# Analysis of Deformations of the Tunnel Excavation Face via Simplified Calculation Methods
###### Abstract  
The stability of the excavation face during the excavation of a tunnel is necessary to safeguard the lives of workers and to guarantee regular and rapid progress of the works. In this paper, the stability conditions of the tunnelling face were analysed using simplified numerical calculation methods (one-step approach) and analytical methods, in order to evaluate the extrusion of the tunnelling face as a representative quantity of the mechanical behaviour of the rock ahead of it. A comparison between the numerical method and the hemispherical method was illustrated, which was able to demonstrate how the latter can be used with high reliability in this type of study. An extensive parametric analysis of the typical conditions encountered in the excavation of tunnels in weak rock made it possible to determine the extent of the face extrusion and the effect on it of the main considered geometric and geomechanical parameters. Thanks to the comparison of the extrusion values obtained from the calculation with the limit value indicated by the scientific literature, it is possible to arrive at a rapid assessment on the stability of the face. A specific study on the role of the pressure applied to the face on the extent of the extrusion then made it possible to understand how to proceed to define the intensity of the main stabilisation systems (TBM thrust and longitudinal fiberglass bolts) to avoid the risk of collapse of the excavation face. t 20232020233/13/1268  
The stability of the excavation face in shallow tunnels is affected by several factors such as soil type, overburden pressure, and groundwater level. Soft soils, such as clays and silts, are particularly prone to face instability due to their low strength and high compressibility. Additionally, the overburden pressure exerted by the soil above the excavation face can cause deformations and failure of the face [10]. In some cases, groundwater seepage can also contribute to the destabilization of the excavation face, particularly in regions with high water table levels [11]. To prevent or mitigate face instability in shallow tunnels, various measures can be adopted such as using reinforcement techniques and grouting. Reinforcement techniques such as fibreglass dowels and shotcrete can be used to stabilize the excavation face and prevent its collapse. It is also possible to improve the strength and stability of the soil around an ahead of an excavation face by grouting: in this procedure, cement-based grout is injected into the soil in order to fill voids and enhance shear strength [12]. Tunnel face instability can also be encountered during the construction of deep tunnels, i.e., tunnels where the depth of the tunnel axis is greater than 10-12 times its radius [13; 14; 15; 16]. In modern tunnel construction, full-face excavation is becoming a popular method to improve the efficiency and effectiveness of the tunnelling process. By utilizing large-scale machinery and equipment, this approach aims to optimize the use of available resources and streamline the construction process. This method is being adopted in various scenarios to make tunnel excavation more systematic and efficient [17]. Experts have long recognized the advantages of full-face excavation, dating back at least a century, and have advocated its use wherever possible. This method is highly beneficial in improving the efficiency of the tunnel construction process and can lead to significant time and cost savings. Despite its age, the practice remains relevant and valuable in modern tunnel construction [18]. However, the stability of the tunnel face is a critical concern for ensuring tunnel stability in numerous circumstances [19]. The stability of the tunnel face can be influenced not only by geological factors, such as the presence of unstable ground or poor-quality soil that is prone to squeezing, but also by the effects of construction techniques and environmental conditions. This adds to the intricacy of studying and ensuring tunnel stability [20].