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Horizontal directional drilling (HDD) techniques are used for steerable installation of new pipelines, ducts and cables. Using the correct type of drilling rig, bores can be carried out between pre-excavated launch and reception pits or from the surface by setting the machine to drill into the ground at a shallow angle.

Installation of the product pipe or duct is usually a two-stage operation. A pilot hole is drilled along the required path to provide steering information, and the bore is then back-reamed in a single or multi-stage operation, depending on the ground conditions and project requirements, to a larger diameter to accommodate the product pipe. During the final ‘pullback’ stage, the product pipe is attached to the reamer by means of a swivel connector, and is pulled into the enlarged bore as the drill string is withdrawn. A load monitoring device may be used to prevent overstress of the product pipe.

In earlier years, HDD was used mainly for the installation of pressure pipes and cable ducts, where precise gradients are not usually critical. In recent years drilling machines and guidance systems have offered improved accuracy, and it is expected that the technique will become increasingly popular for gravity pipelines. Equipment capabilities have also improved recently, both in the power and diameter of installation available and in the wider range of ground conditions that can be bored.

Both surface-launched and pit-launched drilling machines are available, the choice depending on the nature of the project. Machines range from compact rigs suitable for small bores and operation in restricted spaces, to extremely large units designed for large diameter, long distance crossings.

Most guided boring machines use a drilling fluid, which lubricates and stabilises the bore, and also conveys the excavated material in suspension.

In recent times the relative cost of HDD has fallen to below that of trenching for many applications, meaning the popularity of this technology will continue to rise.

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