Most pipelines installed by this process are fusion-welded continuous HDPE pipes, although steel, ductile iron, and PVC pipes are also installed by this process. HDD is often employed when an open-cut excavation is unsuitable, such as at a railway crossing or river crossing. During the last stage of reaming, the service pipe is pulled back into the bore. Successive reamers are then pulled back to enlarge the bore diameter to the desired size. The bore is steered to create an initial hole at the required line and grade. First, a pilot bore is made with a suitably sized drilling rig. Horizontal directional drilling (HDD) consists of several installation steps. Thomas Walsh, in Applied Plastics Engineering Handbook (Second Edition), 2017 32.25.2 Horizontal Directional Drilling The recent introduction of fusible PVC piping has allowed PVC piping to be more widely used in HDD installations of water pipe, telecommunications, and electrical conduit that were previously dominated by PE pipes. Most pipelines installed by this process are fusion welded continuous HDPE pipes, although steel, ductile iron, and PVC pipes are also installed by this process. HDD is often employed when an open cut excavation is unsuitable such as at a railway crossing or river crossing. First, a pilot bore is made with a suitable-sized drilling rig. Horizontal directional drilling consists of several installation steps. Thomas Walsh, in Applied Plastics Engineering Handbook, 2011 34.29.2 Horizontal Directional Drilling (HDD) The Plastic Piping Industry in North America
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