Directional boring
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Directional boring, commonly called horizontal directional drilling or HDD, is a steerable trenchless method of installing underground pipes, conduits and cables in a shallow arc along a prescribed bore path by using a surface launched drilling rig, with minimal impact on the surrounding area. Directional boring is used when trenching or excavating is not practical. Directional boring minimizes environmental disruption. It is suitable for a variety of soil conditions and jobs including road, landscape and river crossings. Installation lengths up to 6,500’ (2,000m) have been completed, and diameters up to 56" (1,200mm) have been installed in shorter runs. Pipes can be made of materials such as PVC, polyethylene, Ductile iron, and steel if the pipes can be pulled through the drilled hole.
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[edit] Technique
Directional boring is used for installing infrastructure such as telecommunications & power cable conduits, water lines, sewer lines, gas lines, oil lines, product pipelines and environmental remediation casings. It is used for crossing waterways, roadways, shore approaches, congested areas, environmentally sensitive areas, and areas where other methods are costlier. It is used instead of other techniques to provide less traffic disruption, lower cost, deeper and/or longer installation, no access pit, shorter completion times, directional capabilities, and environmental safety.
The technique has extensive use in urban areas for developing subsurface utilities as it helps in avoiding extensive open cut trenches. The use however necessitates that the operator must have the complete information about the existing utilities so the he can plan his hole alignment to avoid damaging those utilities. Since uncontrolled drilling can lead to such damages different agencies/government authorities owning the urban 'right-of-way' or the utilities have formed their rules for safe work execution. For standardization of the techniques different trenchless technology promoting organizations have developed guidelines for this technique.
[edit] Process
The method comprises a three stage process wherein first stage is of drilling a pilot hole on the designed path, second stage is of enlarging the hole by passing a larger cutting tool termed as Reamer the third stage is of placing the product or casing pipe in the enlarged hole. The directional control capabilities assist the rig operator in making necessary changes in the directions of the drilling head.
Horizontal directional drilling is done with the help of a viscous fluid known as drilling fluid. It is a mixture of water and, usually, bentonite or polymer continuously pumped to the cutting head or drill bit to facilitate the removal of cuttings, stabilize the bore hole, cool the cutting head, and lubricate the passage of the product pipe.
[edit] Locating and guidance
Location and guidance of the drilling is very important part of the drilling operation as the drilling head is under the ground while drilling and in most cases not visible from the ground surface. An uncontrolled or unguided drilling can lead to substantial destructions. By properly locating and guiding the drill head such destruction can be avoided.
There are two types of locating equipment for locating the bore head: the ‘walk-over’ locating system or a 'wire-line' locating system. In both of the systems a sonde, or transmitter, behind the bore head registers angle, rotation, direction and temperature data. This information is encoded into an electro-magnetic signal and transmitted through the ground to the surface in a walk-over system. At the surface a receiver (usually a hand-held 'locator') is manually positioned over the sonde, the signal decoded and steering directions are relayed to the bore machine operator. In a wireline system, this information is transmitted through the cable fitted within the drill string. Both systems have their own merits and depending upon the site requirements a particular system is chosen.
