Pilot Bore

Published: July 30, 2017 | Last updated: October 31, 2023

What Does Pilot Bore Mean?

A pilot bore is the initial horizontal hole drilled along the intended route for the final pipe product. The main purpose of the pilot bore is to map the predetermined path of the pipe installation.

During pilot boring, the drill is controlled by an operator from a remote location. Real-time locating technologies are used to assist the operator in steering the pilot bore to avoid underground obstacles, such as water, wastewater, and electrical utilities.

Pilot boring is typically associated with horizontal directional drilling (HDD). Before drilling begins, relatively small workspaces are excavated at the start and endpoints of the pipe installation. These excavations, also known as exit and entry pits, are used to facilitate drilling equipment and installation personnel.

The entry pit usually contains the drilling rig, power unit, drill pipe skid, mud recycling unit, and the control cabin. Erosion and noise control features may also be located at the entry pit as required.

The pilot bore process starts by inserting the drill string and cutting head assembly into the entry pit. The pilot bore penetrates the wall at the entry pit and advances until the cutting head reaches the exit pit.

Once the pilot bore is complete, the hole is enlarged with a larger cutting tool to the diameter required to install the product pipe. This process is known as reaming or back-reaming.

A pilot bore is also known as a pilot hole.

Trenchlesspedia Explains Pilot Bore

Figure 1: The horizontal drilling process showing the drilling of the pilot bore in phase 1 (source)

During the drilling of the pilot bore drilling mud (or drilling fluid,) is pumped through the drill string and into the pilot hole via nozzles in the cutting head. The drilling fluid mixes with the surrounding soil, creating a slurry that helps suspend and remove the cuttings. In addition to removing entrained cuttings, the drilling fluid also lubricates the drilling equipment and stabilizes the surrounding soil in the pilot bore.

One of the main features of the pilot boring process is its ability to be steered and controlled. This helps outline the path for the final pipe while avoiding obstacles that may interfere with the installation process. To assist the operator in locating the drill in real-time, several locating technologies may be used. The most common of these are:

Walk-Over Locating Systems

In walk-over locating systems, personnel located on the ground level use hand-held equipment to track the pilot bore’s progress as it moves through the ground. A transmitter located on the drill head transmits detailed data about the location of the pilot drill bit to a receiver at ground level. This data is sent to the controller, who adjusts the path of the pilot bore as needed.

Wire-Line Locating Systems

These locating systems are similar to their walk-over counterparts. However, instead of being wireless, wire-line locating systems use insulated cables to power the transmitter. Since these systems do not use onboard batteries, the battery life of wire-line locators is superior to walk-over systems.

Like walk-over systems, depth and location readings are transmitted to a receiver at the ground level and sent to an operator.

Gyro-Guided Drilling Systems

Gyro-guided systems operate differently from both walk-over and wire-line systems. Instead of transmitting electromagnetic signals, a system of gyroscopes is used to determine the pilot drill head’s location-based parameters. Since the gyroscopes do not need to determine magnetic north to function, they are unaffected by surrounding magnetic disturbances.


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