Table of Contents




Main features and application range

Diameter range

Depth of installation

Drive length

Type of pipe

Required working space

Soil condition





    Horizontal directional drilling (HDD) was pioneered in the United States in the early 1970s by an innovative road boring contractor who successfully completed a 183 m (600 ft) river crossing using a modified rod pushing tool with no steering capability (DCCA 1994). By integrating existing technology from the oil well drilling industry and modern surveying and steering techniques, today's directional drilling methods have become the preferred approach for installing utility lines, ranging from large-size pipeline river crossings to small-diameter cable conduits.

    The HDD industry is divided into three major sectors--large-diameter HDD (maxi-HDD), medium-diameter HDD (midi-HDD), and small-diameter HDD (mini-HDD, also called guided boring)--according to their typical application areas. Although there is no significant difference in the operation mechanisms among these systems, the different application ranges often require corresponding modification to the system configuration and capacities, mode of spoil removal, and directional control methods to achieve optimal cost-efficiency. Table 1 compares typical maxi-, midi-, and mini-HDD systems.

Table 1. Comparison of main features of typical maxi-, midi-, and mini-HDD (Iseley and Gokhale 1997)

System Description Product Pipe Diameter Depth Range Drive Length Torque Thrust/ Pullback Machine Weight (including truck) Typical Application

600-1,200 mm

(24-48 in)

< 61 m (200 ft)

< 1,818 m

(6,000 ft)

< 108.5 kN-m

(80,000 ft-lb)

< 445 kN

(100,000 lb)

< 267 kN

(30 ton)

River, Highway crossings

300-600 mm

(12-24 in)

< 23 m (75 ft)

<  274 m

(900 ft)

1-9.5 kN-m

(900-7,000 ft-lb)

89-445 kN

(20,000-100,000 lb)

< 160 kN

(18 ton)

Under rivers and roadways


50-300 mm

(2-12 in)

<  4.5 m (15 ft)

< 182 m

(600 ft)

< 1.3 kN-m

(950 ft-lb)

<  89 kN

(20,000 lb)

< 80 kN 

(9 ton)

Telecom and Power cables, Gas lines

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    Directional drilling methods utilize steerable soil drilling systems to install both small- and large-diameter lines. In most cases, HDD is a two-stage process. Stage 1 involves drilling a pilot hole approximately 25 to 125 mm (1 to 5 in) in diameter along the proposed design centerline. In stage 2, the pilot hole is enlarged to the desired diameter to accommodate the pipeline. The pilot hole is drilled with a surface-launched rig with an inclined carriage, typically adjusted at an angle of 8 to 18 degrees with the ground for entrance and 8 to 12 degrees for exit angle (Miller the Driller 2002). The preferred minimum radius in feet for steel pipe is typically 100 times the diameter of pipe in inch. For plastic pipe, the multiplication factor is 40, i.e., 40 times of diameter of pipe in inch. 

Most systems adopt either fluid-assisted drilling or a high pressure fluid jetting method to create or enlarge the bore hole. In a few instances, some mini-HDD systems utilize dry bore systems (with compressed air) in hard, dry soils and calcified or soft rock formations.

    The progress of the pilot hole is monitored by a specially designed surveying system, either a walkover system or an electromagnetic down-hole navigational system. In a walkover system, the drill head is equipped with a sonde (also called a beacon) transmitter behind the drill bit. The sonde is powered by battery and emits signals continuously. These signals can be picked up on the ground with a hand-held receiver. The receiver provides data on the position, temperature, depth, and orientation of the drill bit (movie (This wmv file is 2.8 MB. Windows Media Player is required to run this file.)). An alternative detection system, the electromagnetic down-hole navigational system can be used in conjunction with a series of four electrical cables positioned directly above the desired path and secured in place. The cables, which can be laid directly on top of the street or highway, do not interfere with traffic flow. The cables transmit an electromagnetic signal that is picked up the navigational instruments in the drill head. These instruments determine the position of the drill head relative to the center of the cables and relay this information continuously to a computer on the operator's console. In case of deviations from the desired path, the operator can make necessary adjustments (Iseley and Gokhale 1997).

    After the drill head (or pilot string and washover pipe) exits at the desired location, reaming devices are attached for the pullback operation. This stage involves enlarging the pilot hole to the desired diameter to accommodate the pipeline. The utility pipe is attached to the reamer, with a swivel to ensure that the rotation (torque) applied to the reamer is not transmitted to the utility. The reamer enlarges the bore hole to the required size, and the utility is installed. For large diameter (greater than 500 mm (20 in.)), an intermediate prereaming may be required before pulling the utility into place. Prior to the pullback operation, the pipeline is usually assembled to its full length and tested.

The drilling process in HDD can be described as follows:

1. Site preparation

    The construction site is prepared before the main drilling operation. A drilling rig is set up at the proper location. Slurry is prepared to stabilize the borehole and to lubricate the surface of borehole. A transmitter is inserted into the housing provided on the pilot drilling string near the drill bit. Other equipment and facilities such as generators, pumps, storages, and offices are prepared at this stage.

2. Pilot hole drilling

    Drilling the pilot hole can be the most important phase of a HDD project, because it determines the ultimate position of the installed pipe. A small diameter (25 to 125 mm (1 to 5 in.)) drilling string penetrates the ground at the prescribed entry point at a predetermined angle routinely between 8 – 18 degrees. The drilling (picture, movie (This wmv file is 4.0 MB. Windows Media Player is required to run this file.)) continues under and across the obstacle along a design profile. 

3. Prereaming

    In general, the final size of the bore should be at least 50% larger than the outside diameter of the product pipe. This overcut is necessary to allow for an annular void for the return of drilling fluids and spoils and to allow for the bend radius of the pipeline. To create a hole that accommodates the required size of pipe, prereaming is necessary. 

    Typically, the reamer is attached to the drill string at the pipe side and pulled back into the pilot hole. Large quantities of slurry are pumped into the hole to maintain the borehole and to flush out the soil cuttings (DCCA 1994). The type of reamer varies based on the soil type. A blade reamer is used for soft soils, a barrel reamer for mixed soils, and a rock reamer with tungsten carbide inserts is used for rock formations.

4. Pullback

    Once the prereaming is completed, the pipe or conduit can be pulled back into the reamed hole filled with drilling fluid. The pipe is prefabricated and tested at the pipe side. If the pipe is made of steel, it is recommended that the pipe be placed on rollers to reduce the friction and to protect pipe coating. However, this operation is usually not required for High Density Polyethylene (HDPE) pipe installation. 

    The drill pipe is connected to the product pipe using a pull head or pulling eye and a swivel (picture). The swivel is a device used to prevent the rotation of the pipeline during pullback. A reamer is also located between the pull head and the drill string to ensure that the hole remains open and to allow lubricating fluid to be pumped into the hole during the pullback. The pullback operation will continue until the pipe or conduit surface at the drill rig. The pull head is disconnected, the drill rig removed, and clean-up and tie-ins are started. For small diameter pipes, the prereaming process and pullback process can be performed at the same time (picture, movie (This wmv file is 3.3 MB. Windows Media Player is required to run this file.)).                Back to Top

Main Feature and Application Range

Diameter range

In maxi- and midi-HDD, the size of pipe installed can range from 75 mm (3 in.) to 1,200 mm (48 in.) in diameter. Multiple lines can be installed in a single pull, but only in the case of small-diameter pipes. The installation procedure for multiple lines is the same as for single lines, with the bundle being pulled back as a single unit along the prereamed profile. A significant multiple line crossing is more than 600 m (2,000 ft) in bore length and consists of five separate lines, pulled as one, ranging in size from 150 mm (6 in.) to 400 mm (16 in.). The largest pipe that can be installed by the mini-HDD system is 300 mm (12 in.) in diameter.

Depth of installation

Mini-HDD can install pipes up to 4.5 m (15 ft) in depth. This depth limitation comes from the restriction in the capacity of walkover tracking system. However, for the larger machines, such as midi- and maxi-HDD, the maximum installation depth for HDD is 61 m (200 ft).


Drive length

The length of bore in HDD is determined by the type of soil and site conditions. Bore spans can range from 120 m (400 ft) to 1,800 m (6,000 ft) for maxi- and midi-HDD. However, small lengths are not economically feasible because of the high operational costs of these systems. Mini-HDD is capable of installing pipelines and utilities 180 m (600 ft) in one continuous pass to a specified tolerance. 


Type of pipe

In general, the pipe to be installed is limited to one that can be joined together continuously, while maintaining sufficient strength to resist the high tensile stresses imposed during the pullback operation. In maxi- and midi-HDD, steel pipe is the most common type of casing used. However, butt-fused, high-density polyethylene pipe (HDPE) also can be used. HDPE pipe, small-diameter steel pipe, copper service lines, and flexible cables are some of the common types of pipe materials being used today in mini-HDD.

Required working space

The directional drilling process is a surface-launched method; therefore, it usually does not require access pits or exit pits. If utility installation is being undertaken, pits may be required to make connections with the existing utility. The rig working area should be reasonably level, firm, and suitable for movement of the rig. For maxi- and midi-HDD, an area of 120 m (400 ft) b y 60 m (200 ft) is considered adequate. The equipment used in mini-HDD is portable, self-contained, and designed to work in congested areas. 

Soil condition

Clay is considered ideal for HDD methods. Cohesionless fine sand and silt generally behave in a fluid manner and stay suspended in the drill fluid for a sufficient amount of time; therefore, they are also suitable for HDD.

High-pressure fluid drilling systems (mini-HDD and midi-HDD) normally do not damage on-line existing utilities and thus are safe for subsurface-congested urban areas. Fluid cutting systems, which are most suitable in soft soil conditions, have been used widely in sand and clay formations. Although small gravel and soft rock formations can be accommodated by higher fluid pressure and more powerful jets, steering accuracy might suffer.

Generally, mechanical drilling systems (mini-HDD) can be applied in a wider range of soil conditions than fluid jetting methods. A pilot hole can be drilled through soil particles ranging from sand or clay to gravel, and even in continuous rock formations, by using suitable drill heads; however, problems might occur in spoil removal, pilot hole stabilization, and backreaming operations. Today's technology enables large drilling operations to be conducted in soil formations consisting of up to 50 percent gravel.


HDD systems have the highest pilot hole boring rate of advancement among all trenchless  methods for new installations. For mini-HDD rigs, a three-person crew is sufficient. In suitable ground conditions, a pipeline as long as 180 m (600 ft) can be installed in 1 day by a regular work crew.        

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Directional Crossing Contractors Association (DCCA). (1994). “Guidelines for a Successful Directional Crossing Bid Package.”

Iseley, T. and Gokhale, S. (1997). “Trenchless installation of conduits beneath roadways.” NCHRP Synthesis 242. Transportation Research Board/National Research Council, Washington, D.C., 36.

Miller the Driller (2002). <http://www.millerthedriller.com>