The terms Horizontal Directional Drilling (HDD), Under-road Boring or Trenchless Installation, all refer to a method whereby underground services are installed without the need to disturb surfaces, structures or other pre-existing services. Directional boring is used for installing infrastructure such as telecommunications and 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 or not possible.
Collectively referred to as Trenchless Technology, this method entails the creation of an underground tunnel along a prescribed bore path which is large enough to accommodate whatever pipes, cables or conduits are required, and the subsequent installation of those services.
Such tunnels are created by various means using an array of specialized equipment (as further detailed below). Equipment utilised is dependent on the diameter of piping to be installed, the length of the bore run, ground conditions and surface conditions. Some bores may need to be at a specified gradient, some curved, others winding to avoid existing services. Any of these can be achieved from surface without the need for large excavations prior to commencement of works.
The main advantages of Horizontal Directional Drilling are:
- Installation of services where no viable alternatives exist
- Speed of installation and project completion
- Minimal to zero environmental or visual impact
- Zero subsequent rectification works – those typical collapsing trenches following rainy spells
- Can be completed from surface without the need for large excavations
- Less traffic disruption,
- Ability to complete deeper and/or longer installations
- Directional/steerable capabilities
In short the HDD process can be summed up as below:
Pilot Hole Drilling
Utilising a Horizontal Directional Drilling guidance system, a pilot hole is drilled, usually from surface along the pre-determined bore path to the exit hole or surface location. Within the drill head which is attached to the drill stem, a ‘sonde’ transmits a signal to the guidance system which is interpreted by the Locator and is then utilised to ensure that the designed bore profile is maintained throughout.
Enlargement of the Pilot Hole
In some instances, where the conduit required to be installed has a small diameter, no enlargement or ‘pre-reaming’ of the pilot hole is required and the pipe/conduit will be installed as soon as the pilot hole has been completed.
In most instances however, it will be necessary for the pilot hole to be enlarged until it is suitable for the size of the pipes/conduits that needs to be installed. Also called ‘pre-reaming’, reamers of varying sizes will be attached to the drill stem and pulled/pushed through the drill hole to enlarge the original space. As during the pilot stage, a blend of water and drill muds i.e. bentonites and polymers will be utilised during the process to assist in the removal of soil/cuttings, to stabilize the bore hole, cool the drill head, and lubricate the passage of the product pipe.