What is horizontal directional drilling?
Horizontal directional drilling, or HDD, installs pipe along a steerable underground path from one small pit to another, so nothing between the two ends gets dug up. It is how conduit and service lines get under roads, driveways and streams without cutting them open.
A bore goes in as three passes, and most of what confuses people about HDD is that they picture only the first one.
The pilot bore
A drill head on the end of jointed rods is pushed and steered from the launch pit to the exit pit. The head is slanted, so rotating it drills straight and pushing it without rotation steers toward the slant. The operator tracks it from the surface with a locator reading a transmitter in the head, and calls out depth and pitch as each rod goes in. Pitch is the reason the bore calculator exists: the drop per rod at a given pitch is what tells you whether you will pass under the obstacle or into it.
The backream
The drill head comes off at the exit and a reamer goes on, then the rig pulls the reamer back through the pilot hole, enlarging it. How much bigger is a judgment call. A common rule of thumb is a final hole around 1.5 times the outside diameter of what you are installing, which is what the reamer size calculator works from. A bigger hole pulls easier and costs more fluid and more spoil.
The pullback
The product pipe or conduit bundle is attached behind the reamer with a swivel, so the reamer can spin without twisting the pipe, and the whole thing is drawn back to the launch pit. On a long shot this is the pass with the risk: it happens once, and a pipe that gets stuck partway is a genuinely bad day.
When boring beats digging
Under a paved road, a finished driveway, mature trees or a stream, boring usually wins before you even price it, because restoration is what makes open cut expensive. In open ground with no surface to protect, a trench is normally faster and cheaper. Rock, cobble and very soft saturated ground all push the decision back toward trenching, because they are the conditions where steering and hole stability get unreliable.