4.1 First Side by Side Geometry
The first model shown in figure 3 was for twin tunnels already constructed at a centre to centre spacing of 3D. This model helped to identify the influence of construction sequence on surface and subsurface ground displacement upon excavating the first and second tunnels underneath the existing twin tunnels. Figure 3: First Side by Side twin tunnels
4.1.1 Surface Displacements
The surface displacements at distances of 4.75m, 7.125m, 9.5m and 11.875m from the centre line of tunnel 1 were computed and then compared with the results calculated from the empirical equations. The displacements were tabulated as shown in table 4 to check for accuracy and consistency of the model. The prediction of surface displacements by Hunt (2005) and Ocak (2014) gave similar results, however there was a discrepancy with the results from the numerical model as shown in Table 3. This is due to many assumptions that were made when using finite element analysis such as the soil was considered as isotropic and linear, and also the volume loss of 0.21% could be the source of error in the results as it is assumed based on past experience on similar projects and on the type of machine and the quality of workmanship (Hunt. D, 2017) . Even though the discrepancy was observed the results were fair for …show more content…
Since the new empirical equation by Ocak (2014) only applies to surface displacements, subsurface displacements were calculated by Equation 2.7 and the two results are shown in table 4.
Table 4: Subsurface displacement Results for the first twin tunnels The subsurface settlement was greater than the surface settlement as observed in table 4 due to the fact that the subsurface is closer to the tunnels than the ground surface where the volume loss
The first model shown in figure 3 was for twin tunnels already constructed at a centre to centre spacing of 3D. This model helped to identify the influence of construction sequence on surface and subsurface ground displacement upon excavating the first and second tunnels underneath the existing twin tunnels. Figure 3: First Side by Side twin tunnels
4.1.1 Surface Displacements
The surface displacements at distances of 4.75m, 7.125m, 9.5m and 11.875m from the centre line of tunnel 1 were computed and then compared with the results calculated from the empirical equations. The displacements were tabulated as shown in table 4 to check for accuracy and consistency of the model. The prediction of surface displacements by Hunt (2005) and Ocak (2014) gave similar results, however there was a discrepancy with the results from the numerical model as shown in Table 3. This is due to many assumptions that were made when using finite element analysis such as the soil was considered as isotropic and linear, and also the volume loss of 0.21% could be the source of error in the results as it is assumed based on past experience on similar projects and on the type of machine and the quality of workmanship (Hunt. D, 2017) . Even though the discrepancy was observed the results were fair for …show more content…
Since the new empirical equation by Ocak (2014) only applies to surface displacements, subsurface displacements were calculated by Equation 2.7 and the two results are shown in table 4.
Table 4: Subsurface displacement Results for the first twin tunnels The subsurface settlement was greater than the surface settlement as observed in table 4 due to the fact that the subsurface is closer to the tunnels than the ground surface where the volume loss