Editor: Good morning! In today’s interview we will delve into the fascinating world of trenchless technologies, focusing specifically on the gravity drilling rig. My guest is expert Krystian Frank, who will help dispel any doubts and explain the nuances of this innovative method. Let’s start with the basics:
General Questions about the Technology
Editor: What exactly does the gravity drilling rig technology involve and what distinguishes it from other methods?
Expert: Gravity drilling rig technology is an advanced trenchless installation method for networks that uses a two-stage drilling process with precise slope control. A key element is the use of a bentonite slurry, which stabilizes the borehole, facilitates spoil transport, and reduces resistance when pulling the pipe. In the first stage we perform a pilot bore that lays out the exact route. Then, in the second stage, the drilling equipment is reconfigured and the target network is pulled in. The entire process takes place underground, minimizing surface disruption, which is crucial, especially when working in extremely confined spaces.
Editor: In which situations is this technology recommended instead of conventional directional drilling or traditional open-cut methods? When does the gravity drilling rig become the preferred solution, particularly in the context of urban challenges and compactness?
Expert: The gravity drilling rig performs exceptionally well in highly urbanized areas where traditional excavations are difficult or impossible due to dense buildings, critical traffic arteries, or an extensive network of existing underground utilities. It is an ideal solution for work on narrow streets and in extremely confined spaces because of the compactness of the technology. Moreover, for greater depths where traditional methods would require extensive ground replacement, our technology is much more efficient.
Editor: Every technology has its limitations. What are the typical limits and challenges faced by the gravity drilling rig technology?
Expert: Indeed, there are some limitations. The minimum slope required to perform the bore is 0.5%. The maximum diameter of installed networks is 500 mm, and for that diameter the bore length is limited to about 30 meters. For smaller diameters, depending on the rig used, bore lengths can range from 40 to 60 meters.
Editor: Regarding bore lengths — what are the minimum and maximum sections that can be accomplished with this method?
Expert: As for minimum lengths, there really aren’t any — the technology is flexible. As mentioned, maximum bore lengths reach up to 60 meters, depending on soil types and the specifications of the rig used.
Editor: What tolerances for slope and axial deviation are permissible with this technology? Can we expect high precision?
Expert: Gravity drilling rig technology is characterized by very high precision. Permissible slope tolerances are as little as ±0.1%, which allows for extremely accurate installation of gravity networks.
Editor: Is this technology suitable for cohesionless soils, or does it perform better in cohesive soils?
Expert: Yes, the technology is effective in cohesionless soils as well. Thanks to the bentonite slurry, which forms a stabilizing film on the borehole walls, we minimize the risk of collapse and maintain the stability of the drilled hole.
Editor: What types of networks can be installed using this method? Are there any material limitations?
Expert: We can install all types of pipes intended for bore-and-pull installations. This applies to both plastic pipes and other materials, provided they meet the appropriate strength and structural requirements for underground networks.
Editor: Can the gravity drilling rig be used to install only gravity networks, or also pressurized networks?
Expert: This is a very important question. The answer is: yes, pressure pipes can be installed. Although the name suggests gravity, the technology allows precise placement of pressurized networks, which broadens its applications.
Editor: Let’s look at economic aspects. What are the main cost differences compared to traditional methods?
Expert: Cost differences are significant and often favor the gravity drilling rig. First and foremost, there are no costs for demolition and surface restoration, which is a huge saving in urban works. Additionally, we do not incur costs for soil replacement, which is often necessary with deep excavations. There are also reduced dewatering costs, since we do not need to lower groundwater levels on a large scale.
Editor: To conclude this section, please outline the key technological stages of executing a gravity bore.
Expert: 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 under the network. Then, in Stage II, the drilling equipment is reconfigured and the target network is pulled into the previously prepared hole.
Design Questions
Editor: Now let’s move on to design aspects. What geotechnical data are absolutely required to properly design a bore for a network?
Expert: Key requirements are geological boreholes drilled along the bore route, which must include detailed information on soil stratigraphy and, importantly, precise determination of groundwater levels. This allows us to select the appropriate rig and slurry parameters.
Editor: Is geophysical documentation or CPT sounding also needed for design? What is their significance?
Expert: Geophysical documentation and CPT (cone penetration) soundings are very useful because they provide additional detailed information about soil properties, enabling even more accurate project planning and risk minimization. However, they are not absolutely essential if sufficient data from boreholes are available.
Editor: The gravity drilling rig technology is known for its compactness. In terms of design, how should start and reception chambers be designed to maximize this feature and in most cases avoid the need to construct traditional launch pits?
Expert: This is one of the greatest advantages of our technology! Standard reinforced concrete manholes of appropriate dimensions most often serve as start and reception chambers, but our technology in the majority of cases eliminates the need to build dedicated launch pits. We can use prefabricated start chambers, for example those produced by our company, which are designed to minimize space. This translates into significant time and cost savings and also enables work in places where traditional chambers would be impossible due to very limited space.
Editor: Can gravity bores be executed on a curve, or only in a straight line?
Expert: In gravity drilling rig technology, bores can be executed only in a straight line. Precise slope and axis control require a straight trajectory.
Editor: Can hybrid solutions be used, for example installing part of the network by open trench and part by drilling technology?
Expert: Absolutely yes. We often use hybrid solutions, combining traditional open-cut methods with gravity bores. This allows optimization of costs and project time and helps overcome difficult sections where open cuts are impossible.
Ground and Site Conditions
Editor: How does the gravity drilling rig perform in water-bearing soils? Is a high groundwater level a problem?
Expert: Yes, our technology works in water-bearing soils. Nevertheless, the feasibility of using the technology under specific soil-water conditions is always assessed individually. We analyze the level and dynamics of groundwater to select appropriate rig and slurry parameters.
Editor: Can bores be carried out under wetlands or in the immediate vicinity of rivers?
Expert: Yes, this is possible. Trenchless technology is especially beneficial in such locations because it minimizes the risk of destabilizing the terrain and negative impacts on the ecosystem.
Editor: How does the equipment behave in clayey or sandy soil? Is one of these soil types more favorable or more challenging?
Expert: The equipment operates well in both clayey and sandy soils. Of course, each soil type requires appropriate adjustment of drilling parameters and the composition of the bentonite slurry, but there are no major contraindications.
Editor: Does the presence of boulders in the ground make bore execution impossible?
Expert: Yes, the presence of boulders unfortunately makes the bore impossible. Our technology, like many other trenchless methods, is not designed to drill through large, hard obstacles.
Editor: What measures can be applied in heterogeneous soils to ensure work continuity?
Expert: In heterogeneous soils, it is crucial to adjust the bentonite slurry composition and select the appropriate drilling equipment and its parameters. Thanks to the flexibility of our technology, we can meet the challenges presented by variable soil conditions.
Execution Aspects
Editor: What is the average time to complete a bore up to 30 meters in length?
Expert: The average completion time for a bore up to 30 meters is usually 1 to 2 working days, depending on soil conditions and project complexity.
Editor: What technical facilities must the contractor provide on site to enable efficient bore execution? Does the compactness of the gravity drilling rig simplify logistics?
Expert: The contractor must primarily provide access to water for drilling operations. Optionally, depending on conditions, a vacuum truck for spoil removal may be required, dewatering in case of high groundwater inflow, and also a system for pumping wastewater when rehabilitating an active collector. The compactness of the technology greatly facilitates logistics, especially on narrow streets and in areas with limited space, because our equipment occupies minimal area.
Editor: What does the mobilization and demobilization process of equipment on site look like?
Expert: Usually two vehicles arrive on site: one 18t truck and one 3.5t van. We use a truck-mounted HDS crane to assemble and disassemble the rig in the manhole. The whole process is planned to minimize time and disruption on site.
Editor: Are permits required for occupying the roadway in the case of gravity bore works?
Expert: If the work involves narrowing or closing a traffic lane, then permits are required. Generally, for any works within the roadway, the road authority should be properly notified.
Editor: How many people typically operate a gravity bore rig?
Expert: Typically, the bore rig is operated by a crew of four people.
Editor: How is axis and slope control performed in real time during drilling?
Expert: Control is very precise. Based on the receiver indications and the pilot probe inclination, the operator makes real-time corrections to the route. Corrections are made by adjusting the fin angle on the pilot rod, allowing precise steering of the drilling direction.
Editor: How is correctness monitored to ensure everything proceeds according to plan?
Expert: In the first rod, called the pilot, a probe is placed that transmits data on inclination, temperature and other parameters to the receiver. The receiver can also indicate the exact position and depth of the probe based on an electromagnetic signal, providing full control over the process.
Editor: What happens if an unforeseen obstacle is encountered during drilling?
Expert: If an obstacle is encountered, we first attempt to correct the slope or deviation from the route. If the impact is frontal and the obstacle is not too large, we attempt to drill through it. Otherwise, if the obstacle is too large or too hard (e.g., a boulder), it may be necessary to change the route or use another method.
Networks and Materials
Editor: What types of networks can be laid using this technology? Are there specific material requirements?
Expert: We can lay any pipes suitable for pulling, i.e., those designed for trenchless technology. Additionally, when drilling from manhole to manhole, it is possible to install pipes that can be joined within the confined space of the chamber, which is extremely important when working on narrow streets.
Editor: How is the tightness of joints between network segments ensured?
Expert: Joint tightness is ensured by using a double-lip gasket, which guarantees a durable and hermetic connection resistant to leaks and infiltration.
Editor: What are the standard diameters of networks that can be installed with the gravity drilling rig?
Expert: Standard PP (polypropylene) pipe modules we can lay are: 110, 180, 225, 280, 315, 400, 450 and 500 mm. For PE (polyethylene) pipes, diameters up to 500 mm are possible.
Safety and Supervision
Editor: What are the health and safety rules when using this technology? Are there any specific guidelines?
Expert: All H&S rules are detailed in our Safe Work Execution Instruction (IBWR). We make every effort to ensure that work is carried out safely for personnel and the surroundings, in accordance with applicable regulations and standards.
Editor: Can work be carried out near active railway tracks or heavily used streets?
Expert: Yes, work can be performed near active tracks or roads. This is one of the main advantages of trenchless technology — we minimize disruption to traffic and infrastructure. Of course, close cooperation with infrastructure managers and appropriate precautions are required.
Editor: What are the acceptance procedures after bore completion?
Expert: Standard acceptance procedures include a TV inspection of the installed network interior to verify its condition and correct installation, and a surveying inventory that confirms the exact position and slope of the bore.
Editor: Are as-built reports from the bore available?
Expert: Yes, the tracking systems we use have the capability to generate as-built reports. They contain data on the bore route, depth, slope and other parameters, which provide valuable documentation for the investor.
Ecology and Sustainable Development
Editor: How does the technology affect the natural environment compared to traditional methods?
Expert: Gravity drilling rig technology has a minimal or no impact on the natural environment. Unlike traditional excavations, it does not require removal of large amounts of soil, does not disturb the soil structure on a wide scale, and does not generate significant emissions.
Editor: What are the differences in CO₂ emissions compared to open excavations? Is this method more ecological?
Expert: Definitely yes. CO₂ emissions are significantly lower with drilling technology compared to open excavations. This results from lower fuel consumption by machines, the lack of need to transport large volumes of spoil, and minimized traffic disruptions.
Editor: What about site restoration after works?
Expert: With gravity drilling rig technology, there is no need for large-scale site reclamation. Minimal surface intervention means that upon completion it is usually sufficient to backfill the small start and reception chambers, and the area around them quickly returns to its original state.
Other Practical Issues
Editor: Gravity drilling rig technology is known for its ability to work in difficult conditions. Are implementations possible in winter conditions, enabling elimination of seasonality?
Expert: Yes, this is a very important advantage of our technology. Implementations are possible in winter conditions, which significantly eliminate seasonality and allow project continuity year-round. Of course, at very low temperatures close to freezing, there may be some limitations due to the use of water-based slurry, but in most cases winter does not prevent bore execution.
Editor: What are the most common design mistakes with this technology?
Expert: One of the most common design mistakes is the lack of information about the need to deepen technological manholes, if they are even planned. Incorrect depth of the start and reception chambers can hinder or make it impossible to correctly set up and operate the rig.
Editor: Is the technology suitable for modernizing old sewer systems?
Expert: Absolutely yes! Gravity drilling rig technology is well suited for modernizing old sewer systems. It enables replacement of degraded channels, and even their axial enlargement, which is critical when retention and transport demands increase.
Editor: Thank you very much for such comprehensive and valuable information. This interview has certainly clarified many issues regarding gravity drilling rig technology, highlighting its compactness, ability to work on narrow streets and in confined spaces, elimination of the need for dedicated start pits in most cases and the possibility of winter operation, eliminating seasonality.
Expert: The pleasure is mine. I am glad I could present this innovative and efficient method to you.
