Watch a horizontal directional drilling crew for a few minutes and the obvious question comes up: how do they know where the drill head is? The bore is underground, out of sight, sometimes far below a road or a river. Yet the drill head comes up in the exit pit right where it was planned, at the depth the engineer called for, without hitting the utilities already in the ground.
There is no guesswork involved. HDD is a steady loop of measuring, steering, and checking, repeated every few feet from one end of the bore to the other.
Step One: Planning the Path
Every bore starts with a planned profile: where it enters, how steeply it dives, how deep it runs under the obstacle, and where it comes back up. The plan accounts for the pipe being installed, since steel and plastic pipe can only bend so sharply, and for existing utilities along the route.
Before drilling, existing lines are marked through the 811 call-before-you-dig system, and critical crossings are often exposed by potholing, which uses vacuum excavation to dig a small hole and see the utility with your own eyes. The bore path is then set to clear them.
Step Two: Knowing Where the Drill Head Is
Just behind the drill bit sits a housing that holds an electronic transmitter, often called a sonde. The sonde sends out a signal that carries its depth, its pitch (how steeply it is angled up or down), and its roll (which way the bit face is pointing, read like a clock face).
On most bores, a crew member walks the surface above the drill head with a receiver, called a locator. The locator picks up the sonde’s signal and shows exactly where the head is, how deep it is, and which way it is heading. The reading is relayed to the operator at the rig, so both people see the same information in real time.
Where nobody can walk over the bore, such as under a wide river, a busy interstate, or a building, crews use wire-line or gyro steering systems. These send position data back through the drill string or calculate it from instruments in the tool, so the bore can be tracked without standing on top of it.
Step Three: Steering
The drill bit has an angled face, a bit like the slanted tip of a spoon. That angle is the secret to steering.
- To drill straight, the rig rotates the drill string while pushing. The angled face spins in a circle, so it cuts evenly in every direction and the bore runs straight.
- To turn, the operator stops rotating, sets the bit face to the clock position pointing the direction they want to go, and pushes without rotating. The angled face deflects the head that way.
Every drill rod is a check-in. After each rod is added, the locator takes a new reading, the crew compares it to the plan, and the operator makes small corrections. Because adjustments happen constantly, the bore never has the chance to drift far off course.
Step Four: Reaming and Pullback
Once the pilot bore breaks out at the exit point, the small pilot hole is enlarged. A reamer is attached to the drill string and pulled back through the bore, opening it to a diameter larger than the pipe being installed. Large bores may take several reaming passes.
Finally, the product pipe is attached behind the reamer with a swivel, so the pipe does not twist, and pulled back through the finished hole. Throughout the process, drilling fluid supports the bore wall, cools the tooling, and carries cuttings back to the surface, where it is collected and managed.
What Can Throw Off a Reading
Locating signals can be affected by interference from power lines, rebar in concrete, buried metal, and conductive ground. Saltwater and brackish soils, which are common along the Gulf Coast, can weaken the signal. Experienced crews know these conditions, calibrate their equipment for the site, and choose the right locating system for the job. That attention to detail is what turns “drilling blind” into drilling on target.
Put HDD to Work on Your Project
GriffinCo runs HDD crews across coastal Alabama, Mississippi, and Florida for water, sewer, gas, power, and fiber projects. If you have a crossing to price, send us the details.