HDD Method

Horizontal Directional Drilling (HDD) is a construction technique whereby a tunnel is drilled under a waterway or other designated area, and a pipeline or other utility is pulled through the drilled underground tunnel. The underground tunnel follows an arc line from the entry point, down under the special crossing area, and then resurfaces on the opposite side. Using advanced technology and highly trained technicians, a drill head guides the drilling pipe electronically to ensure the angle, depth, and exit point adhere to carefully designed engineering plans. Throughout the drilling process, the tunnel is kept open and lubricated by circulating a watery mud-slurry mixture, typically composed of about 95% water and 5% bentonite clay—a natural, non-toxic substance. The drilling mud also helps coat the walls of the tunnel and remove drill cuttings. When the tunnel is 12 inches wider than the diameter of the pipe going through it, the welded pipeline segment can be pulled through the underground arched tunnel from one end to the other end. These pipeline segments are typically made of thicker steel and have a protective first layer of coating to prevent corrosion and a second abrasionresistant coating to protect the pipe during the pull through the tunnel. The two ends of the pipeline segment can then be welded together with the rest of that pipeline system.

Long-distance underground pipelines are built across open areas, such as rural farmlands and urban utility corridors. Typically, construction crews clear an approved route, dig a specially prepared trench, lower welded sections of pipeline into the trench, and carefully replace removed soil for the restoration process. Usually, this method is preferred for open area installation and can be completed in a way that minimizes environmental and stakeholder impact, even in sensitive areas. For large rivers or certain sensitive crossings, directional drilling is an appropriate and preferred construction method, depending on geologic conditions and other factors. HDD construction is considered suitable for large river crossings, extreme congestion (e.g. crossing rail yards), or other unique site-specific situations because it minimizes impacts on the area above the drill. Although land around the drill entry and exit locations is disturbed during HDD activities, it will be restored to its pre-drill state.