A Step-by-Step Guide to HDD

By Denise Sullivan
Published: June 26, 2018 | Last updated: July 20, 2023
Key Takeaways

Horizontal directional drilling requires pre-site planning. Once planning is complete, workers drill a pilot hole, ream it to the appropriate diameter and then remove the pipe string.

Horizontal directional drilling (HDD) is a conventional method employed by cities and construction workers to install utilities and plumbing underground. Unlike traditional underground installation, HDD does not require open trenches to complete.


Equipment Needed for HDD

As horizontal directional drilling requires both entry and exit sides, there are multiple pieces of equipment necessary depending on the side. For the entry point, side workers need:

  • Power units and generators
  • Water pump
  • Drill mud supply
  • Mud mixing tank
  • Mud pump and drill pipe racks
  • Drill pipes
  • The rig itself

If mud is required, mud handling and a cleaning system are necessary.


On the exit point, side workers need:

  • Cutting settlement tanks
  • Exit mud containment tanks
  • Rollers and pipeline handling equipment
  • Side booms
  • Pipe racks
  • Product pipes


Free Download: An In-Depth Look At the Horizontal Directional Drilling Process


Other equipment necessary include pipelines, coating, welding, and testing equipment.

Horizontal Drilling Rig Components

As for the rig itself, there are a couple of components that are solely dependent upon the overall boring plan. These are the drill rod and drill bits. Drilling rods, also known as drill stems, come in a variety of lengths most commonly 3.0, 4.6, and 9.1 meters. The segments have female and male threading on opposite ends to allow for the attachment one to another.


Overall rod length is calculated based on the entry and exit angles in degrees, depth in meters, length of an obstacle to be circumvented, and curve radius in meters for both the entry and exit point. Calculations must be considered carefully as exceeding the bend radius can cause damage to the rod and eventual failure. Failure results in additional costs for replacement and downtime of the machine.

The type of rock workers are drilling determines the choice of the drill head. For soft ground, such as clay, soft limestone, shale, unconsolidated sand, and red bed, a drag or fixed cutter bit is best. These bits are a single solid piece that rotates with the drill string. There are no bearings and workers can use either cutting fluid or air to remove broken ground.

For medium or hard ground, such as limestone, calcites, cherty limestone, hard shale, mudstone, or dolomites, a three-cone rolling cutter is best. The shape, angle and material determine their use. Bits with long widely-spaced teeth are for the medium ground while shorter, tightly spaced teeth are made to break up hard soil. Bits for hard and medium ground use drilling fluid to excavate rock chips. (Learn more in “The Right Drill Bit for Soft, Medium & Hard Ground Conditions.”)

Steps Required for HDD

For horizontal directional drilling, there are four necessary steps to complete. To begin there is the pre-site planning. The planning is followed by drilling the pilothole, expansion of the shaft via reaming, and then pullback of the pipe string.

Pre-Site Planning

Pre-site planning begins with a geotechnical report. This report includes an examination of past geological surveys. In addition to reviewing historical information, surveyors take samples of the ground at random intervals.

Surveyors may collect these samples via hand auger or drilling machine. The soil samples are then sent for analysis, which tells the location, elevation, depth of the example. The report returns with indications of soil types encountered at each depth.

In addition to soil classification, the geotechnical report indicates the soil strength and any groundwater conditions. Engineers use this information to plan for additional drainage, and possible uplift pressure or foundation seepage that may occur.

Once the geotechnical report is complete, engineers determine the entry and exit points for the HDD rig. They use the report data to map the drill path. After planning the route, the project planners refine entry and exit points.

Drilling the Pilot Hole

After all the pre-site planning is complete, it is time to bring the equipment to the site and set it up accordingly. The geotechnical report indicates the bearing capacity of the soil, so workers are guaranteed not to bring machinery too heavy for the site. The survey results also help engineers in the selection of the appropriate bits and rod length for the job. (See “Choosing Drill Rods for Trenchless Tunneling.”)

Once the equipment is in place, workers drill a pilot hole along the predetermined path. A probe situated close to the bit sends readings back to the controller periodically. These readings indicate the vertical and horizontal coordinates a long the hole. These readings are in relation to the initial entry point. Operators use them to ensure they are staying on the path and avoid deviation. (Read “Working With Drilling Deviations.”)

Often, while drilling the pilot hole, workers will inject drilling fluid into the hole. This fluid helps to provide stability to the borehole and transport drill cuttings out. It also helps to clean the build up on the drill and cool down the bit while reducing friction between drill and the wall.

Expansion of the Hole

Once the drill bit pierces the exit location ground, the downhole, workers remove the downhole assembly. They then attach a back reamer to the drill string. The string gets pulled back through the borehole, enlarging the diameter.

To reach the desired diameter, workers may have to make additional passes through the line with the reamer.

Not all pilot holes need expansion. Smaller diameter pipelines do not use reamers. This is due to the pilot hole being an adequate size to pull the pipe string back through.

The Pullback of Pipe String

The pipe string is the drag section, which is slightly longer than the drill length. The line is pulled over rollers into the exit hole and pulled back to the rig until the entire pipe string has moved through the borehole. Often this piece is connected to the reamer.

The external coating on the pipe string is visible and allows workers to inspect the line for damage upon pullback completion. Workers then complete internal inspection to ensure there was no damage to the pipeline during retraction.

Once the pipeline is confirmed successful, equipment is demobilized and dismantled.

Horizontal directional drilling projects require careful planning. Geotechnical reports allow planners to determine what drill bits to use, how long rods need to be, and the site can manage heavy equipment. Once a plan is in place, workers can drill the pilot hole and expand it for pipeline use. (See “The Process of Borehole Expansion.”

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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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