In the construction of pipelines and laying of utilities in densely built-up metropolitan areas, passing through water and other natural and artificial obstacles, geological exploration expeditions and mining operations, horizontal directional drilling (HDD) technology is used. It enables underground surveys and crossings to be realised quickly, economically and with high quality.

The method was developed as an alternative to other open pipe-laying methods. The main difference is drilling in the horizontal direction without opening an above-ground trench.

The directional drilling method (DD), which is alike as HDD, also allows deviations from the horizon line. Despite the similarities, there are fundamental differences between HDD and DD in the areas of application, the equipment used and the general methodology used.

Horizontal directional drilling

Horizontal directional drilling is used for trenchless laying of utilities. This is especially true in congested and metropolitan areas. HDD is effective for laying utilities under water obstacles, roads and railways.

Horizontal directional drilling uses self-propelled equipment (drill rigs). Depending on the reach and pulling power, rigs are classified accordingly. The main elements of drilling rigs are: undercarriage, body, rig frame, drilling rig, accumulator, control and instrument panel, rod feeder device, gun carriage.

There are 5 phases of HDD pipe-laying work:

• Preparatory work – during this stage, research work is carried out to analyse the natural conditions, geology of the construction area, as well as to prepare all permits.
• Test (pilot) drilling – this phase is considered the most critical. The success of the entire construction project depends on the quality of the test (pilot) drilling. During pilot drilling the drill head, attached to the drill rod, is lowered into the ground. At this point the machine operator has to work his way around any obstacles (encrustations of solid rock, boulders or other utilities) with millimetre precision. At this point the borehole is filled with drilling mud, which compensates friction and prevents the working head from overheating. A navigation system is installed on the working head to ensure that the work is carried out accurately.
• Reaming the hole – at this stage, the drilling head is removed from the flexible pipe and the reamer is installed. A rimmer is a special tool used to enlarge the diameter of the borehole. Rimmer is most often used, the diameter of which is 100-150 mm larger than the diameter of the target pipe.
• Pumping through the pipe – it is important that the drilling of the pipes starts on the side opposite to that of the drill rig. The lines are connected to the pipe by a swivel, which ensures that the rotation of the rimmer is not transmitted to the pipe but that the pipe is pulled into the drilled hole by the pulling force.
• The final stage is the final step in which the cavity between the pipe and the drilled hole wall is filled and stabilised with cement mortar, which is pumped to the hole at high pressure using a special machine.

Trenchless pipe-laying using horizontal directional drilling can take place using two methods: piercing and punching.

The piercing method is predominantly used in ductile soils (clay, loam). This method has clear limitations on the size of the hole (for pipes up to 600 mm in diameter and up to 60 m in length).

Through the use of jacks and high-performance drilling rigs pressure of up to 3000 kN is created on the rock. At this point the ground is pressed through a tube with a conically shaped steel head, which ensures the penetration into the ground. Depending on the capacity of the drilling equipment a penetration rate of up to 40 m per hour can be achieved.

The punching method, as opposed to piercing, can be used in all rock types. Punching allows to lay the pipes with diameters of up to 1200 mm and lengths of up to 100 metres. The main difference is that method needs no a capping device on the pipe: jacks are used to push the pipe through the rock to the required distance. The hydraulic jacks are positioned around the circumference of the pipe, operating synchronously with the same power.

Directional drilling

With the development of horizontal directional drilling technology and equipment, another method of trenchless utility installation emerged: the directional drilling method. This technology was first used in the geological surveying, mining and then later applied to the installation of utilities under riverbeds. And only in USA over the last 30 years more than 2.5 million pipeline and utility crossings have been made using this technology.

The directional drilling method allows the borehole to be deviated from the horizon. The directional drilling method has therefore proved to be an effective way of constructing tunnels and pipelines under rivers with ribbon-like channel arrangements.

Despite its effectiveness, directional drilling still involves higher financial costs compared to HDD: this is due to the more expensive equipment required for the construction of pipelines with this method. However, directional drilling technology is indispensable for trenchless laying of utilities in boulder and silt soils as well as in karst soils.

*the photos are for illustrative purposes only