The Volume Loss: Real Estimation and Its Effect on Surface Settlements Due to Excavation of Tabriz Metro Tunnel

A case study including monitoring data of surface settlements induced by tunneling in Tabriz metro line 2 is presented. This study focuses on about 4.6 km EPB-TBM tunneling works from west depot to S5 Station, where the tunnel was being excavated in sandy and fine grained alluvial, generally below the water table. Although volume loss (V_L) has an important effect in estimating the ground movements, the prediction of optimum V_L relies on the data from previous case studies with similar ground condition. Realizing unavailability of V_L data based on Tabriz geological conditions, from the actual occurred settlements, V_L was back-calculates by Gaussian empirical equation to get the V_L percentage. Also a back-analysis was done to obtain settlement trough width constant K in both longitudinal and transverse sections. Furthermore, face support and tail grouting pressures were monitored, and compared with COB method. Likewise, a 3D finite element multi-step simulation was carried out by ABAQUS to predict surface settlements. There was concluded that, in longitudinal sections, numerical simulation is not capable of reproducing the recorded S_max, applying Mohr–Coulomb model. S_max can be estimated accurately by Gaussian curves, but the distribution of longitudinal surface settlement doesn’t match with these curves. The obtained V_L varies from 0.13 to 1.35%, with an average value of 0.46%. The range of the V_L increases significantly with the decrease of the tunnel depth. For the studied case, in shallow tunnels (C/D ≃ 1) high values of V_L are occurred despite good control of face and grout injection pressures.