HDD vs. open-cut trenching: which fits your utility project?

When a utility line has to go underground, there are two common ways to get it there. You can dig an open-cut trench along the route, place the line and backfill it. Or you can use horizontal directional drilling (HDD), which bores under the surface and only digs at the entry and exit points.

Neither one is always better. Each is the right call in its own place, and the choice depends on what sits on top of the route, what is in the ground and what you need at the end of the job.

Side-by-side comparison

When HDD is the right call

HDD earns its place when the surface above the line is already built and worth protecting:

• Crossing a road, sidewalk or driveway. The bore passes underneath, so there is no pavement to cut and less traffic to manage.
• Going under water or a drainage channel. HDD is widely used for waterway crossings.
• Working in a crowded, finished area. Where yards, landscaping, other utilities and finished concrete fill the route, a bore avoids tearing them up.
• Keeping restoration small. With only pits to close, there is far less surface to repair.

When trenching is the right call

Trenching is the everyday tool of new construction:

• New subdivisions and commercial sites. Before the streets, driveways and landscaping go in, there is little finished surface to protect, and an open trench is often the most direct way to put utility conduit and pipe in place.
• Several lines on one route. Utilities often publish joint-trench layouts, where several lines share one trench in set positions, where their standards allow.
• Open ground with easy access. Nothing valuable on the surface, and room for the equipment.
• Ground that fights a drill. Coarse gravel, cobbles and boulders are known problem soils for HDD.
• Lines that need to be seen as they are laid, for example with many connections close together.

Open trenches come with their own work: shoring or sloping where depth requires it, keeping people away from the edge, and backfilling properly.

Many projects use both

A new development is often trenched across the open site and bored where the route crosses an existing road, driveway or utility. Planning the whole underground scope as one job lets each part of the route get the method that fits it. See infrastructure builds.

Both methods depend on good utility information

Whatever method you pick, the biggest risk is hitting something that is already buried. A Federal Highway Administration summary of a Purdue University study notes that existing records of underground site conditions are usually incorrect, incomplete or otherwise inadequate. The same summary lists the benefits of knowing where utilities really are, including fewer unforeseen utility conflicts and fewer claims and change orders.

That is why every job, bore or trench, starts with an 811 locate and a careful look at the plans. Read do you have to call 811 before digging in Texas? for the timing rules.

Questions to ask before you choose

• Is the site still open ground, or is it already paved and landscaped?
• Does the route cross a road, a driveway or water?
• How many lines need to go in, and can they share a route?
• What is the soil like? Are there known cobbles, boulders or rock?
• How many existing utilities are close to the route?
• Will traffic control or permits be needed? (See traffic control and permitting.)
• What record of the finished line does the owner need at the end?

The short answer

If the line has to pass under something already built, HDD is usually the right call. If the site is new construction or open ground, trenching usually is. Many jobs use some of each, and a contractor who looks at the plans and the site can tell you which part fits which method, and why.

UGA, based in El Paso, Texas, provides horizontal directional drilling, trenching and infrastructure builds. If you would like us to look at your route, request a bid.

This article is general information. Every site is different, and the right method depends on the plans, the soil and local requirements.