Horizontal directional drilling or HDD (known also as Guided Boring) techniques are used for the trenchless installation of new pipelines, ducts and cables. The bore path may be straight or gradually curved, as the direction of the drilling head can be adjusted at any stage during the initial pilot bore to steer around obstacles or under highways, rivers or railways. 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. Many machines are designed to operate purely as surface launch machines.
In terms of scale and capability, HDD tends to fall between the techniques of impact moling and microtunnelling. The term HDD is frequently used to describe the heavier end of the market such as major river, canal and highway crossings often covering long distances, but there is now such an overlap in equipment capabilities that it is probably unnecessary to draw a line between the this term and that of Guided Boring.
Installation of the product pipe or duct is usually a two-stage operation. A pilot hole is first drilled along the required path using position-monitoring equipment to steer information the drill head. The bore is then back-reamed, in a single or multistage 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is attached to the reamer by means of a swivel connector, and is pulled into the enlarged bore as the drill string is withdrawn.
HDD has been used mainly for the installation of pressure pipes and cable ducts, where precise gradients are not usually critical. However, in more recent years, with improvements in drilling machines and guidance system accuracy, the technique is becoming increasingly popular for gravity pipelines.
Equipment capabilities have also improved, both in the power and diameter of installation available and in the wider range of ground conditions that can be bored. Apart from the obvious environmental benefits of trenchless installation, the relative cost of HDD has fallen to below that of trenching for many applications, even ignoring the social costs of traffic disruption and delay.
Most, but not all, HDD machines use a fluid-assisted drill head that is pushed through the ground on the end of a string of drill pipes. The drill head is usually angled; so that constant rotation of the drill string produces a straight bore. Alternatively, keeping the head in one position causes the line to deviate. A sonde or beacon is usually built into the drill head and signals emitted by this are picked up and traced by a receiver on the surface, so allowing the direction, depth, and other parameters to be monitored. These are commonly known as ‘Walk-over’ systems.
Animation of a basic HDD installation
Animation courtesy of ISTT
Hard-wire guidance systems are also used, with the cable running through the drill string, particularly in cases where the bore path cannot readily be traced on the surface (across rivers, for example) or where the depth of the bore is too great for accurate location by radio-frequency methods.
A Bentonite/water mix is often used as the drilling fluid or ‘mud’, which carries bore debris in suspension and may be filtered through a recycling system.
Some systems are designed for dry operation without the use of large quantities of water or drilling fluids. These are simpler to operate, create less mess and do not require as much on-site equipment, but there may be restrictions on the sizes that can be installed and on the ground conditions that the machines can cope with.
An increasingly common feature is the use of percussive action to complement axial force and rotation. This can be achieved either with a percussive hammer at the boring head, or by generating the percussion at the machine on the surface and transmitting it along the drill string. Either way, this can improve significantly the ability of HDD machines to punch through difficult ground or hard inclusions.
Manufacturers throughout the world are numerous and offer a variety of equipment, ranging from compact rigs for small diameters and short lengths, to very large machines capable of installing well over a kilometer of large diameter pipes. An equally extensive range of bore guidance systems; drill heads, reamers and accessories is also available.
There are two essential features of any guided boring machine. The first is a powered rack, which pushes the drill string through the ground to bore the pilot hole, and then pulls it and the product pipe through the bore during the backreaming operation. Typically, the inclination of the rack on a surface launched rig can be adjusted between about 10° and 20° to the horizontal. The second feature is a motor and drive system to rotate the drill string (together with the attached bore head or back-reamer) and provide rotational torque.
Pit-launched machines are fixed in position within the launch pit, using the rear and front faces of the excavation to provide reaction to the thrust and pullback forces. Surface-launched rigs have some form of stake-down system to anchor them to the ground. On the more sophisticated machines, the stake-down system may be hydraulically powered.
Some surface-launched machines are self-contained, having on-board mixing tanks and pumps for the drilling fluid, together with associated power supplies, valves and control systems. Alternatively, separate fluid mixing and pumping units can be provided. The fluid is pumped through the hollow drill string to the bore-head, and returns through the space between the drill string and the walls of the bore. The fluid, together with the excavated material mixed with it, is usually pumped into a recycling unit for separation and re-use.
Drilling rigs, especially surface-launched machines, may incorporate an automatic drill pipe loading system in which the lengths of drill pipe are contained in a ‘carousel’ or on-board storage rack and are automatically added to or removed from the drill string as boring or back-reaming progresses. This usually operates in conjunction with an automatic vice arrangement which screws the drill pipes together or unscrews them during back-reaming. Automatic pipe handling has become increasingly common, even on smaller machines, since it speeds up installation, improves safety and reduces manpower requirements.
The pullback capacity range of surface launched HDD machines have widened immensely since the introduction of the first rigs some 25 years ago. Thrust and pull back capability now runs between 3.5 t for the smallest of rigs up to 600 t with some of the Mega-Rigs as they are known, and torque capacity available is up to almost 50,000 Nm depending on speed of rotation. This range of capacity makes it possible under the right conditions to install pipe diameters from as little as 50 mm with the small rigs up to as much as 1,500 mm with the largest rigs. Installations are possible over distances of up to 1,500 m under the right conditions with the larger rigs although most installation are between 50 and 500 m.
The capabilities of HDD machines vary considerably according to the type of ground through which they are drilling. In general, homogeneous clays are the most favorable soils, whilst sand can present problems especially if it is below the water table or is not self-supporting. Gravel can be difficult. Mud motors, powered by the drilling fluid, can be used to drive rock-cutting heads, and some smaller rigs have specialy designed multiplt drill rod systems to handle rock drilling. Another way of improving performance in hard ground is to use percussive action in conjunction with forward thrust and rotation.
The choice of back reaming tools and accessories is also very wide, and most have particular design features that are claimed to enhance performance. Most reamers are bullet shaped with an arrangement of tungsten carbide teeth and fluid jets. The rear of the reamer has a coupling to which a towing head can be attached for pulling in the product pipe. Special designs are available for difficult ground conditions, including hole-openers for reaming in rock.
Using a basic operation that is exactly the same as with Fluid-Assisted Boring, Fluid Jet Boring has but one basic difference from that outlined above. In this technique instead of utilising a mechanical cutter for ground excavation at the boring head, a series of high-pressure fluid jets are used. The jets may be of water or a drilling fluid mix. The fluid is projected forward out of the end of the angled cutting head in such a way as to dislodge the ground ahead of the drill string. The angled head allows for steering adjustments in the same way as with Fluid- Assisted Boring. This system is less widely used than Fluid-Assisted Boring and tends to be more applicable to softer ground suited to excavation by the high pressure jets.
Some systems are designed for dry operation. Both surface-launched and pit-launched versions are available, and dry boring machines tend to be more compact and simpler than most fluid-assisted rigs. Instead of relying entirely on thrust and rotation generated at the rig, dry boring machines use a high-frequency pneumatic hammer at the bore head to penetrate and compact the ground for the pilot bore.
An intermediate technique between fluid-assisted and dry boring is to incorporate a water/polymer mist lubrication system into the airflow of a dry boring machine. This helps to moisten and loosen the soil, and can increase productivity in dry soil conditions. Water/polymer mist lubrication can be used during both pilot boring and back reaming.
Both fluid-assisted and dry boring methods have their merits in appropriate conditions. Whilst fluid-assisted boring has greater versatility in terms of ground conditions and maximum diameters, it requires more equipment and involves dealing with mud-filled excavations and the disposal or recycling of materials. Dry boring is essentially a displacement technique, and should perhaps be described as ‘guided moling’. As such, it is best suited to compressible, self-supporting soils, and may not be appropriate for sands and gravels at bore diameters above about 75 mm. The risk of surface heave should also be considered, especially in granular soils.
Another area in which HDD has been utilised successfully is the installation of a ‘Pipe Arch’. Pipe Arches are utilised where a series of casing pipe are bored into the ground, often below road ways or rail tracks, to form a preliminary support mechanism separating the ground within the arch from that surrounding it. Once the pipe arch is competed, the soils within the pipe arch are excavated within the confines and safety zone created by the arch, minimising the potential for ground movement during excavation and reducing the potential for subsidence at surface and therefore in the infrastructure beneath which the new tunnel will pass.