also specialized equipment required for using the product. Screens, hydro cyclones, tanks and pumps are all part of a drilling mud system.

HDD Limitations

Most of the disadvantages of HDD are related to the use of drilling mud and ground conditions. Some ground conditions have loose gravel or soil, which makes it difficult to seal the bore. Under these conditions, it may be better to install a casing followed by the product pipe.

Drilling mud can also burst through the tunnel into the surrounding soil because of high pressure or incorrect drilling mud composition. This failure can cause subsidence and environmental damage, leading to delays in the project while rectifying the situation.

Benefits of HDD

HDD installations do not require entry and exit pits. The piping is installed from the surface at an angle into the ground. This access means no construction of entrance and exit pits, but it may still require a large footprint for the run-up of pipe into the bore. (Read also: It's the Pits: Pits and Excavations in a Trenchless Project.)

A significant benefit of HDD is the ability to follow curved trajectories. Older trenchless construction methods like auger boring can only follow a straight line. Curved trajectories allow piping to dip beneath a river, rail or road and then rise again on the other side. There are no underground elbows to worry about and no right-angle changes in the flow path. The development of HDPE piping has also contributed to this benefit because the piping is flexible enough to follow a curved trajectory, unlike steel piping.

Case Studies and Safety

A project in St. Albert, Canada, highlights the speed and efficiency of HDD installations. The contractor installed three parallel sewers under the Sturgeon River in just four days. There were two 16 inch and one 26 inch pipes, each of which traversed 420 feet underground.

As with any trenchless construction method, one of the risks is colliding with existing underground infrastructure. Drilling through power lines, gas lines, or other utilities could lead to disastrous consequences. In one incident, a restaurant worked was killed when an HDD rig struck a natural gas line close to the restaurant. Another incident also resulted in a fatality when contractors tried to pull a stuck pipe out of the bore. Safety awareness and training are critical for the safe operation of HDD rigs.


HDD is a versatile and well-established trenchless construction method. It is especially suitable for curved trajectories under rivers and other obstacles.

Drilling mud systems are crucial to the success of HDD installations and should be well-planned and engineered as part of the project preparation. Technology developments in HDD steering are also improving the accuracy of HDD installations compared to the steering options available when the method was first launched.

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Written by Phil Kendon | Technical Writer @ Trenchlesspedia

Phil Kendon

Phil Kendon has an undergraduate degree in engineering along with a masters in vocational practice. He has ten years of manufacturing experience in the oil and gas sector along with ten years of experience with non profits. Phil lives on the idyllic paradise island of Mauritius with his wife, Leigh, and 3 children, Timothy, Hannah and Luke. Here he pursues his work with non profits as well as his passion for writing.

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