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.

A proper assessment of the stability leads to proactive decision making for the method of construction, the shape and size of the cross-section of the facility and the adequate support. The stability is directly dependent on the physical, mechanical and technical characteristics of the rock mass. The traditional assessment-based approach and the classification of stability of underground facilities is based on both experience and experiments with previously constructed facilities. To ease the process for the early phase of design, there was a need for the development of a mathematical model for assessment of the stability. Ambiguity in mining engineering, directs the development of the model using a fuzzy logic system and fuzzy "if-then" rules. The model was developed based on very scarce input data that is most often the only data available during the design stage. Fuzzy model results have shown, through the random selection of input data in the model, that in 70% of cases, there is high numerical congruence with the traditional method for determination of coefficient of stability S; and in 90% of cases, there is also congruence in accordance with the traditional approach to the classification of underground facilities based on the stability assessment.

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

The application of multichannel surface wave analysis (MASW) has proven to be an effective method for non-invasive investigation of shallow geological structures, especially under complex conditions with heterogeneous layers and the presence of groundwater. In the technical stone quarry field in Čelina, MASW enabled precise soil characterization, identification of transition zones and anomalies, and determination of seismic resistance of layers based on the Vs30 parameter. The investigations confirmed the presence of a fault structure within the deposit, with the wider fault zone proving to be hydrogeologically significant, potentially acting as an aquifer. The geological composition of the carbonate terrain, characterized by lateral variability, required a conservative approach to the analysis based on the fundamental dispersion mode. The creation of a three-dimensional model from combined MASW profiles provided accurate spatial insight into the mechanical properties of the soil, facilitating engineering decision-making for future interventions. The results also indicated the need for further investigations, including the potential drilling of an exploration borehole in the fault zone to confirm the presence of an aquifer layer. MASW is confirmed as a reliable, economical, and practically applicable method for a wide range of engineering and hydrogeological studies. Its ability to quickly profile the soil makes it suitable for urban and inaccessible areas, and when combined with other geophysical methods, it can contribute to an integrated approach to subsurface analysis.

This study investigates road tunnel stability in heterogeneous flysch formations using the Zenica Tunnel as a case study. A hybrid research framework integrating empirical classification, analytical modeling, and numerical simulation was applied. The approach combines the Rock Mass Rating (RMR) system, the Convergence–Confinement Method (CCM), and nonlinear two-dimensional finite element (FEM) analyses. Statistical evaluation of the results reveals a strong exponential relationship between the stability factor N_s and measured tunnel convergence, with coefficients of determination (R²) between 0.89 and 0.96. Particular attention was given to sections classified as Category V rock mass. The analysis indicates that when RMR values fall below 25, the stability factor N_s exceeds the critical value of 5, marking the onset of pronounced squeezing behavior. The results show that analytical methods provide conservative estimates of tunnel stability, while numerical modeling enables improved calibration of support system stiffness. The proposed integrated methodology contributes to more reliable stability assessment and support design in road tunnels excavated in complex flysch formations.

Excavation of tunnels with a small overburden inevitably implies surface settlements,especially when they are carried out in soft soil or weak rock masses. Subsidencebecomes a critical aspect when the construction of tunnels is realized in narrower urbanareas, directly below occupied residential buildings. Taking as an example the KobiljaGlava tunnel, which will represent part of the main project connecting Vogošća withSarajevo and at the same time the connection of the narrowest city center of Sarajevowith the A1 motorway on the Vc corridor, surface deformations were monitored during theexcavation and installation of the primary support of the left tunnel tube. Special attentionwas dedicated to excavation from the entrance side on a certain section of the tunnelwhere the height of the overburden was less than 2D. During the excavation of the lefttunnel tube of the Kobilja Glava tunnel, which was carried out from the entrance portal atchainage km 3+543,202 in the direction of Vogošće, multiple surface settlements weremeasured from the very entrance to the place where the excavation was stopped(chainage km 3+615,56) compared to the predicted values obtained by assessment. Themain reason for stopping the further progress of the excavation of the left tunnel tubefrom the chainage km 3+615.56 lies in the fact that the direction of the geological layerscoincided with the direction of the progress of the excavation, which had a significantnegative impact on the movement of the soil itself, and therefore on the increase surfacedeformations. After a comprehensive analysis of the above, as well as consideration ofoptional possibilities, it was decided to approach the excavation of the left tunnel tubefrom another attack point, i.e. to start the excavation from cross passage No. 1 in thedirection towards the entrance. In this way, it was possible that the direction of thegeological layers under these cirrcustamces positively contributes to the reduction ofsurface subsidence caused by the progress of the tunnel excavation by over 50%.