Horizontal Directional Drilling
Horizontal directional drilling is a mud rotary method of guided drilling that is used to install pipe, conduit and well screen underground. It is an alternative method to jack-and-bore and auger drilling with unique advantages over those methods. It can be used to install a single pipe or multiple pipes of varying sizes in one borehole, and many types of materials can be installed including PVC, HDPE, and Ductile Iron pipe for sewer, water, product, gas, electrical, or air transmission.
Trenchless uses various directional drilling equipment (depending on project requirements) to complete many different types of installation. The drilling process begins at land surface at a small entrance pit with the drill head entering the ground at between 10 and 30 degrees. The drill string (bit and drill pipe) continues down to the target depth where it then transitions to the horizontal portion of the bore. The drill string is advanced across the site until it is directed to ascend into a small exit pit on land surface. Drilling mud is circulated down the drill string and out the drill bit to clear cuttings, lubricate and cool the drill bit and to stabilize the borehole from collapse.
The drilling process is guided by utilizing a special, slanted drill bit which when pushed in the bore without rotation causes the drill string to curve in the direction of the slanted head of the bit. Once the appropriate course correction is achieved, rotation begins again and the borepath is further advanced in a straight line.
Tracking of this process is accomplished through capturing the signals from a transmitter located in the drill head which gives the tracker (person with a handheld device receiving transmitted data) information on depth, lateral location, inclination and declination, and the orientation of the slanted drill bit head. The orientation of the slanted drill head is important to aid the steering process described above. Tracking of the borepath at 5’ to 30’ intervals allows for accurate installation within a few inches of the desired course and for producing useful record drawings upon completion of the bore.
Once the pilot hole has been completed, the drill bit is removed and a reaming bit is attached to the drill string followed by a swivel and the pipe, conduit or well material. Then the drill string is rotated and the hole is enlarged by pulling the reamer back through the pilot hole followed by the pipe material. Once the reaming bit emerges from the entrance pit, it is removed and the installation is complete.
The advantage of this installation process is that it is non-destructive to the site, no large pits or dewatering is required, it can be steered, hence avoiding underground obstacles such as utilities, and long distances can be drilled in one “shot”.
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