# Numerical Modeling for Tunnel Lining Optimization
## 3 Tunnel Instrumentation  
Six convergence measurement stations (EM-7, EM-6, EM-5, EM-4, EM-3, EM-2, and EM-1) were installed, each with three measurement lines (L1, L2, and L3), and topographic references. In addition, at EM-4, a rod extensometer, three radial pressure cells and two tangential pressure cells were also deployed at this control section, as shown in Figure 6 and Table 2.  
Figure 4: Schematic tunneling process [29]. (I) Stage 1, (II) Stage 2, (III) Stage 3, and (IV) Stage 4. Figure 5: Excavation of the complete upper half section with a central core. It shows the poor quality of the soil and the presence of water seepage. Stations EM-5, EM-6, and EM-7 were installed at 5 m, 10 m, and 20 m upstream of the control section (EM-4), respectively. Stations EM-3, EM-2, and EM-1 were located at 5 m, 10 m, and 20 m downstream of the control section (EM-4), respectively. These measurements were complemented with reference surface points to monitor the settlements along the tunnel axis. In addition, a tunnel section (29+300) was instrumented with one extensometer, radial, and tangential pressure cells to establish the load acting on the PL. First, an extensometer was installed before the excavation face reached the instrumented section. Then, radial pressure cells (RPCs) were installed between the ground and the shotcrete of the primary lining (SFRC) to measure the radial stress, and tangential pressure cells (TPCs) were embedded within the shotcrete to measure the axial stresses. Figure 6b shows the configuration of the reference surface points installed to monitor the settlements along the tunnel axis. A topographical survey of both the primary and secondary lining was carried out to establish convergences and divergences. Table 2 presents a summary of the installed sensors. To ensure proper contact between the soil and the radial and tangent load cells, prior to the placement of the shotcrete reinforced with fibers, the soil surface was smoothed out \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sensor ID** & **Instrumentation** & **Variables Measured** \\ \hline L1, L2 and L3 & Convergences and divergences & Tunnel distortion \\ \hline TSR-01 & Reference surface points & Surface settlements \\ \hline Ext-01 & Extensometer & Tunnel crown settlements \\ \hline RPC-L, RPC-R and RPC-C & Radial pressure cells & Radial stress on primary lining \\ \hline TPC-L and TPC-R & Tangential pressure cells & Axial stress on primary lining \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Instrumentation sensors and variables measured. Figure 6: Schematic representation of the tunnel instrumentation: (**a**) inside the tunnel and (**b**) topographical elevation survey references, modified from [29]. with a thin layer of mortar at the interface between the instruments and the soil, according to the following procedure:  
1. Preparation of the installation area:  
Cleaning was carried out, removing loose material and dust from the surface where the pressure cells were installed. Subsequently, the ground surface was smoothed, using a mortar to obtain a uniform surface to ensure optimal contact with the load cells. 2. Marking of the location:  
The specific points where the radial and tangential pressure cells were installed were marked. 3. Drilling and hole preparation to fix the load cells to the soil:  
Hole drilling on the ground was carried out at the marked locations to place the load cells. Subsequently, the holes were cleaned to ensure that they were free of obstructions, to allow an accurate installation of the sensors. 4. Installation of the cells:  
The load cells were placed, ensuring that they were correctly oriented to measure radial and tangential pressures. Subsequently, the area around the cells was sealed with concrete mortar to protect and fix the sensors. 5.