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

Abstract

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 (VL) has an important effect in estimating the ground movements, the prediction of optimum VL relies on the data from previous case studies with similar ground condition. Realizing unavailability of VL data based on Tabriz geological conditions, from the actual occurred settlements, VL was back-calculates by Gaussian empirical equation to get the VL 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 Smax, applying Mohr–Coulomb model. Smax can be estimated accurately by Gaussian curves, but the distribution of longitudinal surface settlement doesn’t match with these curves. The obtained VL varies from 0.13 to 1.35%, with an average value of 0.46%. The range of the VL increases significantly with the decrease of the tunnel depth. For the studied case, in shallow tunnels (C/D ≃ 1) high values of VL are occurred despite good control of face and grout injection pressures.