Gravity drill rig technology – modern solutions in construction

General questions about the technology:

Łukasz Madej: What exactly is gravity drill rig technology and what sets it apart from other methods?

Krystian Frank: Gravity drill rig technology is an advanced trenchless installation method that uses a two-stage bore process with precise slope control. A key element is the use of a bentonite slurry, which stabilizes the borehole, helps transport spoil and reduces resistance when pulling in the pipe. In the first stage we perform a pilot bore that defines the exact route. In the second stage we reconfigure the drilling equipment and pull in the final pipe. The entire operation is carried out underground, minimizing surface disruption — a crucial advantage when working in highly constrained spaces.

Ł.M.: In which situations is this technology recommended instead of conventional guided bore or open-cut methods? When does the gravity drill rig become the preferred solution, especially considering urban challenges and compactness?

K.F.: The gravity drill rig performs exceptionally well in highly urbanized areas where traditional excavations are difficult or impossible due to dense development, critical road arteries or an extensive existing underground utility network. It is ideal for narrow streets and very restricted sites thanks to the compactness of the equipment. Moreover, for deep installations where conventional methods would require full soil replacement, our technology is considerably faster and more cost-effective.

Ł.M.: Every technology has limitations. What are the typical limits and challenges associated with the gravity drill rig?

K.F.: Indeed, there are some constraints. The minimum slope required to perform the bore is 0.5%. The maximum bore diameter is 500 mm, and for that diameter the section length is limited to about 30 metres. For smaller diameters, depending on the drill rig used, bore lengths can range from 40 to 60 metres.

Ł.M.: And what about the length limits — what are the minimum and maximum sections achievable with this method?

K.F.: For minimum lengths there’s really no strict lower limit — the technology is flexible. As for maximum lengths, as I mentioned, they reach up to 60 metres depending on soil conditions and the specifications of the rig used.

Ł.M.: What tolerances for slope and axial deviation are acceptable with this technology? Can we expect high precision?

K.F.: The gravity drill rig technology delivers very high precision. Permissible slope tolerances are as tight as ±0.1%, which allows for extremely accurate installation of gravity-fed systems.

The gravity drill rig performs exceptionally well in highly urbanized areas where traditional excavations are difficult or impossible due to dense development, critical road arteries or an extensive existing underground utility network

Ł.M.: Is this technology suitable for non-cohesive soils, or does it work better in cohesive soils?

K.F.: Yes, the technology is effective in non-cohesive soils as well. By using a pressurised bentonite slurry that forms a stabilising film on the borehole walls, we minimise the risk of collapse and maintain borehole stability.

Ł.M.: What types of networks can be installed with this method? Are there material restrictions?

K.F.: We can install any type of pipe intended for trenchless installation and suitable for pulling. This includes plastic pipes and other materials, provided they meet the required strength and construction standards for trenchless installation and underground networks.

Ł.M.: Can the gravity drill rig be used only for gravity systems, or also for pressure systems?

K.F.: That’s a very important question. The answer is: yes, pressure pipes can be installed. Despite the name, the technology allows precise installation of pressurised networks as well, broadening its range of applications.

Ł.M.: Let’s look at the economics. What are the main cost differences compared to traditional methods?

K.F.: The cost differences are significant and often favor the gravity drill rig. Primarily, there are no demolition and surface restoration costs, which in urban works represent a major saving. Additionally, we avoid soil replacement costs that are often required for deep trenches. De-watering costs are also limited because we do not have to lower groundwater on a large scale.

Ł.M.: Finally for this section, please outline the key technological stages of a gravity bore installation.

K.F.: As I mentioned, the process consists of two main stages. Stage I is the pilot bore, during which we precisely mark and drill the route for the network. Stage II involves reconfiguring the drilling equipment and pulling the target pipe along the path created by the pilot string.

Design questions

Ł.M.: Moving on to design aspects. Which geotechnical data are absolutely required to correctly design a bore for a network?

K.F.: The key data are geotechnical boreholes carried out along the bore route that must include detailed information about soil stratigraphy and, importantly, an accurate determination of the groundwater level. This allows us to select the appropriate rig and slurry parameters.

Ł.M.: Is geophysical documentation or CPT testing needed for design? What value do they add?

K.F.: Geophysical documentation and cone penetration tests (CPT) are very useful because they provide additional detailed information on soil properties, enabling more precise planning and risk reduction. However, they are not absolutely required if we already have sufficient borehole data, including predicted soil bearing capacity.

Ł.M.: Gravity drill rigs are known for compactness. From a design perspective, how should start and receiving chambers be designed to maximise this advantage and, in most cases, avoid traditional start manholes?

K.F.: This is one of the biggest benefits of our technology. Standard reinforced concrete manholes sized DN1200 or DN1500 often serve as start and receiving chambers, and in most cases our technology eliminates the need for dedicated technical chambers. We can use prefabricated start chambers produced by our company, designed to minimise footprint. This results in considerable time and cost savings and enables work in locations where traditional manholes would be impossible due to very limited space.

Key are geotechnical boreholes carried out along the bore route that must include detailed information about soil stratigraphy and, importantly, an accurate determination of the groundwater level

Ł.M.: Can gravity bores be executed on a curve, or only as straight lines?

K.F.: With the gravity drill rig, bores can only be executed in a straight line. Precise control of slope and axis requires a straight alignment.

Ł.M.: Can hybrid solutions be used, for example installing part of a network by open-cut and part by drilling?

K.F.: Absolutely. We often use hybrid solutions, combining traditional open-cut methods with gravity bores. This allows optimisation of costs and schedules and helps overcome difficult sections where open-cut excavations are impossible.

Ground and site conditions

Ł.M.: How does the gravity drill rig perform in waterlogged ground? Is a high groundwater level a problem?

K.F.: Our technology copes with waterlogged ground. However, the feasibility in specific ground-water conditions is always assessed individually. We analyse groundwater level and dynamics to determine any additional works required, such as dewatering, and to select appropriate rig and slurry parameters.

Ł.M.: Can bores be performed under wetlands or in close proximity to rivers?

K.F.: Yes, this is possible. Trenchless technology is particularly advantageous in such situations because it minimises the risk of destabilising the terrain and adverse impacts on the ecosystem.

Ł.M.: How does the machine behave in clay or sandy soils? Is either soil type more favourable or more challenging?

K.F.: The machine performs well both in clayey and sandy soils. Naturally, each soil type requires appropriate adjustment of drilling parameters and the bentonite slurry composition, but there are no significant contraindications for either type.

Ł.M.: Does the presence of boulders in the ground prevent bore execution?

K.F.: Unfortunately, yes — the presence of large boulders can prevent bore execution. Our technology, like many trenchless methods, is not designed to drill through large, very hard obstacles.

Ł.M.: What measures can be taken in soils with heterogeneous structure to ensure smooth progress?

K.F.: In heterogeneous soils, it is crucial to adjust the bentonite slurry composition and to select the appropriate drilling equipment and its operating parameters. Thanks to the flexibility of our technology, we can meet the challenges posed by variable ground conditions.

Execution aspects

Ł.M.: What is the average time to complete a bore up to 30 metres long?

K.F.: The average completion time for a bore up to 30 metres is usually between 1 and 2 working days, depending on soil conditions and project complexity.

Ł.M.: What technical support must the contractor provide on site to enable efficient execution of the bore? Does the compactness of the gravity drill rig simplify logistics?

K.F.: The contractor must primarily ensure access to water for the drilling process. Depending on conditions, a vacuum tanker for spoil collection may be required, dewatering in case of high groundwater inflow, and a system for pumping wastewater if working on an active sewer being rehabilitated. The compactness of the technology significantly eases logistics, especially on narrow streets and constrained sites, since our equipment occupies minimal area.

Control is very precise. Based on the receiver's readings and the pilot probe inclination the operator corrects the route in real time

Ł.M.: How is equipment mobilisation and demobilisation handled on site?

K.F.: Typically two vehicles arrive on site: a heavy truck (18 t) and a van (3.5 t). We use the truck-mounted HDS crane to assemble and disassemble the rig within the manhole. The whole process is planned to minimise time and disruption on site.

Ł.M.: Are permits required to occupy the carriageway when carrying out gravity bores?

K.F.: If the works involve narrowing or closing a traffic lane, permits are required. In general, any works in the road corridor require agreements with the road authority and relevant services.

Ł.M.: How many people typically operate a gravity bore setup?

K.F.: We typically need four people to operate the gravity bore setup.

Ł.M.: How is axis and slope control performed in real time during drilling?

K.F.: Control is very precise. Based on the receiver’s readings and the inclination of the pilot probe the operator continually corrects the route. Corrections are made by adjusting the deflection fin on the pilot rod, which allows precise steering of the bore direction.

Ł.M.: How is quality monitored to ensure the work proceeds according to plan?

K.F.: In the first rod, the pilot, a sonde transmits data on inclination, temperature and other parameters to the receiver. The receiver can also indicate the probe’s exact position and depth based on an electromagnetic signal, providing full control over the process.

Ł.M.: What happens if an unexpected obstacle is encountered during drilling?

K.F.: If an obstacle is encountered, the first step is to attempt to correct slope or deviation from the route. If the impact is frontal and the obstacle is not too large, we may attempt to drill through it. If the obstacle is too large or too hard (for example a boulder), it may be necessary to change the route or use a different method.

Networks and materials

Ł.M.: What types of networks can be laid with this technology? Are there specific material requirements?

K.F.: We can lay any pipes suitable for pulling — i.e. designed for trenchless technologies. Additionally, when drilling from manhole to manhole, it is possible to install pipes that can be connected within the confined space of the chamber, which is critical when working on narrow streets.

Ł.M.: How is joint tightness between network sections ensured?

K.F.: Joint tightness is ensured by using a double-lip seal, which provides a durable, watertight connection resistant to leakage and infiltration.

Ł.M.: What are the standard diameters of networks that can be installed with the gravity drill rig?

K.F.: Standard polypropylene (PP) pipe sizes we can install are: 110, 180, 225, 280, 315, 400, 450 and 500 mm. For polyethylene (PE) pipes diameters up to 500 mm are possible.

Safety and supervision

Ł.M.: What are the health and safety rules when using this technology? Are there specific guidelines?

K.F.: All H&S rules are described in detail in our Instruction for Safe Execution of Works (IBWR). We take every measure to ensure work is carried out safely for personnel and the surroundings, in accordance with applicable regulations and standards.

Ł.M.: Can work be carried out next to active railway tracks or heavily trafficked streets?

K.F.: Yes, work can be carried out adjacent to active tracks or busy streets. This is one of the main advantages of trenchless technology — it minimises disruption to traffic and infrastructure. Of course, close cooperation with infrastructure owners and adherence to safety measures is required.

Ł.M.: What are the acceptance procedures after completing the bore?

K.F.: Standard acceptance procedures include TV inspection (television inspection / CCTV) of the installed pipe to verify condition and correct installation, and a geodetic survey that confirms the exact position and slope of the bore.

Ł.M.: Are post-construction reports from the bore provided?

K.F.: Yes, the tracking systems we use can generate post-construction reports. They contain data on the bore path, depth, slope and other parameters, providing valuable documentation for the client.

Ecology and sustainability

Ł.M.: How does the technology impact the natural environment compared to traditional methods?

K.F.: Gravity drill rig technology has minimal or no impact on the natural environment. Unlike open-cut excavations, it does not require removing large volumes of soil, does not disrupt ground structure on a wide scale and does not generate significant emissions.

Ł.M.: What are the differences in CO2 emissions compared to open trenches? Is this method more environmentally friendly?

K.F.: Definitely. CO2 emissions are significantly lower with drilling technology compared to open trenches. This is due to lower fuel consumption of machines, no need to transport large amounts of spoil and minimised traffic disruption.

Ł.M.: What about site restoration after the works?

K.F.: With the gravity drill rig technology broad-scale site restoration is not required. The minimal surface intervention means only minor works are needed after completion.

Other practical matters

Ł.M.: The gravity drill rig is known for its ability to operate in challenging conditions. Is winter execution possible, allowing projects to avoid seasonality?

K.F.: Yes, that is an important advantage of our technology. Execution in winter conditions is possible, which greatly reduces seasonality and allows projects to continue year-round. Of course, at very low temperatures near freezing there may be some limitations due to the water-based bentonite slurry, but in most cases typical winter conditions do not prevent bore operations in our region. 

With gravity drill rig technology broad-scale site restoration is not required. The minimal surface intervention means that after completion it is usually sufficient to backfill the small start and receiving chambers and the surrounding area quickly returns to its original state

Ł.M.: What are the most common design mistakes with this technology?

K.F.: One common design error is the lack of information about the need to deepen technical manholes where they are planned. Incorrect depth of the start and receiving chambers can hinder or prevent proper positioning and operation of the rig.

Ł.M.: Is this technology suitable for upgrading old sewer systems?

K.F.: Absolutely. Gravity drill rig technology is well suited for upgrading old sewer systems. It enables replacement of degraded pipes and even allows increasing their diameter while maintaining the axis alignment, which is important when addressing growing retention and conveyance needs.

Ł.M.: Thank you very much for such comprehensive and valuable information. This interview certainly clarified many aspects of gravity drill rig technology, highlighting its compactness, suitability for narrow streets and restricted sites, the fact that dedicated start manholes are not required in most cases, and the ability to operate in winter, which removes seasonality from works.

K.F.: My pleasure. I’m glad I could introduce you to this innovative and efficient method.