Trenchless technologies, method 1: Horizontal directional drilling

March 29, 2017 Keivan Rafie

Understanding how and when to use Horizontal Directional Drilling (Part 2 in a series)


In the civil engineering world, finding safer and faster methods to complete the projects with minimum impact on environment and public is the focus for most designers and contractors. I believe Horizontal Directional Drilling (HDD) is one of the most interesting technologies that let us install pipelines, sewer, water mains, power and telecom through steerable and trenchless method where conventional open-cut techniques are not feasible, such as crossing the major highway or a river for utility installation in order to minimize public impact and protect the environment.

So, how does it work? HDD equipment make a horizontal bore hole from one location to another. The process begins with boring a small pilot hole along the desired centerline of a proposed profile under the crossing obstacle (e.g. a highway) with a continuous string of steel drill rod. The position of the drill head can be accurately controlled with devices and sensors that monitor and track the drill head. without directional capabilities, HDD could never have become the primary trenchless construction method for installing underground utilities that it is today. Through small adjustments made by the operator, the drill head can go around trees, under foundations, buried utilities, lakes, or other obstructions.

When the bore head and rod emerge on the opposite side of the crossing, usually a special cutter, called a back reamer, is attached, and pulled back through the pilot hole. The reamer bores out the pilot hole so that the pipe can be pulled through. The pipe is usually pulled through from the side of the crossing opposite the drill rig.

Drawing showing how HDD is done


One of the important aspects in the design of the drill-path is creating as large a radius of curvature as possible within the limits of project. The drill-path normally has curvature along its vertical profile. Curvature requirements are dependent on site geometry (crossing length, required depth to provide safe cover, staging site location, etc.). The degree of curvature is also limited by the bending radius of the drill rod and the pipe.

The reaming operation usually consists of using an appropriate tool to open the pilot hole to a slightly larger diameter than the carrier pipe. Normal over-sizing may be from 1.2 to 1.5 times the outside diameter of the carrier pipe. Several reaming passes may be needed for large-diameter pipes which require large boreholes that cannot be created in a single pass. Usually a “drilling mud” such as fluid bentonite clay is injected into the bore while cutting and reaming to stabilize the hole and remove soil cuttings.

A good design practice for HDD pipes in my opinion, should consider, all temporary construction loads and long term service loads. An experienced team should assess pipe properties to ensure smooth installation and operation. Design team must check and determine the competence of pipe in terms of its structural performance.

Here are few extra tips on HDD method:

  • Select the crossing route to keep it to the shortest reasonable distance.
  • Find routes and sites where the pipeline can be constructed in one continuous length.
  • The HDD process takes very little working space versus other methods.
  • Long crossings with large diameter pipe need bigger, more powerful equipment.
  • On the pipe side of the crossing, sufficient temporary space should be rented to allow fusing and joining the carrier pipe if required.

Interested to learn more about HDD? I recommend reading these articles…

  • Trenchless Excavation by Horizontal Directional Drilling (HDD) - Mr. SEIT Kin Fun, Raymond
  • Handbook of PE Pipe (Second Edition)
  • Horizontal Directional Drilling Guide (2015)
  • Planning Horizontal Directional Drilling for Pipeline Construction [CAPP Publication 2004-0022]

About the Author

Keivan Rafie

Keivan Rafie’s engineering expertise is in tunneling, mining, and ground-improvement projects. For the last 15 years, he’s worked across the globe in Asia, Australia, the Middle East, and North America. Keivan’s knowledge and experience go deep with tunneling methods, such as drill and blast, NATM, and TBM, and he is involved in all stages from design to manufacturing and construction.

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