Research on the phenomenon of increasing borehole diameter at the installation of rod anchors in marl using wet technology compared to dry drilling procedure

 Corresponding author: Satko Filipović Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Research on the phenomenon of increasing borehole diameter at the installation of rod anchors in marl using wet technology compared to dry drilling procedure Satko Filipović 1, *, Ekrem Bektašević 2, Kemal Gutić 3, Namik Musa 4 and Noris Sakić 5 1 Ministry of traffic - Road Directorate of Canton Sarajevo, Project manager. 2 PPG” d.o.o. Sarajevo, Supervising Engineer. 3 Faculty of Mining, Geology and Civil Engineering, University of Tuzla, Full professor, Bosnia and Herzegovina. 4 PPG” d.o.o. Sarajevo, Assistant to Supervising Engineer. 5 Faculty of Mining, Geology and Civil Engineering, University of Tuzla, Assistant, Bosnia and Herzegovina. Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 Publication history: Received on 23 December 2024; revised on 31 January 2025; accepted on 02 February 2025 Article DOI: https://doi.org/10.30574/gjeta.2025.22.2.0019 Abstract The excavation of the “Kobilja Glava” tunnel takes place in a complex geological environment and in a highly urbanized zone with an extremely large number of residential buildings located above both tunnel pipes, as well as in the very close distance of them. More exactly, at the entrance and exit tunnel portals there is a decaying crust whose thickness varies from 3.50 to 6.0 meters. It is an extremely incoherent material that is exposed to significant degradation processes under the influence of atmospheric conditions, which significantly create difficulties while prediction of the actual characteristics of material. Also, the excavation job is being carried out in a substrate that is made of marl, dark gray colour with a distinctly layered structure. For the purposes of this work, exploratory drillholes were drilled using two different technologies. The first implies that the drillholes are made using a wet procedure (flushing of the material from drillhole is done using water under pressure), with the measurement of the diameter of the drillhole at three reference locations inside the drillhole. For this technology the reference value of the drillhole diameter would be achieved by statistical processing. The second technology implied that the drilling of exploratory drillhole was carried with the same equipment, using a dry method (the mud of the material is carried out using compressed air), and that the diameter of the drillhole was recorded in the same way, under identical conditions. The aim of research was to establish how the applied wet drilling method affect to the degradation of the physical-mechanical characteristics of the marl, which has a several chain effects on the construction of the tunnel itself (increasing convergence due to the decreasing the physical-mechanical characteristics of the rock mass, the need for by applying additional supporting elements, increased consumption of injection mixture that results in an increase of negative impact on the environment, etc.) Keywords: Exploratory drillhole; Increasing the diameter of drillhole; Degradation of marl under the influence of water; Wet drilling process; Dry drilling process 1. Introduction The rapid development of urban areas in recent decades contributes to increasing needs for using the underground space[1]. Tunnel construction is a demanding interdisciplinary work [2]. Contemporary design and construction of tunnels requires appropriate techniques and technologies in all phases of the tunnel project [3]. Excavation of the tunnel can be considered as the most important working phases due to the series of consequences it can cause [4]. “Kobilja Glava” tunnel is part of the main project of connecting the trans-European road network at the City road named the First transversal in Sarajevo Canton. It is a two-tube tunnel with separate traffic directions, which is located in a close urban Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 2 area where there is a large number of residential and infrastructure facilities. An additional aggravating circumstance during the construction of this tunnel is the fact that it is located very shallowly below the mentioned buildings, so it is an extremely small layer above the tunnel itself. This fact greatly complicates the construction of the tunnel itself, since by disturbing the "natural state of stress" (in-situ stress) that inevitably occurs during tunnel excavation, there is a redistribution of stress accompanied by displacements, which do not only occur at the level of the tunnel excavation and in the immediate vicinity of it, but depending on the type of soil mass in which excavations are carried out, it is transferred to a wider area around the excavation itself. In a situation where the building are located in the immediate vicinity, it is extremely important that during the excavation of the tunnel, all available measures are applied, which will enable the maximum reduction of the above-mentioned impacts on the surrounding soil, and therefore the reduction of potential damage to residential and infrastructure buildings. For this reason, during the construction of “Kobilja Glava” tunnel, research was started with the main goal of determining how the drilling of drillholes for rod anchors using wet process technology, which involves flushing out the material from the drillhole using pressurized water, affects the weakening of the rock material. Of particular interest was the issue of increasing the diameter of the drillhole, which results in a series of unwanted effects during the construction of the tunnel, both from the point of view of additional weakening of the rock mass, and from the point of view of increased consumption of the injection mixture for filling the mentioned drillhole. The geographic location of “Kobilja Glava” tunnel on the route of the Sarajevo-Vogošća road is shown in Figure 1. Figure 1 Geographical location of “Kobilja Glava” tunnel on the route of the Sarajevo-Vogošća road [1] 2. Research methodology 2.1. Engineering geological characteristics along the route of "Kobilja Glava" tunnel During the daily mapping of the open faces during the excavation of “Kobilja Glava” tunnel, the established lithological geological structure of the material along the route of the tunnel consisted of surface cover and substrate. Furthermore, it can be stated that two groups can be observed in the surface cover itself, more excitably that are man-made creationsembankment created by the construction of the surrounding residential and infrastructure facilities, and the eluvialdeluvial cover. Both of these groups represent an extremely unfavourable environment for carrying out construction operations. In the geological substratum, two groups are also observed, more exactly, the decaying crust of the geological substratum and the undisturbed geological substratum. The first of these two groups is built from clayey marl materials, and as such is extremely susceptible to various influences, especially the effect of water and moisture from the environment. The undisturbed geological substrate is made of marl, dark gray colour, with a distinct layered structure. Such substrate has significantly better properties than decay crust, but it is still a material that generally has largely uneven physical and mechanical properties, which also significantly depend on exposure to water. The basic characteristic of marl is that with little exposure to external influences, they change their intact properties extremely quickly, i.e. they degrade [5]. Regarding to this, it would be more correct to treat marl as a soft rock mass. According to the RMR classification (Bieniawski's classification) of rock masses, marls at “Kobilja Glava” tunnel construction site can be classified into V and IV categories. Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 3 More exactly, in the portal zones, considering the extremely small layer above the tunnel, and the fact that the slope above the tunnel has a slight increase in height, the V category is predicted. But in the zones where the layer above the tunnel is thicker, a more compact rock mass is expected, which was not exposed to the processes of surface decomposition, and therefore has significantly better characteristics, that zone of the tunnel is characterized as category IV rock mass according to the RMR classification. In accordance with the above mentioned, extensive analyses were carried out on the basis of which the zoning of the tunnel itself was carried out, to which the supporting elements recommended for use during tunnel excavation were adjusted. Based on the analyses carried out, both tunnel pipes were divided into five sectors related to the application of different supporting elements, as shown in Figure 2. Figure 2 Presentation of the division of the tunnel into sectors by type of supporting elements In the portal zones of both tunnel pipes, a category V rock mass was found, while in the remaining part of the tunnel, category IV rock mass was found, all based on extensive analyses carried out according to Bienawski [6], Marinos et al. 2007, Marinos and Hoek, 2001[7], NATM recommendations, ONORM 2203 standard, as well as stress and deformation analysis of the rock mass around the underground excavation using the PLAXIS 2D software package (with evaluation of 3D impact during excavation)[8]. According to above mentioned, it has been rationalized and adequate support, consisting of shotcrete, reinforcing mesh, steel belts of appropriate dimensions, and SN and IBO anchors with appropriate diameters. 2.2. Geotechnical characteristics of underground excavation stabilization “Kobilja Glava” tunnel was excavated using the New Austrian Tunnelling Method (NATM). Bearing in mind the frequent changes in the engineering-geological characteristics of the rock mass in the part of the tunnel excavation, NATM enabled the application of multi-phase excavation, while at the same time securing the excavation with a primary support [9]. NATM (New Austrian Tunnelling Method) represents the general concept or philosophy of tunnel construction. It is a procedure that is based first on the basis of scientifically established ideas and facts that were later confirmed through practice, with the task of achieving optimal safety in the synergy of the capacity of the rock mass with the applied supporting elements, and at the same time the economy of the tunnel construction process itself. Therefore, NATM represents a successful system of improving the geotechnical characteristics of the rock mass by applying support, which takes advantage of the primary strength of the rock, the rationality of the supporting systems, and the mandatory control of the support by measuring the deformations of the excavated and supported tunnel. In “Kobilja Glava” tunnel, support types for category IV and V were applied with occasional modifications through geotechnical missions. The decision to apply modified types of support was additionally confirmed and supported by calculations through individual geotechnical missions, which were carried out during the recording of rock mass changes during the tunnel excavation works. The basic supporting elements of category IV are shown below (Figure 3): • Length of the excavation step 1.0 – 1.2 m depending on the conditions of the environment through which the excavation is carried out; Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 4 • IBO anchors ϕ32 length l=4m in the ceiling part of calote, • IBO anchors ϕ32 length l=6m in the sides of calote, • Lattice girder PS 95/20/30, • shotcrete C25/30 layer thickness ds=24 cm, • ribbed reinforcing mesh Q257 Figure 3 Support type for rock mass category IV 2.3. Basic supporting elements of category V are shown below • Pipe shield – steel pipes ϕ114 mm length l=12m to protect the ceiling of the excavation (overlap of 4,0m), • Length of excavation step 0,5 – 0,8 m depending on the conditions of the environment through which the excavation is carried out, • Shotcrete C25/30 layer thickness ds=30 cm, • Ribbed reinforcing mesh Q257, • Lattice girder PS 95/20/30, • Ibo anchors ϕ32 length l=6m, • Shotcrete thickness ds=25 cm, • Ribbed reinforcing mesh Q257. In addition to the basic supporting elements in the IV category of rock mass, during the excavation in certain sections of the tunnel, Ø32mm steel rods were also applied, if necessary, to protect the ceiling of the calote in lengths of l=3 and 4m. As additional support measures in the V category, we can single out the elephant foot in the form of an extension of the support zone in calote area, and SN anchors 51 mm. In regard to SN 51 mm anchors, they were applied after it was determined that there was a shear failure of the installed designed anchors, which resulted in the appearance of uneven settlement along the contour of the tunnel excavation. These anchors did not have a tensile load bearing function, but their primary function was to suspend the straps to reduce uneven convergences, i.e. greater subsidence of one side compared to the other [9]. For the purpose of stabilizing the face of the excavation in the left tunnel pipe on a certain section of the tunnel where longitudinal movements were observed during excavation at the face of the excavation, IBO anchors Ø32 12 meters long were used to strengthen the rock mass in the face as an additional security measure (Figure 4). Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 5 Figure 4 Support type for rock mass category V 2.4. Technology of installing rod self-drilling anchors Self-drilling IBO-SDA anchors are a special type of rod anchors that represent a combined system of rock anchors and drilling rods. During drilling, the hollow IBO anchor is used as a drill rod. At the end of the anchor, a drilling crown is placed, which can be of different sections [10]. The basic elements of these anchors are: hollow steel pipes of suitable outer and inner diameter (R32, R38, R51), supporting plates of suitable dimensions, nuts which can be dome and hex (R32, R38, R51), connectors used to connect hollow steel rods, whose diameter matches the diameter of the rod itself, the spacer whose primary function is to maintain the central position of the anchor in the well itself, and the associated disposable drill bits (depending on the diameter of the hollow steel rod itself). The following picture shows the installation of self-drilling anchors (Figure 5). Figure 5 Basic elements of IBO-SDA self-drilling anchors Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 6 The use of anchors is inevitable in situations where it is necessary to increase or maintain the stability of rock mass or soil. They have found their wide application in tunnel construction, where they are used as elements of the primary substructure, thereby increasing the shear strength of the surrounding soil. At that stage, the anchors are predominantly loaded in tension. The drilling of the wells for these anchors is performed using a disposable crown and the anchor rod, through which the well is flushed out either by applying water or air. Drilling is carried out using specialized machinery (Figure 6). The machine has two arms that allow drilling at specified locations at the head of the tunnel excavation. An anchor of the designed diameter with a suitable disposable crown is placed in the arm of the machine, and the drillhole is drilled to the designed depth. During drilling, if it is a wet process, water is injected under pressure, which is used to wash out the excavated material. According to the same principle, drilling is also carried out using a dry process, with the fact that in this case, instead of water, a pipe is connected to the machine through which air is pumped in under pressure. In this way, the cleaning of excavated material from the drillhole is done with compressed air. Each of the procedures has certain disadvantages. When it comes to the wet process, the disadvantage is reflected in the fact that large amounts of water are introduced into the drillhole, so materials that are exposed to the degradation process under the influence of water, leads to the weakening of the rock material. Another disadvantage is a significant accumulation of water in front of the tunnel head, which complicates the work and must be promptly removed from the specified location. The problem with the removing water from this location is even bigger if the direction of progress of the tunnel excavation coincides with the decreasing level of the tunnel. When it comes to the dry process, disadvantage increasing dust, which needs to be urgently removed so that it does not negatively affect the health of workers. Also, in addition to dust removal, it is necessary to adjust the ventilation system in such a way as to ensure the supply of sufficient quantities of air for workers. Figure 6 Layout of the drilling rig manufactured by Epiroc - Boomer Injecting the drillholes with the injection mixture through the anchor body as well as the crown can be done either after the completion of drilling (Figure 5) or during the drilling procedure [11]. 3. Determining the increase of drillhole diameter in marl using wet drilling technology compared to dry drilling technology Due to the above mentioned deficiencies, especially when it comes to the wet process, and the construction of “Kobilja Glava” tunnel takes place in marl, a material that is extremely susceptible to degradation under the influence of water, we assumed that flushing the wells with water when drilling would lead to an increase in the diameter. In this connection, we conducted research work paying special attention to this issue. For the purposes of confirming the basic hypothesis expressed in the title of this paper, which is the increase of drillhole diameter when applying wet process compared to the dry process, drilling of exploratory drillhole was carried out at station km 4+049.69 in LTC. Based on its characteristics the rock mass is classified as category IV. In “Kobilja Glava” tunnel, most of the anchors that were installed as a supporting element were anchors with a length of 6 m, the necessary time for the installation of the anchors was standardized. Standardization has been established that the time duration of drilling the drillhole for Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 7 installation of rod anchors of 6 m is 3.05 minutes. based on the average value of monitoring the installation of three anchors, Table 1. Table 1 Standardization of installation of 6m long anchors in “Kobilja Glava” tunnel No. Type of anchors IBO ϕ32 Unit of measure Established time of installation 1. L=6m min 3.20 2. L=6m min 2.57 3. L=6m min 3.40 Aver.time 3.05 min At the beginning of the tunnel excavation in the left tunnel pipe at station km 4+049.69, 6 drillholes were first marked, which will be used to confirm the hypothesis. After that, drilling was started using specialized machinery. For the purposes of confirming the basic hypothesis, two series of drillholes were made, marked with the numbers 1 and 2. Number 1 indicates drillholes that were drilled using the dry drilling method, while the number 2 indicates drillholes that were drilled using the wet drilling method (Figure 7). Figure 7 Marked places for drilling at the head of the tunnel excavation A total of 6 exploratory drillholes were drilled, three using the dry method (using compressed air for drilling mud), and three using a wet procedure (flushing the wells with water). It is important to point out that when drilling wells with water, the drilling period is adapted to the established drilling time of drillholes of 6m, which means that the drilling time of experimental drillholes with a depth of 1.0m was carried out reciprocally to the drilling of drillholes of 6m (approx. 3.0 min.). The view of drilling drillholes is shown in Figure 8. Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 8 Figure 8 Presentation of drillholes drilling: a) dry process and b) wet process Drilling was done in an alternating order, more exactly first a dry one was drilled, then a wet one, and so on. The drilling was carried out according to the previously defined time operations, which precisely defined the length of drilling the drillholes in minutes, with the known consumption of water for flushing the drillholes. For the purposes of this research, and in order to be able to read the diameter of the exploratory drillholes, they were drilled to a depth of approx. 1.0 m. Furthermore, by using a handy (simple) accessory made of two metal profiles (approx. 1 cm), in such a way that they were connected in the middle by a shaft that allows the movement of these profiles, more precisely opening and closing the angle between the profiles, it was possible at reference depths of 15, 50 and 80 cm inside the drillhole, measure with a digital calliper, actually the diameter of the drillhole at those three positions. The obtained values were averaged for each individual drillhole. A schematic representation of diameter measurements at reference depths inside the drillhole is shown in Figure 9. Figure 9 Schematic presentation of diameter measurements at reference positions inside the drillhole The next step was to determine the final average value on the basis of the average values for each of the series of three drillholes made using the same method, in order to be able to compare the obtained values, which was also to obtain the final difference in the size of the diameters obtained by applying two different drilling methods. It is extremely important to note here that the same rod anchor crown with a diameter of 50.5 mm was used for drilling all the drillholes, which is presented in Figure 10. Global Journal of Engineering and Technology Advances, 2025, 22(02), 001–014 9 Figure 10 Rod anchor crown used for drilling exploratory wells 4. Results A presentation of all recorded measurement results at reference locations within all six drillholes is given in Table 2 and Figures 11. and 12. Table 2 Obtained results of measuring the diameter of all six exploratory drillholes Measurement location in the drillhole (cm) TYPE OF DRILLHOLE dry process wet process dry process wet process dry process wet process 1 2 1 2 1 2 15 5.07 5.18 5.05 5.20 5.06 5.22 50 5.05 5.23 5.05 5.24 5.02 5.26 80 5.04 5.30 5.04 5.32 5.08 5.34 Averg. time. drillhole 5.05 5.24 5.05 5.25 5.05 5.27 Avrg. drillhole diameter value - dry process (ds) 5.051 Avrg. drillhole diameter value - wet process (dm) 5.254 Percentage increase in diameter - dry/wet process 4.03%