Horizontal Directional Drilling (HDD) is not a new technology when it comes to the installation of pressure pipe systems for water distribution or sewage conveyance. Until recently, the installation process has typically been more expensive than traditional open-cut methods for the same length of installed pipe because it requires highly specialized drilling equipment to complete the installation.
In this article we are going to develop a basic understanding of the technique and explore some challenges and successes from the design engineer’s perspective.
WHAT IS HHD?
HDD is a trenchless method of installing pipe and cable systems underground along a predetermined path. Over the last 30+ years the technology has evolved from its origin as a preferred pipeline construction method for the crossing of major waterways to a technique that can be employed in very congested utility corridors along busy highway systems as well as over longer distances where traditional open-cut methods would require work hour restrictions and/or adversely impact activities along the path.
Current technology limits a single bore length to around 8,000 linear feet and pipe diameters to 48-inches or less. The HDD process, in general terms, follows three basic stages that are described below.
The first stage of the process involves drilling of the pilot hole. During this phase of the process, the pilot-hole path is monitored, and periodic readings of the inclination and azimuth of the leading edge of the drill are taken in conjunction with measurements of the distance drilled since the last survey point. This information is used to calculate the horizontal and vertical coordinates along the pilot hole relative to the initial entry point on the surface. The operator can then adjust the drill inclination as needed to come up out of the ground at the pre-determined exit point.
The second stage of the process involves pre-reaming the pilot hole in preparation to install the pressure pipe. Although not required, many experienced contractors repeat this intermediate step to minimize the force required to pull the pipeline along the initial pilot hole without overburdening the drilling equipment. Reaming tools typically consist of a circular array of cutters and drilling fluid jets and are often custom configured for a particular bore hole size or soil type.
The final stage involves pulling the pressure pipe through the enlarged pilot hole. This is accomplished by attaching the prefabricated pipeline pull section behind the reamer assembly at the exit point and pulling the assembly back to the drilling rig. A swivel is utilized between the reamer and the pressure pipe being installed to minimize torsion transmitted to the pressure pipe as it is being pulled through the enlarged pilot hole into its final position. After the pipe is pulled through the bore, it is typically allowed to “relax” for a period of time to self-correct any elongation that may have occurred during the installation process.
Designers who are considering HHD as an installation option need to be aware of geological conditions along the predetermined path. Sandy soils and shallow bedrock present separate, different installation challenges to the HDD process. One of KCI’s case studies involved extending a sewer forcemain though a high-traffic golf course and driving range in sandy soils. During the first stage of the HDD process, the contractor experienced several blow-outs because the initial pilot hole was located too close to the surface. The unconsolidated sandy soil in the area was pushed upward, resulting in the drill head deviating from the designated path. The solution was to back up and dive a little deeper in the sandy soil areas to avoid additional blow-outs. Drilling through shallow bedrock presents a separate set of challenges. Drilling heads may need to be constructed with special alloy materials, and shallow angle bedrock strata can cause challenging drill defection scenarios from the predetermined path.
Congested utility corridors and/or limited utility right-of-ways (ROW) also present a challenge for HHD methods. The design engineer is strongly encouraged to utilize subsurface utility engineering services to perform Quality Level A designation of existing utilities that cross the predetermined path. KCI recently completed a project that required the extension of a pump station forcemain through a very narrow State Highway Administration ROW that already had several underground high-priority communications lines and a primary high pressure natural gas line. Successful mapping of the existing utilities allowed the HDD subcontractor to quickly install the forcemain extension and minimize traffic disruption to the busy state highway.
SUCCESSES AND ADDITIONAL BENEFITS
Both projects described above utilized shorter HDD runs (i.e., shorter distances between entry and exit points) because of the soils in the project areas. This approach gave the contractor the best opportunity to follow the designed route. HDD installation techniques can also reduce environmental impacts because they typically require less earth disturbance, which is generally limited to the area surrounding the entry and exit locations. This approach can also shorten planning, permitting and installation schedules. Specifically, less earth disturbance requires only basic erosion and sediment controls and a basic permit at the local levels. In the instances above, construction time was also decreased because the subcontractor was able to install the runs in fewer hours with less equipment than traditional open-cut technologies.
Directional drilling can be a viable alternative to consider for pressure pipe installations in areas that typically require only traditional cut-cover installation techniques. In addition to expediting the project schedule, this construction technique can also minimize earth disturbance and traffic impacts, which may have positive benefits on project permitting. HDD is becoming more cost competitive versus traditional methods for water distribution and sewer force main installation.