What is horizontal directional drilling (HDD)?

Horizontal directional drilling, or HDD, is a way to put pipe, conduit or cable under the ground without digging a long open trench. A drilling rig sits at the surface, bores along a path the operator steers, and comes back up at a planned exit point. The utility line is then pulled back through the hole.

Because only the two ends of the bore are dug, the street, sidewalk, yard or creek between them stays where it is. That is the main reason HDD has become a standard tool for utility work in built-up areas.

HDD at a glance

The three steps of a bore

Every HDD job follows the same basic order.

1. The pilot hole. The rig drills a small-diameter hole along a planned path from one surface point to another. The drill head is angled so the operator can steer it up, down, left or right as the rods are pushed in.

2. Reaming. If the pipe or conduit is larger than the pilot hole, a reamer is pulled back through to widen the bore to the size needed. Small installations can sometimes skip this step.

3. Pullback. The product pipe or conduit is attached behind the reamer and pulled into the finished hole. When it is done, the new line runs underground and is only exposed at its two ends.

How the drill head is tracked

On most utility bores, a small transmitter called a sonde rides just behind the drill head. It reports data such as the head’s angle, rotation and direction. A crew member walks above the bore with a handheld receiver, reads that signal, and the operator steers from what the receiver shows. This is called a walkover system.

Modern locators, such as the DigiTrak Falcon F5 family made by Digital Control Inc., can also store a record of each rod drilled. That record becomes the starting point for the bore log and the as-built record after the job. We explain this in more detail in what a walkover locator does on a drilling job.

Why contractors choose HDD

Trenchless methods are built to install underground lines with as little digging as possible. In practice, that brings a few clear benefits:

• Less surface damage. There is no long trench to cut through pavement, landscaping or concrete.
• Less traffic disruption. Roads and driveways can often stay open while the bore runs underneath.
• Crossings that a trench can’t make. HDD is used to go under waterways, roadways and congested areas.
• Less restoration. With only entry and exit pits to close, there is less surface to repair at the end of the job.

For a side-by-side look, see HDD vs. open-cut trenching.

Where HDD needs extra care

HDD is not the right answer for every job. Coarse gravel, cobbles and boulders are known problem soils. Some hard, abrasive or badly fractured rock can make a bore impractical. And because the drill is working blind, the crew depends on good utility records, an 811 locate and careful tracking to avoid existing lines.

Cost also varies a lot. Soil type, bore length and pipe diameter all change how much work a bore takes, so there is no single price per foot that fits every job.

What a good HDD job looks like

A well-run bore starts before the rig shows up. The plans are read, existing utilities are located and marked, the entry and exit points are set, and traffic control is arranged if the work is near a road. During the bore, the crew tracks the drill head rod by rod. After the bore, the surface is restored and the contractor hands over an accurate record of where the line actually went.

That last step matters more than it looks. An accurate as-built record helps the next crew that digs nearby. It also helps the owner of the line find it again years later.

UGA, based in El Paso, Texas, provides horizontal directional drilling and fiber and telecom conduit boring, with restoration after the bore. If you have a project in mind, you can request a bid.

This article is general information about the HDD method. Every job is different, and the right approach depends on the site, the soil and the plans.