Horizontal Directional Drills

Horizontal directional drilling machines are powerful rotary units designed for use in a trenchless pipe installation method known as horizontal directional drilling (HDD). The method has been in use in the U.S. for more than 45 years. Its advantages over other pipe and utilities installation methods are due to its ability to create directionally controlled pipe paths long distances underground without disturbed surface features, including building, roadways, streams, lakes rivers and sensitive environmental areas. For wider bores, the machine drills an initial pilot hole and then pulls back a hole reamer or widener. It is also used to pull back pipe, conduit or cables to complete the installation. Any pipe material durable enough for such application is suitable. Common materials include PVC, polyethylene, polypropylene, ductile iron, and steel.

HDD machines are capable of making bores thousands of feet long, depending on ground conditions, the rig and tooling’s size, and thrust and pullback capabilities. For instance, a small unit not much bigger than a riding mower used for short utility installations might offer 50 ft-lb of rotary torque with a thrust and pullback capability of 5,500 ft-lb. At the other end of the spectrum, a “maxi-rig” might be rated for 100,000 ft-lb maximum rotary torque with up to 1,300,00 ft-lb (more than 650 tons) of thrust and pullback force, capable of making bores and pulling back pipe over distances greater than 2 miles.

HDD machines and their tooling allow the operator to “steer” the pipe from a relatively shallow entry angle to their horizontal path before turning the pipe to the surface again or to an excavation. With the use of locating equipment, the operator can also steer the pipe around underground obstacles, within the tooling’s radius capacity. A variety of locating strategies are used for navigation, including walk-over systems, wire-line locating systems, and gyro-based locating systems. Site requirements determine which system or combination of systems is most advantageous.

Some ground conditions are problematic. Loose-flowing soils such as gravel, cobbles, and boulders can make it difficult to maintain bore walls and allow drilling fluid to escape. On the other hand, rock strength, abrasiveness or porosity can make the project too expensive or even render it impossible. However, various innovations in products and techniques have extended the range of the method’s feasibility, making HDD a commonly preferred alternative for pipe and utility installation, particularly when job requirements make other installation methods impractical.