## 4 Results and Discussion
### Influence of Ventilation Duct Position on Dust Dispersion  
The study was conducted at seven different vertical positions along the height of the tunnel (\(h\) = 1.1 m, 1.4 m, 1.7 m, 2.0 m, 2.3 m, 2.7 m, 3.0 m) to determine the dust concentration distribution at cross-sections located at distances of 1 m, 3 m, 6 m from the tunnel wall va vi tri cach nen lo 1.5 m. These sections correspond to the working areas where laborers are engaged in construction activities within the tunnel over time. The average breathing height of tunnel workers in Vietnam was taken into consideration. Furthermore, to assess the influence of the ventilation duct's position on the dust dispersion pattern, the dust dispersion law was determined at different positions along the tunnel's height over corresponding time intervals. The results of the maximum dust concentration at various positions are shown in the following figure:  
Figure 19 reveals that at a distance of 1.0 m from the source, dust concentration is prominently generated within approximately 10 s. Among the models with different heights (\(h\) = 1.1 m to \(h\) = 3.0 m), the model with \(h\) = 1.1 m exhibits the highest dust concentration, reaching around 1344 mg/m\({}^{3}\), while the lowest concentration is observed in the \(h\) = 3.0 m model at 631 mg/m\({}^{3}\). During the period of 10-100 s, dust concentration sharply declines due to rapid dispersion within the tunnel. From 100 to 600 s, the concentration gradually decreases as particles settle and diffuse outside the tunnel. For models \(h\) = 1.1 m to \(h\) = 2.7 m, it takes around 450-480 s for the dust concentration to fall below 10 mg/m\({}^{3}\), whereas the \(h\) = 3.0 m model achieves this level within about 300 s. Throughout the total simulation time, the average concentration at \(h\) = 1.1 m was 71.9 mg/m\({}^{3}\); at \(h\) = 1.4 m, the average concentration was 71.2 mg/m\({}^{3}\); at \(h\) = 1.7 m, the average concentration was 83.4 mg/m\({}^{3}\); at \(h\) = 2.0 m, the average concentration was 67.3 mg/m\({}^{3}\); at \(h\) = 2.3 m, the average concentration was 86.2 mg/m\({}^{3}\); at \(h\) = 2.7 m, the average concentration was 74.7 mg/m\({}^{3}\); and at \(h\) = 3.0 m, the average concentration was 53.6 mg/m\({}^{3}\). The results indicate that the position at \(h\) = 3.0 m provides better dust reduction, while the dust concentrations at other positions do not show significant differences. Figures 20 and 21 show that at distances of 3 m and 6 m from the face, the results also indicate that within approximately 10 s, the dust concentration is lower for the \(h\) = 3.0 model compared to the other positions. It takes about 270 s for the dust concentration to decrease to 10 mg/m\({}^{3}\) for the \(h\) = 3.0 m model, whereas the remaining models require between 450 and 600 s. At the measurement location 3 m from the face with a total time of 600 s the average concentration at \(h\) = 1.1 m was 19.5 mg/m\({}^{3}\), at \(h\) = 1.4 m the average concentration was 29.5 mg/m\({}^{3}\), at \(h\) = 1.7 m the average concentration was 28.9 mg/m\({}^{3}\), at  
Figure 18: The relationship between the average concentration and wind speed at a position 6.0 m away from the face. \(h\) = 2.0 m the average concentration was 24.6 mg/m\({}^{3}\), at \(h\) = 2.3 m the average concentration was 27.9 mg/m\({}^{3}\), at \(h\) = 2.7 m the average concentration was 22 mg/m\({}^{3}\), and at \(h\) = 3.0 m the average concentration was 12.6 mg/m\({}^{3}\). At the measurement location 6 m from the face, the average concentration at \(h\) = 1.1 m was 17.1 mg/m\({}^{3}\); at \(h\) = 1.4 m, the average concentration was 28.2 mg/m\({}^{3}\); at \(h\) = 1.7 m, the average concentration was 31.4 mg/m\({}^{3}\); at \(h\) = 2.0 m, the average concentration was 27.7 mg/m\({}^{3}\); at \(h\) = 2.3 m, the average concentration was 32.8 mg/m\({}^{3}\); at \(h\) = 2.7 m, the average concentration was 31.6 mg/m\({}^{3}\); and at \(h\) = 3.0 m, the average concentration was 8.12 mg/m\({}^{3}\).