Horizontal Directional Drilling: 7 Key Facts to Know

By Tabitha Mishra
Published: September 13, 2017 | Last updated: July 5, 2023
Key Takeaways

Technical knowledge of the pilot hole curvature, reamer size, drilling fluid viscosity and the workspace needed is paramount when working on an HDD project.

Horizontal directional drilling (HDD) is becoming widely accepted in the construction industry as a convenient way to get a pipeline drilling job done with minimum disruption to everyday life. It has gained interest mainly due to the fact that HDD projects do not involve digging up entire stretches of roads, though the entry and exit points have to be dug and cordoned off.


The rapid development of the trenchless industry can be chiefly attributed to the need for lesser environmental impact, especially when pipelines have to be laid in places that cradle plant, animal, and delicate aquatic life. (Read on in "Why the Oil and Gas Pipeline Industries are Eyeing HDD.")

While HDD is not new in itself, its foray into the field of laying sewer, and water pipelines, and replacement or renewal of old pipelines is becoming popular only now. In fact it is still in its infancy. More contractors are now realizing the benefits of HDD. Here we will take a look at some key facts related to HDD that everyone dealing with, or planning to deal with such projects needs to know.


Horizontal Directional Drilling in Brief

The HDD process involves drilling a pilot hole in the horizontal direction towards the other end of the planned route. This hole is dug using a steel drilling rod. When the drill head emerges on the other end, a back reamer is attached to the string and pulled back through the hole. This reamer is slightly bigger than the diameter of the pipe. Once the reamer is out, the pipe is pulled through and installed.

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Below are some key facts to consider when undertaking an HDD project:

Selection of Drill Route

The route for carrying out an HDD project should be the shortest and straightest possible, so that the pipeline can be installed in a continuous segment. Proper geotechnical investigation should be carried out to study the soil type. This will help in proper designing of the crossing route and will help come up with the most efficient and cost effective operating procedure including appropriate tools to be used. (Learn more in Why a Detailed Geotechnical Report Means Success for Your Trenchless Project.")

The entry and exit points should not be at a difference of elevation of more than 50-feet and should be located within eye range clear of above ground obstructions. All buried structures within 10 to 25-feet (depending on mini or maxi HDD application) of the route, should be located to avoid cross boring.


Work Space for Entry and Exit Points

While it is true that HDD takes up lesser working space, sufficient space should be provided depending upon the size of the project undertaken. A project involving larger pipes will require greater quantity of drilling fluid, larger pumps, mud storage equipment, etc. For example: For a Maxi HDD project requiring a 1000-feet crossing may also require a space of (100×150) feet. On the exit point, sufficient space should be provided so that joining and fusing pipes can be done in a continuous string.

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

Pilot Hole Curvature

While designing the drill path, one of the most important factors to consider is the radius of curvature. Since curvature induces bending stress, it should be kept to a minimum so that the pullback load is reduced. Since the pipe has to be pulled through the hole, too much curvature or too many curves can increase the tensile strength, thus reducing the pipes resistance to collapse. The entry and exit points should be chosen with care such that the maximum radius of curvature is attained in the direction of drilling.

Back Reamer Size

Once the pilot hole is drilled, it needs to be expanded to the appropriate size with sufficient margin to allow the pipe to be pulled in easily. This margin or oversize is about 1.2 to 1.5 times the pipe diameter and depends on factors like soil stability, hydrostatic pressure, depth of drilling, etc. Once the pipe is installed, it will have to withstand vertical earth pressure without immediate support from the surrounding soil. Usually another reamer of the same size is passed back again to remove remaining debris and to compact the wall of the borehole.

Viscosity and Distribution of Drilling Fluid

Drilling fluid consists of a mixture of bentonite and water that is used to lubricate the hole during drilling and reaming. It also acts as a cleansing and stabilizing agent that keeps the hole from collapsing and reduces torque while drilling.

It is essential to have the correct viscosity of the drilling fluid that will allow the cuttings to be suspended, and must be constantly monitored to ensure that the pumps are not overworked and that the fluid is evenly distributed throughout the borehole. Uneven distribution can cause failure of borehole or problems during pipe pullback.

Pipe Handling During Pullback

Pulling the pipe back through the pipe can be challenging because the pipe needs to be pulled in one segment without breakage. To ensure proper pulling, all procedures for fusion welding and inspection should be followed. The pipe should be handled with care by providing proper support to minimize bending. Other factors like exit and entry, rate of mud flow, circulation of drilling fluid, axial tension force, etc. should be recorded at constant intervals.

Safety and First Aid

Safety procedures are usually mentioned at the end of an article to keep it fresh in the mind of the reader. However; it should be the first undertaking at the beginning of any project. All safety procedures with regards to handling of machinery, equipment, chemicals, personal safety etc. should be followed to the letter. Not only will it provide a safe working environment, it keeps away the worry of work place accidents. First aid kit should always be kept handy for minor accidents that can be treated on site.

HDD is fast becoming an essential tool in the contractor’s tool box. Conventional methods of laying pipelines are slowly becoming obsolete thanks to trenchless technology’s no dig methods that reduce traffic congestion, and environmental impact on wetlands, estuaries, rivers, etc. However; it is essential for every contractor undertaking an HDD projectto be familiar with some key aspects as mentioned above to complete a trenchless project.

Employing a knowledgeable contractor and engineer who can follow the design parameters to the dot is essential to ensure successful completion of any project. (Read more in "7 Types of Trenchless Rehabilitation Methods and How They Are Used.")

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Written by Tabitha Mishra | Civil Engineer, Technical Content Writer

Tabitha Mishra

Tabitha has a Bachelors Degree in Civil Engineering from Mumbai University, India, and is currently freelancing as a technical content writer. Prior to writing, she has worked as a site engineer and site manager for various building construction, building rehabilitation, and real estate evaluation projects.

Tabitha is also certified as a Primavera project management professional and is well versed with Auto CAD. In her spare time, she does private consultation for small-sized home builders and assists with plans and permissions.

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