Zbornik radova Građevinskog fakulteta Subotica, Journal of Faculty of Civil Engineering

Research and determination of the blasting damage zone around the profile of tunnels or underground structures is a platform that would serve to confirm or correct blasting parameters and underground assemblies, that is, geotechnical and main tunnel projects. From the obtained research, the stability of the excavation profile, as well as the safety risk for equipment and personnel on the construction site, would be better understood. Considering the importance of the size of the damage zone of the rock mass in terms of its influence on stability, the amount of primary support elements required to achieve the stability of the excavated space, and the additional costs of stabilization of the excavation and the safety of people and equipment, the aforementioned problem was investigated and processed for specific cases and for the rock mass characteristic for the location in question. From the described research, the change in the elastic characteristics of the rock mass and the depth of the damage zone around the tunnel excavation profile were measured and calculated.

E-Zbornik : Elektronički zbornik radova Građevinskog fakulteta Sveučilišta u Mostaru / e-ZBORNIK : Electronic Collection of Papers of the Faculty of Civil Engineering

This paper systematically analyzes the challenges of stabilizing tunnel excavations in zones with low overburden in urban environments, through an engineering-validated case study of the Kobilja Glava Tunnel. A combined approach involving the New Austrian Tunneling Method (NATM) and the pre-installation of steel pipe umbrellas was applied as the primary pre-support measure under complex geotechnical conditions. The design, drilling, grouting, and formation of the temporary support arch were thoroughly documented, along with the implementation of shotcrete, lattice girders, self-drilling anchors, and reinforcement meshes. A numerical analysis was performed using the PLAXIS 2D software package, encompassing the modeling of deformations, shear forces, axial forces, and bending moments, with precisely defined support parameters. Geodetic monitoring recorded maximum surface settlements of up to 70 mm at an overburden of less than 3 m, while deformations were reduced to 28 mm at an overburden of 20 m. The numerical model confirmed soil plasticization within a 3 m wide zone, with maximum displacements reaching 6.3 cm, consistent with field measurements. Calculated tensile strain and angular distortion were classified according to established building damage criteria, confirming minimal structural impact on adjacent buildings. The applied combination of the NATM and the pipe umbrella pre-support system proved to be an effective and reliable solution for controlling deformations and ensuring excavation stability under conditions of limited rock cover and dense urban development. The obtained results provide a verified framework and practical recommendations for future tunneling projects in similar geotechnical and urban conditions, aiming to enhance safety, stability, and cost-effectiveness.

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.)

Miniranje je široko primijenjena metoda iskopa u podzemnoj gradnji u I, II, III i IV kategoriji stijenske mase. Međutim, vibracije izazvane eksplozijom, ako nisu pravilno kontrolisane, mogu negativno utjecati na stijensku masu koja okružuje tunel. Nekvalitetan iskop primjenom tehnike miniranja može uzrokovati povećani prekoprofilni iskop i prekomjerno rastresanje okolnog stijenskog masiva te smanjenje nosivosti stijene koja je osnovni podgradni element. Određivanje dubine zone oštećenja stijenske mase oko profila iskopa miniranjem seizmičkom cross-hole tomografijom imala su za cilj odrediti dubinu zone oštećenja i promjene fizičko-mehaničkih svojstava materijala oko konturne površine podzemnog iskopa u III kategoriji stijenske mase, odnosno odrediti intenzitet oštećenja unutar poremećene zone ovisno o udaljenosti od minskog polja i parametara bušenja i miniranja.