The pilot hole is the most demanding phase of an HDD installation. It establishes the geometry every later pass follows, and it is drilled blind — steered from the surface using downhole survey data and an understanding of how the bottom-hole assembly responds to the ground. This article explains the core mechanism that lets an operator steer a bore hundreds or thousands of feet to a target within a few feet.
The Steering Principle: Bias Plus a Non-Rotating String
Directional control comes from two things working together. First, the leading edge of the drill string is made asymmetric — it has a built-in bias that makes the bit want to curve in one direction. Second, that string is advanced without rotating, so the bias stays pointed in a fixed direction and the bore curves that way. To change direction, the operator simply rolls the drill string so the bias points where the new curve should go. This orientation of the bias is called the tool face.
To drill straight, the operator alternates the tool face between offsetting positions so the small curves cancel out, producing net-straight progress. And where no directional control is needed — through a long tangent in uniform ground — the whole string can simply be rotated continuously, which drills straight and fast.
Creating the Bias: Bent Sub or Bent Housing
The asymmetry is usually a small angular offset built into the assembly — a bent sub or a bent motor housing just behind the bit. That few-degree bend is what points the bit off the string’s axis and generates the steering tendency. The size of the bend, the length of the assembly, and the stiffness of the formation together determine how tightly the bore can turn.
Two Ways to Cut: Jetting vs. Mud Motor
Jetting (soft soils)
In soft soils, progress is often made by hydraulic cutting — high-velocity drilling fluid from a jet nozzle erodes the formation. The flow can be deliberately offset from the string’s central axis to create the steering bias, either by blocking selected nozzles on a roller-cone bit or with a purpose-built jet-deflection bit. If a hard spot is hit, the operator can rotate the whole string to grind through it, then return to oriented jetting.
Mud motors (harder ground)
Harder soils and rock need mechanical cutting, supplied by a downhole mud motor. A positive-displacement mud motor — the type typically used in HDD — converts the hydraulic energy of the pumped drilling fluid into bit rotation through a spiral stator and a lobed rotor. Because the motor turns the bit without turning the drill string, the tool face can still be held fixed for steering while the bit cuts rock. Positive-displacement motors are preferred over turbine motors for the torque characteristics HDD needs.
Knowing Where the Bit Is
Steering is only as good as the position data behind it. A survey probe housed in a non-magnetic collar near the bit reports inclination and azimuth after each joint; combined with the measured drilled distance, those readings are integrated into a running set of coordinates. A wash pipe is sometimes rotated concentrically over the steerable string to keep it from sticking and to preserve the hole if the assembly must be pulled. The math that turns raw angle readings into a bore position is covered in our article on downhole survey methods, and the geometry the operator is trying to hit is set by the bore path design.